Importance of Atmospheric Sciences in Stone Heritage Conservation Study in Italy and Mexico

Abstract

The preservation of heritage is crucial to successfully accomplish the Sustaining Development Goals (SDGs) because it leads to social unity; therefore, evaluating the decay mechanisms of stone-built heritage sites is critical to understanding the role of atmospheric conditions towards their conservation and to developing effective policies in the preservation of heritage and hence, community strength. In this paper, the differences of material decay between Italy’s and Mexico’s atmospheres and the perspectives to implement a more active role of the atmospheric sciences in the conservation of built heritage are presented. The risk assessment methodology proposed by the International Center for the Study of the Preservation and Restoration of Cultural Property (ICCROM) is used to present the reviewed published research because it is one of the few methodologies suggested for amply analyzing cultural property. Currently, in Europe, most research is aimed either to estimate the future decay of built property or to assess the main threats to a single site, on sites for which building materials have been studied previously and on sites for which forecast models have been developed, and the results are useful towards a preventive conservation approach, although the damage functions were developed considering a single climate and might not perform well under different conditions. Built property, however, is present worldwide and the conditions of those studies are not met in most developing countries, as the variation in materials and atmospheric conditions has not been researched yet. This article intends to reduce this gap by comparing both approaches, acknowledging possible common courses of action, and highlighting the role of built heritage in SDGs.

1. Introduction

Stone-built heritage refers to the “unique and irreplaceable landmarks that contribute to the definition of the identity of cities and communities” [1]. This concept has changed during the last century from considering built heritage as only landmarks (i.e., monuments, single buildings) to any space (public squares, markets, neighborhoods, or even cities). All built heritage, regardless of type, is subject to different approaches to conservation which hold important values that might change [1,2,3,4,5,6,7]. It rarely consists of isolated buildings. In fact, there are numerous examples of the United Nations Educational, Scientific and Cultural Organization (UNESCO) listed sites that refer to groups of buildings. These “ensembles” [8] include a broader urban context, including geographical setting.

In general, heritage “enables social cohesion, fosters socio-economic regeneration and poverty reduction, strengthens social well-being, improves the appeal and creativity of regions, and enhances long-term tourism benefits” [9,10], and is included in Target 11.4 of the 2030 Agenda of Sustainable Development Goals (SDG) of the United Nations (UN). However, built heritage can also contribute to targets 3 (good health and well-being), 4 (quality education), and 9 (industry, innovation, and infrastructure) [10]. Conservation and reuse of built assets is one way to assure the well-being of the population; guarantee a quality education; enhance industry, innovation and infrastructure; develop sustainable cities and communities, due to the society’s sense of belonging; create opportunities for communal life, especially in pedestrianized areas; and contribute to the resilience and adaptability of the community by serving as a psychological shelter when facing a disaster [9,10].

Hence, SDGs are directly related to built heritage conservation. In this paper, approaches for built heritage conservation in Italy and Mexico are compared and discussed by using the six-step ICCROM methodology [11]. Throughout several decades, methodologies to understand the decay-relevant variables and mechanisms of built heritage materiality have been developed, mainly in Europe. Global interest in the subject has risen significantly over the present century, utilizing the same developed analytical techniques and methodologies; however, although different building types and atmospheres are present, few have been studied.

1.1. Material Decay

It is important to mention the mechanisms and the atmospheric deterioration agents [10] which act on building materials. The definition of material decay [12] is “The degradation over time of the material’s properties (physical, chemical, mechanical, etc.) and characteristics (texture, mineralogical composition, etc.), leading to the failure of the material as a building component” [12]. In addition, the decay interfaces are highlighted at two scales: macroscale, focused on building pathology; and microscale, focused on the kinetics and the thermodynamics of the phenomenon. To study the material decay, non-destructive or micro destructive analytical techniques are preferred (e.g., XRD, XRF or FTIR [13]) as they allow future availability of the building materials; however, the decay itself might be an indicator of the atmosphere to which the material was exposed [14].

Built property can have up to 27 different pathologies [15], divided into 5 categories: cracks and deformation, detachment, features induced by mass loss, discoloration and deposit, and biological colonization (see Appendix A).

However, those categories describe the visual appearance of different events and do not describe the process the material suffers until it starts to suffer damage. Hence, in this paper, only the pathologies in which the atmosphere plays an important role are considered. Pathologies were grouped depending on the main physicochemical decay drivers: weathering if it is controlled by the climate, or atmospheric deposition-driven if it is controlled by the pollutants in the atmosphere. Both act physicochemically [16].

Weathering drivers are different decay mechanisms caused by atmospheric agents such as relative humidity, temperature, and UV/visible light. These can act through different mechanisms, collectively called physical weathering, any of which results in the fracture of the material due to a repetitive cycle of stress. These decay mechanisms include phenomena such as thermoclastism, sometimes mistakenly called and evaluated as thermal shock, e.g., [17,18]. Thermal shock would mean an abrupt change in temperature, producing the sudden failure of the material [19], which rarely happens in the atmosphere; instead, in thermoclastism, the material suffers repetitive cycles of mechanical stress due to volume changes in mineral grains caused by heating and cooling, eventually producing its failure, and so it must be called thermal fatigue [20,21,22,23,24]. Cryoclastism occurs when a water solution penetrates the material and the temperature reaches its freezing point (below 0 °C, for it is not pure water) and becomes ice; afterwards, temperature increases, the water melts, and the cycle is repeated [19]. Expansion/contraction of hygroscopic minerals, where a physicochemical reaction of minerals occurs with liquid water, makes the material part of the mineral lattice and produces mechanical stress due to the increase in both volume and friction between interphases [25]. Finally, salt recrystallization is due to the migration of salts in a solution passing through the porous system of building materials, with subsequent phase change. It can produce efflorescence or sub-efflorescence. Even if the decay is due to a physical process, it is considered a physicochemical mechanism because different salts preferentially recrystallize depending on the chemical nature of the deposited and building materials [26,27,28].

Another important decay mechanism related to the natural environment is biodeterioration, caused by the effects that living organisms might have on the building materials. These effects can be chemical due to the solubilization of the substrate, physical due to the growth of roots, or both [29,30,31,32,33,34,35].

