- freely available
Geosciences 2011, 1(1), 26-43; doi:10.3390/geosciences1010026
Abstract: An overview of issues with environmental relevance that arise from the interaction between pollutants and surfaces of the built environment is presented in this paper. Two broad perspectives are considered: decay of materials and recording of pollution characteristics. In relation to the former, we consider the possible implications on human activities restrictions, materials and morphological options, consumption of resources and release of pollutants resulting from the alteration of materials, conservation and restoration procedures. In terms of pollution recording, the interest of the stony materials as passive monitors of pollution, the question of heterogeneous conditions on buildings and the interest of qualitative and quantitative studies are highlighted. The importance of longitudinal studies on new and cleaned surfaces is considered, both for the understanding of materials decay and for the assessment of pollution conditions. The use of tracers to record the characteristics of pollution sources, interaction with materials and pathways of pollutants is also discussed. Finally, some recommendations are presented, based on the issues discussed on this paper that might be relevant for environmental management programs, including environmental education.
Materials are an essential part of our culture and way of life being applied in a great diversity of surfaces of the built environment, from extensive outdoors walls to kitchen countertops. As has been recognized for a long time, they experience transformations after emplacement (Herodotus, Vitruvius and the Bible refer to alterations of materials). These alterations are related to characteristics of the materials and conditions of application, including environmental conditions and buildings.
Diverse pollutants can interact with surfaces in the built environment causing unwanted alterations and leaving marks of these interactions. In this paper, we attempt to present an overview of the possible relevance of the study of these interactions for environmental policy, regarding both impact on materials (including materials with cultural value) and the use of decay features such as sources of information on pollution.
The following two main sections of the paper represent different perspectives in relation to the built environment and pollutants. In the next section environmental issues arising from the study of the behavior of materials applied in the built environment are considered, both from the perspective of the conservation of the old (cultural heritage), applications on new constructions and building materials as sources of pollutants. This is followed by a discussion on the potentialities and problems on the use of built surfaces as records of pollution. The paper ends with some final considerations and recommendations based on the contents of the previous sections.
2. Decay of Materials
Extrinsic agents can affect materials' surfaces causing alterations of their initial characteristics, either from changes in the initial substances, accretion of matter or erosive loss, resulting in macroscopic evidences. A detailed classification of the features resulting from alteration of stony materials (that can be applied to other materials) is found in . These processes affect the generality of materials, namely porous materials such as stony materials [2-4] and woods [5,6], but also metals [7-9], polymers [10,11], glass [12,13], paints [14,15] and paintings [16-18] in both very ancient and recent works.
In the case of alteration of building materials that are elements of cultural heritage, conservation would comprise measures for reduction of the decay process of building materials. When the processes are linked to pollutants resulting from antrophogenic activities, these measures could include reduction of emissions from vehicles and industry which might even imply the closure of roads and facilities (the impact of vibrations from circulating vehicles must also be considered). Favrel and Hecq  include the impact on buildings in their assessment of costs related to air traffic. Watt et al.  discuss the possible implications of the effects of pollution on buildings for the definition of air quality standards. As another example, the Portuguese municipal assembly of Batalha, in Portugal, recently expressed concern for the possible impact of increasing vehicle traffic near the Batalha's monastery. The circulation and access of people to museums and cultural elements, as well as climatic options, activities and materials may also impact such places of interest [21-24].
In this context, it is essential to assess the real impact of decay processes. For example, biological colonization can have a marked visual impact leading to visual hindering of decorative elements (and therefore it could obliterate its cultural value); however its impact in terms of physical transformation and material loss is usually minimal. Several studies have attempted to establish relations between decay of materials and pollution conditions through laboratory studies aiming to define damage or dose-response functions [9,13,20,25-28]. However, the application of these studies to actual structures is hindered by questions regarding size and time effects as well as the multiplicity of agents that are involved and the complexity of the interactions. There have been fewer studies comparing observed decay of materials in actual structures to the characteristics of the pollution load in the area (e.g., as was done by Nord et al. ).
The evaluation of the decay of a material can be a matter of great controversy as is illustrated in Mostafavi and Leatherbarrow  where several points of view are considered, including the perspective that weathering can be seen as productively modifying a building over time. One can even find examples of intentional weathering [30-32]. The assessment of the alteration of a built element should consider intensity and extension (see Fitzner and Heinrichs ), the distribution of alteration features and the age of the element. For example irregular straining can be considered as dirt or soiling [30,33] especially in recent constructions (see Figure 1), while a generalized biological coating in older constructions can be considered as “patina” that mark the passage of time (for further considerations see Mostafavi and Leatherbarrow  and Kirkwood ).