Atmospheric deposition-driven decay is caused by the interaction between the stone and the pollutants present in the atmosphere. When gases and particles are removed from the atmosphere and settle either by physical or chemical means on different receptors, including ecosystems, buildings, animal and vegetal species it is called “atmospheric deposition” [36]. It can be dry or wet: dry deposition includes physicochemical mechanisms such as sedimentation, turbulent diffusion, and electrical migration, and is controlled by meteorological factors, particle, and surface characteristics [37]. In wet deposition, pollutants react with water and are dissolved. Afterwards, precipitation in the form of rain, hail, snow, or even fog remove the dissolved pollutants from the atmosphere [38]. An example is the Karst effect, which is stone decay due to wet deposition in an atmosphere of CO₂, with an acidic pH of approximately 5.6. Even though they are simplified as two different phenomena, dry and wet atmospheric deposition are related: when precipitation reacts with the soiling crust of built heritage, mass can be removed, either by dissolution or decohesion of the damaged layer. However, dry deposition is accepted as the major factor harming cultural heritage [39,40,41]. Atmospheric deposition effects can be studied similarly to geological weathering [42,43], since these mechanisms do not differ substantially from rock weathering in natural environments, but when it comes to stone decay sources, there is an important difference: atmospheric deposition is completely related to anthropogenic sources, whereas weathering is considered to be naturally occurring.

Atmospheric deposition can act mainly through two mechanisms: leaching and soiling [36]. Soiling (also called blackening) refers to the accumulation of atmospheric pollutants, reacting and forming mainly carbonaceous particles on the surface. It is a chemical decay process due to dry deposition [44]. Among most atmospheric pollutants, gases and particle matter (PM) are important due to their harmful effects on the environment, heritage and human health. The impact of PM in human health has been studied widely (e.g., [45]), and models have been developed to evaluate the source (the emitting entity) concentration and the resulting deposition (e.g., [46]). Leaching occurs when the rock is dissolved by an acidic pH in rain (the pH of natural rain is around 5.6). It is thus a chemical decay process resulting in material loss [43]. The effects of atmospheric deposition on different receptors depends on the amount as well as the chemical composition. Therefore, it is important to have atmospheric monitoring networks to evaluate chemical concentrations and emissions trends in order to determine the potential pollution sources [46], hence the impact human activity has on stone decay.

1.2. Building Materials

As built heritage conservation is desirable, one of the first features to be considered is the material characterization and the identification of mechanisms mentioned previously, but its most important threat changes depending upon both its properties and the surrounding atmospheric conditions.

The stone materials used for the construction of built property are innumerable. Among the most used materials worldwide is natural stone from near the building site, due to the difficulty and cost of transporting stone over great distances to the point of use [47,48,49]. Most research of historic built heritage in Southern Europe has focused on carbonate stones (limestone, marble, dolomite), due to their properties (such as a smooth surface, high reflection of light, and the ability to be easily painted) despite their vulnerability to the atmosphere.

Plenty of studies on material decay have been carried out, mainly in Europe, and especially in Italy (e.g., [14,44,50,51,52,53,54,55,56]), with a few examples in countries outside the European Union, such as Vietnam, Panama, Colombia, Brazil, and Mexico [57,58,59,60,61,62,63]. These studies have focused on carbonate stones, but other materials such as mortar, volcanic stone, or granite have also been studied (e.g., [22,50,54,56,57,58,59,60,61,62,63,64,65,66]).

Besides the differences in decay mechanisms due to the materials’ intrinsic properties, weathering also depends on climate. The authors of [25] suggested that the decay of stones does not depend solely on traditional climate parameters, such as mean annual precipitation, but also on more complex variables, such as wind-driven rain. Thermoclastism, which depends on insolation, can be relevant to carbonaceous stones’ decay due to their anisotropy, and also to that of volcanic stones, due to their heterogenous grain size and chemical composition, because each mineral expands and contracts at a different rate, making these stones prone to failure through thermal fatigue [21,22]. Hygric expansion can also be important, since the presence of clay materials in the volcanic tuff can lead to weathering due to micro-porosity and their average pore radius [63,66].

The contribution of this review is not only intended to highlight how atmospheric sciences can contribute on the study of built heritage decay, but also to address punctual aspects that need to be addressed and are normally overlooked. For example, the damage functions are developed for specific climates and are normally used everywhere as if the response of the material would be the same. Furthermore, it classifies the sources consulted within the ICCROM risk assessment, which is not normally addressed, but can become handy for conservators, researchers, stakeholders and decision makers.

2. Research Aim and Relevance

As stated previously, preservation can play an important role in achieving the SDGs: besides ensuring the well-being of the population, it guarantees a quality education, and helps to develop industry, innovation, infrastructure, and sustainable cities and communities. Aside its intangible dimension, it also involves research to identify important hazards for built heritage and innovative solutions to ensure its preservation, which includes the monitoring systems, holistic approach and arguably the development of environmentally friendly coatings.

Damage of stone-built heritage is ineluctable because it is impossible to isolate all the buildings from its atmosphere, and hence, atmospheric sciences play a crucial role in the study of its decay, but although atmospheric conditions are shared in different places, the approach towards built heritage conservation is different depending on the area. Hence, the aim of this paper is to highlight the importance of the atmospheric conditions in the approach towards stone-built heritage conservation through comparing two different countries, discussing their similarities and differences.

The atmosphere plays a crucial role in the material decay of built heritage: atmospheric variables acting jointly control the undergoing deterioration through several mechanisms as the ones mentioned in Section 1.1. However, the goal of this review is not to classify those variables importance in physical weathering nor identify the main atmospheric threat (or threats) towards stone heritage, for it would only be valid only for specific study cases. In this review, it is presented how atmospheric sciences are important to enhance the atmospheric hazards the stone heritage is exposed to, and hence, ensure a longer preservation with a more comprehensive approach. The topic is important because several actions have been developed to diminish threats to cultural heritage, some of them with the aim to prevent the damage. Examples of this are [67] to prepare a plan to manage disaster risks of several kinds, including natural disasters and human-induced (as pollution or war) or the Guide to risk management of cultural heritage [11]. Both explore the idea that, although the harm of cultural heritage might seem inevitable, to establish an action plan does not only ensure its preservation, but also “can contribute positively to disaster risk reduction” [67]. However, most research has focused on the development of coatings, and though undoubtedly useful, they fall short to solve the problem from the root, because the periodic application of coatings is enough to delay its decay, but not to dimmish the threat, and can even contribute to pollution [68], though in the last decade this issue has been discussed and the so called “green conservation” has proposed a number of different ideas to reduce the hazard towards the conservators and the environment [69]. In this very line, a better conservation would mean diminishing the number of potentially-pollutant interventions, and the amount of new material exploited from the quarries to carry out an intervention.

3. Methodology

A systematic literature review structure was used in this work, considering the ICCROM risk assessment methodology [11]. To evaluate if published papers used part of this risk assessment, and considered the exposure of the stone heritage to different environments, Italy and Mexico were selected. Both countries have several built heritage sites with available data on atmospheric deposition and air quality for some regions with different climates, published papers on the decay of building materials, and have different attitudes towards the economic potential of cultural heritage.

An important step in the study of atmospheric-driven decay is to list the decay-relevant variables, which can be used in different steps of the ICCROM methodology to predict cultural heritage decay. For atmospheric sciences, there exists the concept of Essential Climate Variables (ECVs) which are “data records intended to provide reliable, traceable, observation-based evidence” [70]. The surface ECVs are precipitation, temperature, water vapor (commonly measured as relative humidity, one of its concentration units), wind speed and direction, pressure, and surface radiation budget [70]. Although there are only six variables, they can interact among each other to produce different datasets, such as in wind-driven rain. Additionally, they have a strong influence on atmospheric composition, since meteorological conditions are the main factor affecting the dispersion of air pollutants [71].

In this work, a total of 67 articles from 1981–2022 were reviewed: 55 from Italy and 12 from Mexico. Depending on the type of work described in each paper, they were classified according to identification, analysis, assessment, minimization, and monitoring [11]. It should be mentioned that the term “threat” used by the ICCROM is known as “minimization” in atmospheric sciences. Some articles fulfill more than one category, and in those cases, the article was counted multiple times. The list and classification for all of them is presented in

Table 1. Classification of analyzed research papers.

Country	Context	Identification	Analysis	Evaluation	Minimization	Monitoring
Italy	6 [72,73,74,75,76,77]	34 [1,14,17,51,53,54,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105]	29 [1,14,17,20,51,53,54,79,80,81,83,85,87,88,89,91,92,93,94,96,98,99,101,104,105,106,107,108,109]	30 [1,14,17,20,44,51,53,80,81,83,85,87,88,89,91,92,93,94,98,99,101,102,104,106,107,108,109,110,111,112]	21 [1,14,17,44,53,94,99,101,106,107,108,110,111,112,113,114,115,116,117,118,119]	17 [17,24,44,50,52,80,107,110,111,112,114,115,117,119,120,121,122]
Mexico	4 [123,124,125,126]	8 [27,41,61,62,63,126,127,128]	12 [27,41,61,62,63,126,127,128,129,130,131,132]	8 [27,41,61,62,63,126,127,128]	2 [127,128]	1 [133]

, with a graphic summary in Figure 1.

Is important to highlight that, among the 10 deterioration agents listed by the ICCROM in 2016, 4 are atmospheric: 3 natural (temperature, relative humidity, radiation as UV/visible radiation), and the others anthropogenic factors affected by natural conditions such as pollutants.

Risk management should include (1) The knowledge of political, physical, and cultural environments; the legal implications of the asset, the financial, administrative and operational aspects to determine the risks heritage is exposed to; and the possible action courses (“context”); (2) The recognition of deterioration agents such as relative humidity, temperature, UV/visible radiation; anthropogenic factors such as pollutants, physical forces due to manipulation, dissociation of the object with its record, and vandalism; and the effects of fire, pest, and water-as-flooding (“identification”); (3) Estimation of the damage or the decay parametrization by using the ABC scale to evaluate the expected damage (“analyze”); (4) Determine the priority of the menace and different threats (“evaluate”); (5) Minimization or threat to cultural heritage for the risk management plan and (6) Assessment of the effectiveness of the measures taken for diminishing different risks. For atmospheric factors, periodic or continuous monitoring campaigns should be included.

The aforementioned criteria were also used in the discussion to point out the importance of knowing the atmospheric conditions that have an impact on built heritage, in order to better understand the material decay and, additionally, to improve the knowledge for the policy assessment of heritage conservation in comparing the different approaches that Mexico and Italy use.

The literature review that follows examines the extent to which the ICCROM risk assessment steps were followed. It will be emphasized that, although sometimes disregarded, characterizing the atmospheric conditions of built heritage is crucial for obtaining an accurate understanding of material decay and for developing public policies and conservation strategies. In addition, two published articles for each country are highlighted in order to exemplify differences in the risk management approaches between Mexico and Italy.

4. Analyzed Frameworks: Italy and Mexico

The Italian and Mexican approaches to cultural heritage are the result of different views and evolutions of ideas about heritage, although both are aimed to protect cultural assets. For clarity, each country is presented in a different section below. Most of the research carried out in Italy referred to specific monuments or to a building materials regionally used, whereas the research in Mexico has focused on carbonate building materials used in the Yucatan Peninsula.

4.1. Italy

The Ministero della Cultura (Culture Ministry) oversees the conservation of cultural heritage sites. The approach to cultural heritage in Italy is focused on exploiting its economic dimension because, according to the official census, there are more than 100,000 immovable heritage sites, including archaeological sites, architectonic heritage, and museums in Italy [134]. For example, the Istituto Guglielmo Tagliacarne [74] reported on the involvement of companies in cultural issues, including their role in the development of new technologies. Although they mention a lack of investment, the willingness to stimulate private funding and a business-oriented vision is clear (although the social environment might not be prepared yet [75]).

This role has been encouraged during the last decade, promoting the participation of Small and Medium Enterprises (SMEs) and the development of advanced equipment for the diagnosis and research of new treatments for the conservation and protection of cultural heritage. Nowadays, it is more and more common to find private funding in the research of new technologies for the preservation of cultural heritage, resulting in an interesting framework: both public and private money being dedicated to the research of the decay of building materials, which is being performed in public institutions, who later publish their results, and whose knowledge can be then exploited by the funding company.

Some important results of this collaboration might be the use of commercial compounds in the preservation of different cultural assets, developed by a company, then tested by different institutions who later published their results in peer-reviewed journals (e.g., [53,86,135]). This may result in both a commercial and conservational success, or the development of portable analytical equipment (even if that funding is not always declared explicitly, producing a potential conflict of interests), such as the X-Ray Fluorescence portable equipment developed by ENEA Tech and INFN, sponsored by CAEN, a private enterprise [136].