Besides the obvious question of the balance between the value of the cultural elements and the consequences of the proposed restrictions, there is the more complex question of the assessment of the links between decay and agents and the even more complex questions of risk assessment as well as of the effectiveness of the measures. Epidemiological observational studies of the alteration of materials can contribute significantly in this regard. An initial consideration that must be addressed is the possible diagnostic value of decay features. The identification of pollution sources that affect the built heritage suffers frequently from the problem of equifinality that affects many studies in the natural sciences (see discussion in Turkington and Paradise ). While it is hardly possible to relate a specific alteration feature to a specific pollution source, the study of the spatial distribution of decay features and substances (minerals, chemical elements, ions, isotopes) could help to pinpoint pollution sources. The identification of pollution sources would be important for planning of intervention measures.
Some records of the characteristics, effects and pathways of pollutants in the built environment have been obtained by using geochemical tracers used with variable results. In particular, some chemical compounds can be used as tracers of pollution. Regarding particulate tracers, some of them are useful to record the effect of atmospheric pollution. Some atmospheric particles (such as N- or S-rich particles) can react with building materials to originate secondary compounds, products of decay. Other particles can be deposited on the materials' surface, but some of them act as catalysts of sulfation reactions of atmospheric SO2 with Ca-rich building materials .
Some other common gaseous, dissolved and particulate pollutants are sulfur, nitrogen, and carbon oxides. They are key components involved in the deterioration of building materials that cause the deposition of crystallization of neoformation minerals. Stable isotopes of sulfur, oxygen and carbon have been used in studies of migration of water and pollutants into the building system and interaction with the building materials [35-38]. Stable isotopes of N and H and radioactive isotopes are scarcely used as tracers in the built environment but, as is discussed, they could provide useful information on the pathways and sources of pollutants, as in other cases such as atmospheric pollution dispersion, hydrological and geochemical studies in nature (see  for discussion).
Given the possible impact of the proposed measures, it is essential to distinguish contribution of presently acting pollution source from the effects related to pollution events that occurred in the past, a distinction that can be extremely hard to assess in historical constructions since there are interactions of pollution agents over time on the built surfaces. This is illustrated in Figure 2 where several emissions of pollutants are considered as well as different sampling times. In the simplest case, admitting that there is no loss from the system, that deposition equals emission, and that the pollution effects act in a cumulative way, the pollution assessed at a given moment is the sum of the integrals of the pollution emission curve up to that time. In the case illustrated in Figure 2, the later the sampling the higher the contents and the higher the multiplicity and complexity of pollution. Additionally, it has been observed that pollutants deposits might evolve with time requiring more aggressive cleaning techniques . The interaction between pollutants and of them with the substrate, and issues related to transport and deposition, would also need to be considered in real cases.
The previous paragraph illustrated the importance of understanding the relationship between pollution sources and decay processes, of studying new works located in the same area as the affected element and to develop longitudinal studies of materials surface. Considering this aim, longitudinal studies could be carried out on old and new materials surfaces as well as on cleaned surfaces. In Figure 2 cleaning is also taken into consideration. In the illustrated example, and admitting a 100% effective cleaning operation that leaves no residue, the system at S4 would present only the pollutants that were deposited after cleaning. Higher rates of decay after cleaning have been observed  and cleaned surfaces could constitute enhanced sampling elements. Periodically cleaned surfaces could be important instances for longitudinal studies regarding the action of pollutants. In this last case, however, it is necessary to assert the effects of the cleaning, namely whether there is a “reset” of the system, the permanence of residual amounts of pollutants or even the introduction of new substances.
Cleaning and other conservation operations could also introduce restrictions on normal circulation of people and vehicles, causes spending of resources (namely water) and, more worrying, environmental risks, namely associated with some of the substances that are used [22,42,43] and the introduction of pollutants in the treated materials [42,44]. The issue of resources consumption, materials and energy, is a current concern for conservation installations such as museums [45,46]. The need for repeated maintenance would imply additional costs and can have an impact on the materials [20,41]. The previsions of decay rates can be used to estimate the periodicity of cleaning  and simulations of performance–degradation models have been considered in the discussion of maintenance budgeting . Conservation intervention is strongly linked to diagnostics studies in order to define the measures that are adjusted to the problem and avoid excessive use of resources.
Another environmental issue related to cultural heritage is the substitution of decayed materials for new materials, an operation that would imply consumption of new substances for the preparation of the new materials, with the associated financial and environmental costs, as well as environmental risks associated with the extraction operations. This links these previous considerations regarding cultural heritage with the more general issue of materials durability, which is also relevant for new works.