In this approach, all of the involved actors seem to benefit: the funding companies might make a profit, the institutions are able to continue their research on the conservation of cultural heritage, further works in the field are generated, and finally, civil society has its heritage preserved. However, this is only possible with a legislative context in which the actors, functions, contracts, and the related institutions are clearly stipulated, as in [76]. This is mainly because the cultural heritage economic dimension is used not only as an economic pole of development, but also because heritage becomes self-sustainable, with the resources to ensure its preservation is obtained by exploiting itself.

As mentioned previously, most publications consulted do not limit themselves to performing only one risk assessment step. We therefore summarize the reviewed literature as part of each step, but it is important to remember that for each asset, the point at which one step finishes and another begins is not always clear. That is because when studying the conditions or the conservation state of an asset, the ICCROM risk assessment protocol is not necessarily present.

4.1.1. Identification of Risk Assessment

In this step the decay mechanisms, the ECVs producing it, and the building materials themselves are investigated. Concerns regarding the effect of air pollution on the decay of building materials can be traced back to the 1950s [137]. One of the first attempts to reduce its effect was the Convention on Long Range Transboundary Air Pollution in 1979, which resulted in the implementation of national emissions targets for all participating parties [138]. The focus was first on acid rain, because it seemed to be the main atmospheric threat, but research showed the magnitude of sulfur- and nitrogen-bearing compounds’ impact [139,140,141,142,143,144]. The first works considered here are dated back to the 1980s [16,100,144,145,146]. They focused on understanding the airborne particles that were later found on outdoor cultural assets, resulting in a deeper understanding of the growth mechanisms of decay crust in limestone.

4.1.2. Analysis on Risk Assessment of Cultural Heritage in Italy

In the analysis step, it is important to be able to somehow quantify the damage. This is mandatory, because it allows researchers to compare the effect of the atmospheric processes on the decay of building materials., but it is not possible yet to define which is more important.

Following the need to understand and quantify the decay on different materials, the European Union started the International Cooperative Program on Effects on Materials, including Historic and Cultural Monuments (ICP Materials). The network has about 30 exposure sites across 18 countries in Europe and North America. Some groups have tried to recreate possible atmospheric conditions with different materials in controlled chambers (so called “accelerated aging test”), with the restraint of recreating real conditions, but the advantage of significantly reducing the time it takes for a material to decay perceptibly [147].

Important publications of this type include [54,86,140,141,142], in which specific aspects of the pollutants–materials interaction and the resulting decay were reported. The weathering test, either accelerated (using a chamber) or in natural conditions allowed researchers to determine the damage functions (or so-called dose-response) for those condition and materials, and quantify material loss (commonly express as surface recession) [143,144]. These relationships, in which material decay is quantified as material loss or, more commonly, surface recession, apply to specific conditions and materials [139].

Surface recession is an example of a tailored variable: most of the work performed has focused on marble; since it is a material with very low porosity, surface recession is an acceptable indicator for its decay. For materials with a higher porosity (higher than 2%), however, surface recession is not the best decay indicator, because lixiviation happens not only at the surface, but also inside the porous material. Thus, material loss is a better indicator of decay and would also be more comparable among different building materials. The damage functions quantify the damage and rank the sources’ contributions.

Several authors [44,50,52,143,144,148,149,150,151,152] present damage functions. The authors of [44,143] summarized different damage functions and showed that the Lipfert and Livingston functions perform best. Some mathematical models include meteorological variables or wet atmospheric deposition to predict the decay as mass loss, but there are cases in which other decay mechanisms, such as weathering, may be more important, as in [17,28]. Examples of damage functions are presented in

Table 2. Damage functions.

Source	Equation	Involved Variables
Lipfert [144]	$\mathrm{L}=18.8\mathrm{R}+0.016\left[{\mathrm{H}}^{+}\right]\mathrm{R}+0.18 \left({\mathrm{V}}_{\mathrm{dS}}\left[{\mathrm{SO}}_2\right]+{\mathrm{V}}_{\mathrm{dN}}\left[{\mathrm{HNO}}_2\right]\right)$	L = annual surface recession (µm/year); R = precipitation (mm/year), [H+] = H+ concentration in rain (µmol/L), VdS = SO2 deposition rate(cm/s), VdN = HNO3 deposition rate (cm/s); [SO2], [HNO3], concentration (µg/m3)
Livingston [148]	$\begin{array}{cc}\hfill \mathrm{\Delta }\left[{\mathrm{Ca}}^{2+}\right]=\mathrm{\Delta }\left[{\mathrm{SO}}_4^{2-}\right]& +\{{10}^{-11.6}(\frac{1}{{\mathsf{\gamma }}_{\mathrm{r}0}{\left[{\mathrm{H}}^{+}\right]}_{\mathrm{r}0}}+\frac{1}{{\mathsf{\gamma }}_0{\left[{\mathrm{H}}^{+}\right]}_0})\hfill \\ & -\left(\frac{1}{2}{\left[{\mathrm{H}}^{+}\right]}_{\mathrm{r}0}-{\left[{\mathrm{H}}^{+}\right]}_0\right)\}\hfill \\ & +\{{10}^{-11.6}(\frac{1}{{\mathsf{\gamma }}_0{\left[{\mathrm{H}}^{+}\right]}_0}+\frac{1}{{\mathsf{\gamma }}_{\mathrm{r}}{\left[{\mathrm{H}}^{+}\right]}_{\mathrm{r}}})\hfill \\ & -\left(\frac{1}{2}{\left[{\mathrm{H}}^{+}\right]}_0-{\left[{\mathrm{H}}^{+}\right]}_{\mathrm{r}}\right)\}\hfill \end{array}$	Δ[Ca2+] y Δ[SO42−] concentration difference between rain and lixiviate; [H+]0 effective hydronium ion in the absence of anthropogenic pollutants (10−5.6); [H+]r effective hydronium ion on rain; [H+]r0 hydronium ion concentration on lixiviate and activity coefficient. All concentrations are expressed as (mol/L)
Webb et al. [149]	$$\begin{gathered} \mathrm{L} \left(\mathrm{mol}\right)={\mathrm{ADV}}_{\mathrm{dS}}{\mathrm{C}}_{{\mathrm{SO}}_2}+\left(\frac{{\mathrm{K}}_{\mathrm{H}}{\mathrm{K}}_1{\mathrm{P}}_{{\mathrm{CO}}_2}}{2{\left[{\mathrm{H}}^{+}\right]}_{\mathrm{r}}}\right)3\mathsf{\Sigma }\left({\mathrm{A}}_{\mathrm{i}}\mathrm{R}-{\mathrm{E}}_{\mathrm{vap}}\right) \\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}+\left(\frac{{\left[{\mathrm{H}}^{+}\right]}_{\mathrm{i}}}{2}\right)3\mathsf{\Sigma }\left({\mathrm{A}}_{\mathrm{i}}\mathrm{R}\right) \end{gathered}$$	CSO2 = SO2 average concentration during exposition (µmol/m3), VdS = SO2 deposition rate(cm/s), D = duration of exposition (s) Ai = rain exposed area; [H+]i, [H+]r = Average weighted mean of hydronium ion for lixiviate and rain (mol/L); Evap = volume of rain evaporated from rain (mm3); KH, K1 = equilibrium constants of carbonate and bicarbonate at CO2 = 350 ppm; R = precipitation (mm)
Baedecker et al. [150]	$\mathrm{L} \left(\mathrm{mmol}/\mathrm{L}\right)=0.16\left[\frac{1.0-0.015\mathrm{T}+0.0000922{\mathrm{T}}^2}{0.683+0.49\left[{\mathrm{H}}^{+}\right]}\right]$	T = Temperature (°C), [H+] = Hydronium ion concentration (mol/L)
TIdblad et al. [151]	$\begin{array}{c}\mathrm{L}=2.7{\left[{\mathrm{SO}}_2\right]}^{0.48}\exp ({\mathrm{f}}_{\mathrm{Pl}}\left(\mathrm{T}\right){\mathrm{t}}^{0.96}+0.019\mathrm{R}\left[{\mathrm{H}}^{+}\right]{\mathrm{t}}^{0.96}\\ { \mathrm{f}}_{\mathrm{Pl}}=-0.018\mathrm{T}\end{array}$	L = annual surface recession (µm/year); [SO2] = concentration (µm/m3); T = Temperature (°C), R = precipitation (mm), [H+] = hydronium ion concentration (mg/L); t = time (years)
Kucera et al. [152]	$$\begin{gathered} \mathrm{L}=3.95+0.0059\left[{\mathrm{SO}}_2\right]{\mathrm{RH}}_{60}+0.054\mathrm{R}\left[{\mathrm{H}}^{+}\right] \\ \hspace{1em}\hspace{1em}\hspace{1em}+0.078\left[{\mathrm{HNO}}_3\right]{\mathrm{RH}}_{60}+0.0258{\mathrm{PM}}_{10} \end{gathered}$$	L = annual surface recession (µm/year); [SO2] = concentration (µm/m3), RH60 = relative humidity if RH > 60, otherwise 0; R = rain (mm); [H+] = hydronium ion concentration (mg/L); [HNO3] = concentration (µm/m3); PM10 = concentration (µm/m3)

. Some of these functions have a very limited use, such as Livingston and Webber: measuring activities and equilibrium constants in such a complex solution as rain, which changes every day, is virtually impossible.

Damage functions are crucial for estimating damage. Even if the results they offer are indicative, it is important because it is a tool to help decision makers choose which assets are under a more severe menace. However, functions from Table 2 (except for Livingston and Webber) are hindered by the fact that they can only be used on carbonate stones whose porosity is below 2%, and hence represent decay as “surface recession”.

4.1.3. Evaluation on Risk Assessment of Cultural Heritage in Italy

Data from measured or calculated decay are useful to assess stone-built conservation states, and hence the threats can be ranked in importance. There are two important examples: an Atlas [50] which estimates the current damage towards carbonate stone, and the comparison between different climate scenarios, including climate change. The authors of [48] estimated the outdoor and indoor atmospheric conditions. The simulation of indoor conditions is useful to know how aggressive they will be towards the conservation of a collection.

4.1.4. Minimization on Risk Assessment of Cultural Heritage in Italy

When the sources are known and ranked, prevention, control and mitigation measures can be proposed. These measures are termed “minimization” in atmospheric sciences or “threat” by the ICOMOS. Recently, research has focused on the development of new coating materials, which must be reapplied periodically. Most reviewed articles reported the evaluation of products used to preserve the building material, such as cleaning [153] or “protecting” [115,116,117,118] treatments, or the use of a technique for the intervention on building materials [113], or a pilot site to test the materials [1]. However, application of protective coatings implies a higher investment and a less environmentally friendly alternative, due to the use of nano plastics [68], although the possibility of environmentally friendly alternatives has been mentioned [69]. Some other papers reported general recommendations towards preservation of the asset, always using analytical techniques or data collecting to identify the most probable decay sources i.e., [14,116,117].

4.1.5. Monitoring on Risk Assessment of Cultural Heritage in Italy

Monitoring refers to observation and constant evaluation o the measures taken, to determine on their effectiveness and adequacy, but this step also included the research in which forecasts of the decay of built heritage are included. In addition, it is a powerful tool that helps to decide which assets are more prone to decay easily and to focus the efforts and resources on those assets. Research published included monitoring to propose an expected decay on the future scenarios, using damage functions to predict the damage on building materials (e.g., [24,50,52,114,121]). However, Other reserachers reported on the parameters harmful towards the asset, as in [108,115,117,119].

4.1.6. Examples of Published Research at Italy

Two examples are presented to highlight the importance of publishing the research on risk assessment of a single asset, in which atmospheric surveys play an important role. In the first one, the in situ measures are shown: the already-conducted research was used to propose the best-suited intervention methodology towards a long-lasting conservation of the Monza Cathedral façade [1], selecting already-developed treatments and commonly applied analytical techniques to clean, consolidate, and protect building materials, using a monitored pilot area. In the second one, external measures were taken: the results from an atmospheric survey in the Baptistery at Florence [14] was used to convince policy and decision makers to act in favor of built heritage conservation with the pedestrianization of the square in which not only the Baptistery, but also the Cathedral and belltower, are emplaced.

In the study by Gulotta and Toniolo [1], it was argued that a conservation project with a strong knowledge foundation through a multidisciplinary assessment should be considered. The Monza cathedral, originally constructed with marble and Varenna stone (a black limestone), was later replaced with Oira stone (a serpentinite). It was concluded that the stone surfaces suffered decay due mainly to the exposure conditions and its own reactivity; the main decay typologies were black crust and granular disintegration. Among the main threats, anthropogenic pollution is thought to be the main threat, because Monza is densely populated and suffers from severe air quality problems, mainly from PM₁₀, NO_x and O₃. Authors selected a representative area in the façade, to develop the pilot tests considering the condition survey and material characterization, the assessment of the conservation treatments, and the scale-up for the executive project. In the first step, historical data, information on the materials, and assessment and identification of the main mechanisms were identified. This phase included the use of different analytical techniques [154], such as petrography, XRD, FTIR, and “specific diagnostic techniques” to identify biocolonization. In the second step, all of the actions to counteract the decay mechanisms were applied, from the selection of the best suited materials and conditions, to the application of different-scoped products (such as protection coating or consolidation). Subsequently, researchers monitored the area for one full year. They even considered that the results obtained during phase two can be applied for accelerated laboratory tests, in order to finally reach phase three, in which the definitive conservation methodologies for the intervention are proposed and applied to the whole building.

In the second example [14], an aerosol sampling campaign in 2003 was performed at the Baptistery in Florence. This is part of one of the most famous Italian ensembles, including not only the Baptistery, but also Giotto’s belltower and the Santa Maria dei Fiori Cathedral. The results from the campaign showed that the main pollution sources were vehicular emissions, but the number of visitors contributed to the resuspension of particulate matter; it also showed that the chemical composition of particulate matter was carbonaceous carbon, non-carbonaceous carbon, and ions in the Total Suspended Matter (TSP, which includes all the suspended particles present in the atmosphere). The authors concluded that there was a “damage risk from air pollution that was higher than that of other major historic buildings located in pedestrian areas of large Italian and western European cities”. In October 2009, after data collection, and after the campaign, the City Hall pedestrianized the square because at the time, around 2000 daily buses used to pass through it. In 2017, an assessment of the measures taken, and new recommendations, reported the success of the measure [155].

At present, case studies remain useful because, if the material decay is known, and the main variables are either monitored or estimated, the preventive conservation approach can be applied and different methodologies for evaluating interventions can be planned.

4.2. Mexico

In Mexico, there were 42,614 archaeological sites, 18,770 historical monument zones, and 121,531 artistic monuments of national interest registered in 2010, when the last built heritage atlas was published [123]. There are two public institutions obligated to ensure the preservation of cultural heritage conservation: Instituto Nacional de Antropología e Historia, (National Institute of Anthropology and History, INAH) and Instituto Nacional de Bellas Artes (National Institute of Fine Arts, INBA) [124].

In the legislation, the private companies are considered as actors, but there is no public policy encouraging investment in the research or conservation of cultural property; it is established that companies performing conservation procedures must be advised by the aforementioned public institutions. Consequently, the conservation of built heritage is the charge of INAH and INBA, and there are few enterprises on the conservation/restoration branch. Most of these are SMEs. [125]. The economic dimension of cultural heritage is not exploited in Mexico. Explaining the reasons for this is beyond the scope of this paper, but socioeconomical problems, including security, have adversely affected international tourism in Mexico.

In Mexico, there are numerous climates according to the Köppen–Geiger classification system, which depends on mean temperature and precipitation, but other methodologies utilize variables such as solar radiation budget [156]. In our opinion, it is more precise to use the latter, as in Mexico the solar radiation budget is quite different than in European countries [71]. Weathering by solar radiation is a subject that has been completely neglected in the published literature.

Most of the relevant research in Mexico is performed by universities, funded by the Consejo Nacional de Ciencia y Tecnología (Science and Technology National Council, CONACYT). Only qualified experts certified by the INAH and INBA are permitted to perform interventions. This serves as a disincentive for scientists at public universities to publish in peer-reviewed journals on this topic. Nevertheless, university, INAH, and INBA scientists produce and communicate valuable knowledge on the conservation of cultural heritage (for example, in the INAH journal Intervencion). The result is that much of the current knowledge on the conservation of materials or analytical techniques in Mexico is at the INAH/INBA archives.

Although research has been carried out in México, the quantity is still small when compared to that performed in Italy, especially because publication in peer-reviewed journals is not among the main objectives of the INAH and INBA and hence, most knowledge of built heritage is not easily accessible for all.

4.2.1. Identification on Risk Assessment of Cultural Heritage in Mexico

In Mexico, most research has focused on study cases, rather than on comparing different decay mechanisms and identifying the deterioration agents. The works consulted [62,126] include a brief description of the important atmospheric variables, but their identification lies more in the building materials. The authors of [41,61] performed both material identification and surveys to understand the surroundings to which the cultural asset is exposed; meanwhile, the authors of [27] performed accelerated tests to identify the hazards of the mechanical decay of stones used as building materials, as well as identify, ing them. Pérez and Lima [127,128] characterized a volcanic tuff from Guadalajara and investigated the best consolidating material for volcanic stones.

4.2.2. Analysis on Risk Assessment of Cultural Heritage in Mexico

Authors represent material decay in different ways: those of [27,41,61,62,63,131] do not quantify the damage, but classify the most important decay mechanisms; meanwhile, the authors of [130,131,132] measure decay with surface recession (although porosity of the sampled materials is around 10%, well above the suggested limit of 2% for this variable); the authors of [129] quantify the stone decay as compression strength, as measured with a sclerometer. Those of [127] represent decay as the change in compressive strength.

4.2.3. Evaluation on Risk Assessment of Cultural Heritage in Mexico

According to the data consulted, for some researchers [126], acid rain is the main deterioration agent, but for others [41,61], it can be atmospheric pollutants, biodeterioration, or water, depending on the site and material. The authors of [62] focused more on the intrinsic properties of the material and proposed that physical weathering caused by anisotropy was likely the main decay cause. In the work of [27], the swelling of clay minerals as a main decay mechanism was identified. Meanwhile, the authors of [27,62,63] characterized the thermal and hygric expansion properties of different volcanic stones, often used as building materials in central Mexico, and concluded that most natural stones are not at risk because of the surrounding climate. In addition, cryoclastism was not researched because it is negligible, as most climates present in Mexico present few days in which freezing temperatures are reached.

4.2.4. Minimization on Risk Assessment of Cultural Heritage in Mexico

Minimization is a subject that remains to be addressed. In Italy, 17 papers were about the minimization of risks, whereas in Mexico, this occurred in only 2 of the consulted works. This might be because, as was explained previously, the institutions responsible for preserving built heritage test different materials, but often do not publish their research. Two consolidating materials were developed in [127,128], and they selected the best options depending on the volcanic tuff they tested, according to the accelerated weathering tests.

However, two of the consulted papers propose mitigation measures that can help to preserve the building materials. Their recommendations, however, are general (i.e., “the air quality must be improved to preserve the facades of monuments made out of ignimbrite” [119]; “evaluation of a consolidant” [128]), and thus cannot be considered actual conservation strategies, but actions.

4.2.5. Monitoring on Risk Assessment of Cultural Heritage in Mexico

The final step of risk assessment was proposed in only one article published about Mexico [133]. The authors proposed the use of an existing damage function to estimate the damage that built property will suffer in Mexico City. Using the Lipfert function, they calculated the “surface recession” of marble will suffer. Though a great effort, it is not very useful, for there are only five monuments in practically the same area which can be expected to behave the same way. Despite the reduced amount of published work in Mexico, however, risk assessment steps are presently being studied.

4.2.6. Examples of Published Research at Mexico

To exemplify the research carried out in Mexico, two examples are presented: [132] and [61]. They were selected because they both used atmospheric data, but in [128], they used only existing measurements, whereas in [61], the authors performed direct surveys to learn specific characteristics of the atmosphere.

The case of Kahl [132] is the culmination of a series of articles. In Mexico, the effects of atmospheric deposition have received attention since the 1990s, with different groups in the country researching the subject. As an example, the Section of Environmental Pollution of the Atmospheric Sciences Center (Sección de Contaminación Ambiental del Centro de Ciencias de la Atmósfera, SCA-CCA), one of the pioneer groups in Mexico for studying the effect of atmospheric deposition on built heritage, used their expertise in the sampling, monitoring, and assessment of air pollution and atmospheric deposition to conduct a study on the wet deposition in the Gulf of Mexico. There are two regions in the country where atmospheric deposition has been evaluated in a systematic and formal way: Mexico City and the Gulf of Mexico. Mexico City observes acid rain events at most of its wet atmospheric deposition sampling sites, having a sulfate/nitrate ratio of 1.5 [157]. Sampling sites in the Gulf of Mexico region (state of Veracruz) have registered high sulfate levels in measured wet atmospheric deposition and a sulfate/nitrate ratio of 4.9, indicating that sulfur oxides are the main acid rain precursors [158,159,160].

The researchers subsequently used the resulting data to research its effect on built heritage in a variety of ways. The first way is an example of a lack of context for the research, because they studied the effect of acid rain on limestone and reported it as “surface recession”, using material sampled from Tulum and El Tajin, whose porosity is around 10% [131]. In another study, decay was simulated using an accelerated weathering chamber [130]. Later studies used atmospheric deposition measurements to determine its chemical composition, and concluded that acid rain was the most important problem for the building material [130]. One of the most interesting aspects of their research was the determination of the provenance of acid rain precursors using back trajectory models [132], and determining that even if air quality on the shore is well below the air quality of national standards, a probable source is the air pollution present at Mexico’s offshore oil platforms, because of the high concentration of sulfur-bearing compounds present in the rain [159].

The research published [61] starts with a broad description of atmospheric phenomena during wet deposition. Their work is very interesting because, using historical meteorological data, they suggest seasons of the year in which the building materials are at a higher risk of decay. Regarding pollution, they discuss how the deposition of sulfur is not as important on metal corrosion as the presence of marine aerosols, due to its chloride (Cl⁻) content. When it comes to the atmospheric deposition and air quality analysis, they performed several surveys to study the concentrations of SO₂, NO_x, total suspended particles (TSP), particulate matter with an aerodynamic diameter of less than 10 μm (PM₁₀), and Cl⁻. They also surveyed wet deposition (rain) for two years, collecting rain samples and analyzing their chemical composition: sulphate (SO₄²⁻), nitrate (NO₃⁻), chloride (Cl⁻), and calcium (Ca²⁺). Based on these data, they concluded that the volume and intensity of precipitation is an important decay factor, but rain pH is not.

They also performed non-destructive techniques to characterize both the building materials and their decay products from the San Pedro Fort. They identified the materials as limestone with some traditional mortars, and detected the presence of minerals containing oxalate, an indicator of biodeterioration. Gypsum was also identified as a decay product, and they suggested heavy traffic surrounding the Fort as the most probable source.

This study also analyzed the decay of the San Francisco de Asis Convent and Basilica Minor from Havana, Cuba. Contrary to Campeche, Havana presents poorer air quality; in fact, indoor pollutant concentration at the Basilica was higher than the outdoor pollution in Campeche. They measured NO₂ concentration and deposition rates for sulfur-bearing compounds and chloride. The building material was also characterized as limestone, and the same oxalate-bearing decay products were found, indicating that although the SO₂ concentration is high, it is not high enough to act as a microbicide. They did find some decay products related to the presence of SO₂, however.

Among their most important conclusions was the need for air quality and atmospheric deposition data. They also concluded that in Campeche, material decay is related to natural atmospheric agents (with salt recrystallization as the main mechanism), whereas in Havana, the decay is mainly anthropogenically driven due to pollution.

5. Discussion

Currently, research both in Italy and Mexico has focused on single sites. At these sites, researchers typically identify the environmental causes and propose prevention, control, and mitigation measures to prevent further decay. All of this would not have been possible without the effort of numerous research groups worldwide. Italy is the pioneer in exploring the idea that built heritage might suffer due to atmospheric conditions, and each asset should be treated according to its characteristics. As a result, instead of having a building material approach, a case-by-case approach was proposed, which resulted in better-suited measures towards conservation for that particular asset.

Concerning the risk assessment of built heritage, the major differences between Italy and Mexico are in the context and risk minimization approaches. Regarding the context, the legislation is similar in both countries, but the public policies are different. While Italy encourages companies to fund the research on the conservation of cultural heritage, promoting open access to the knowledge and intervention of the assets, and exploiting the economic dimension of the heritage. In Mexico, on the other hand, the funding is mostly public, used for culture and education, the research results are not public and are far from being used as a possible economic development pole. Regardless, neither of these countries justify conservation of built property as an active way to reach the SDGs. Still, as of 2017, 13% of Italy’s Gross Domestic Product (GDP, around 1.96 trillion dollars) comes from tourism-related activities, while in Mexico, it represents 8.7% of the GDP (around 1.16 trillion dollars), according to the Organization for Economic Cooperation and Development (OECD) [161]. Another important issue was addressed by [77] about the Italian context: “advanced maintenance procedures are implemented only if the management is supported by a stable government with long-term vision”. In fact, not only in Italy, but in most of the world, the evaluation of public policies is difficult because they are normally changed when a different politician takes office.

Another important difference among both countries is the climate: in Italy, most of the country presents a temperate climate, whereas in Mexico, it ranges from tropical to temperate. That means that in Italy, the same material will suffer almost the same weathering conditions, but in Mexico, the main climatic threats depend on the site. As an example, in Italy, the freeze-thaw cycles are an important threat all over the country, but although that risk virtually does not exist in Mexico, there are some regions in the country in which this mechanism might be important. It is to be highlighted that thermoclastism must be researched deeper because Mexico is one of the most insolated countries in the world [162]. The consequences of few versus plentiful factors of climate is that in the first scenario the very same decay function can be used, whereas in the second, several decay functions may be needed.

As mentioned previously, in Europe, wet deposition is not considered an important decay variable [39], compared to the other mechanisms. In addition, in Italy, the atmospheric conditions are always included in their studies, but do not consider the ECVs from public networks, even though the government has more than 4500 hydrometeorological monitoring stations. In contrast, in Mexico, the studies focused on the identification of the variables controlling the decay without proposing preservation strategies, or providing only vague recommendations. Despite the differences between Italian and Mexican societies, when the decay source is of anthropogenic origin, similar strategies can be used, such as the use of renewable energy sources, change in urban mobility, or promotion of preventive systems [163]. However, there is considerable room for improvement when it comes to understanding and proposing minimization strategies when the decay sources are mainly ECVs, and is important to always remember that the very idea of cities is different in Mexico and Italy.

The study of material decay, for atmospheric science, ought to lead to monitoring. In that way, it would be possible to predict the most vulnerable heritage sites, and even estimate the time before an intervention is needed, as in [164]. They proposed the idea to include not only the study of the materials, but also the responsibility of decision-makers, including the term of a “tolerable corrosion rate” and produced a map for Vienna regarding which materials will exceed this tolerable corrosion.

Damage functions can only be developed if building materials as well as their behavior under specific conditions are studied. Although silicon-based stones have been used widely as building materials around the world, as in Peru, Ecuador, and Mexico, comparatively little research about decay of building materials other than carbonaceous stone has been performed; however, some already-mentioned efforts have been carried out. An important aspect this article intends to highlight is the fact that damage is presented in many ways, depending on its nature, and it can represent a problem, for decay will not be comparable. The damage produced by salt recrystallization is different from the damage that atmospheric deposition can create; whereas one tends to produce mass loss, the other tends to increase the porosity of the material, so it is difficult to compare damage numerically. To do so, it would be necessary not only to know the pathologies each decay mechanism provokes, but also how to quantify them, and then conduct further research on how to compare them. In the meantime, it would be a positive change to drop the use of surface recession and substitute it with material loss as indicator of damage because of the reasons already explained.

The use of data is another important difference among the analyzed countries. There are two different aspects which need to be addressed related to data: understanding and availability. The understanding of data is one of the main challenges that science faces currently. There are multiple sources of data, which can be combined and interpreted in apparently infinite ways. As an example, some air quality equipment can monitor a single pollutant each minute, so data mining is important for understanding and discerning trends and patterns. There are many different meteorology and air quality networks all around the world, and some regions, such as Europe and the United States, have large amounts of ground-based observational data. For regions outside of these data-rich areas however, data are lacking, and in those cases, remote sensing can be useful. Both meteorological and air quality data can be measured, estimated, or modelled through satellite observations [165,166,167,168,169,170], although accurate regional methodologies are not always available.

To perform a correct monitoring of built heritage decay, in addition to material surveys, it is important to develop damage functions with atmospheric variables, preferably ECVs that can be used to predict the greater risk to built assets.

Finally, in the endeavor for sustainability, understanding and promoting conservation of stone heritage can be crucial, because it can help to strengthen community bonding, improve self-sustainable cities and, in general, create a less-aggressive atmosphere towards building materials which would also encourage the well-being of individuals, by allowing better air quality with different benefits for us all.

6. Future Perspectives

As was discussed in the review, the atmospheric sciences play a crucial role in the understanding and study of building decay, but to fully exploit the potential of this role, multidisciplinary research is needed. That way, not only the technical language, but also the use of important variables, and even data mining would improve greatly.

Damage functions, while they do not necessarily describe all the different processes that the material suffer, are useful because they help to estimate future decay in areas where no research has been performed; however, the amount of building materials used in built heritage for which these mathematical equations have not yet been developed is high, so researching different materials would be a step toward optimizing resources dedicated to conservation. Additionally, research such as that in [88] must be encouraged to promote understanding of the variables involved in how different building materials decay in different climates and at different latitudes [171,172,173,174,175].

A different way to quantify damage must be developed because the use of mass loss neglects the effect of physical weathering. There is a long way to go to parametrize building material decay.

Air quality monitoring must improve greatly worldwide, especially in developing countries, not only because it can be used for conservation purposes, but because information on air pollution is vital when assessing a population’s health and well-being. This would also help to identify areas prone to decay.

7. Conclusions

Built heritage conservation is important when it comes to fulfilling the SDGs.

Understanding the atmosphere is key to addressing the decay of stone-built assets because it determines the main decay mechanisms and the most harmful variables towards building materials.

Atmospheric sciences can be crucial to the study of built heritage conservation, but right now its role its marginal.

The main contributions the atmospheric sciences can give to stone heritage conservation are as follows:

• To explain the processes the materials will suffer from the atmospheric conditions.
• The use of modelling techniques as indicators of past and future scenarios the asset has or will likely suffer.
• To help in the development of innovative solutions towards a better-suited atmosphere, not only to guarantee the preservation of stone heritage, but also for a better-quality atmosphere for society in its integrity.

It is important to encourage reflection on damage parametrization, in order to be able to discover and develop ways to compare the decay produced by different mechanisms.

Surveys specifically regarding the study of a single stone-built asset must be minimized in favor of the use of already-existing data, coming from environmental monitoring networks.

The study of material decay ought to lead to modelling that can foresee the most vulnerable monuments using widely available, remote-sensed measurements of environmental parameters.

Damage functions which have been developed for a single climate might not perform well in different conditions.

One of the most difficult issues when it comes to built heritage conservation is the short-term duration of public policies, whose application is often too short to properly assess their effectiveness.

The study of landmarks outside of Europe will be a good starting point to more broadly study the heritage of humankind and reduce the reliance of accumulated expertise on only a few societies. In this way, the knowledge of different materials, techniques, and viewpoints will enrich people all around the world.

Creating a less polluted atmosphere will impact the conservation of stone heritage, but also societal well-being on the whole.