Therefore, the identification and selection of materials that have higher durability would promote required performance for a longer time and decrease the impact associated with these two aspects (expenditure of resources and release of contaminants). However, this must be balanced with the possible environmental costs of extraction of these more durable materials. The need to use stones with higher durability could imply measures such as the preservation of quarries of stones with higher durability value (with consequences for territory management). For example in Portuguese legislation, the recognition of quality and value of a certain rock types has lead to definition of geological reserves with restriction on the kind of activities that can be developed. Another related issue is the consideration of materials making allowance for the environmental impact of its fabrication as well as its recycling capability .
Besides the intrinsic properties of materials, there are architectural options that can promote depreciation of the built surfaces. There are studies of computer simulation of weathering of materials in relation to morphological features [49,50], an aspect that has been mostly considered in terms of virtual reality for applications such as videogames and movies. Certain characteristics of the built environment that promote infiltration or permanence of water could promote the development of decay features. Characteristics that promote or minimize alteration can be considered in the design of structures in order to minimize weathering processes [7,30,33,51-55].
There are important interrelations between studies of the old and of the new. Historical heritage elements could be important sources of information on the behavior of stones under real exposition conditions (in contrast to laboratory tests, perfectly controlled but limited in size, time and lacking the impact of interactions). The observations of the evolution of new constructions could be an important component for understanding decay of historical cultural heritage, following the old geology motto of “the present is the key to the past”.
Another aspect related to alteration of building materials is the release of pollutants. Building materials can act as pollution sources affecting the surrounding environment (in relation to cementitious materials see ; regarding metallic materials several examples can be found in ). The release of substances from building materials can also affect those materials or other nearby materials. Pollutants from building materials can arise from the pore content such as soluble salts on natural rocks [21,58,59] and in the case of cements in the initial stages of setting (for the main chemical characteristics of the pore solutions in initial stages of setting of cements see  and references therein). In the case of artificial materials prepared with water (such as mortars and pavements) the water that is used is also a potential source of pollutants . In the context of epidemiological studies of materials decay, one needs to study the release of pollutants due to the alteration of the constituents of materials, as mentioned in relation to the weathering of constituents of natural stone, specially calcite [62-68] but also dolomite , silicates [70,71], iron sulfides [72-75] or oxides [76-79] and organic components (; see also several references in ), constituents of set mortars [80,82,83] and metals corrosion . These aspects could be considered in relation to the formulation of materials (and the control of chemical composition of different constituents) but also regarding the way they are applied on the built environment (architectural options).
Finally, one can note that there is currently research  that aims to develop new materials that interact actively with the surrounding environment (“metabolic materials”), promoting the preservation of the building from the action of exterior agents and the positive impacts of the building on the surrounding environment (“living architecture”).
3. Built Environment as Record of Pollution
Elements of the built environment, since they react and fixate pollutants, have the potential to constitute records of the surrounding environment. This is a relatively common notion that is found, for example, when people refer to rust marks in metallic elements in regions near the sea. While the conditions of fixation of pollutants by humans and by materials surfaces have major differences, studies of surfaces can be valuable in the comparison of exposure conditions.
Alterations of surfaces can be easily detectable (namely by visual inspection) indicators of pollution. At an initial stage and in a qualitative perspective one can consider the possible information that can be gained from the occurrence of alteration features and its typology (for a developed proposal of weathering features typology see Fitzner and Heinrichs ). The recording value of decay forms for the study of pollution can be very different. In general terms, one can argue that fixation increases while erosion decreases the recording potential and, in that sense, the most valuable decay features for the characterization of the pollution load are coatings.
There is a great variety of coatings that develop as a consequence of the action of exogenous agents (absorption, reaction, deposition) and, hence, the presence of certain coatings might give general indications regarding the presence of pollutants. In general, soiling aspects constitute indicators of circulation of atmospheric particulate pollution (Figure 3). Coatings formed on façades can give qualitative information on the characteristics of the environment surrounding these buildings [85,86].
Simple dust deposits have been used for studies of pollution in outdoor and indoor locations . On other, more complex, coatings, pollutants are fixated by organisms or by mineral aggregates. A (in)famous kind of coating related to mineral neoformations is the black crust, where gypsum aggregates fixate atmospheric particulate matter (Figure 4). In this case the gypsum aggregates (see Figure 4b) trapping atmospheric particles contribute to very evident visual features. Another type of widespread coating, especially in diverse modern constructions , are carbonate-rich coatings (Figure 5). and the possible environmental monitoring potential of this frequent type of coating, which is usually very stable after formation, remains underexplored (natural carbonate deposits with similar characteristics such as tufas has been used in environmental monitoring even in modern environments, see [88,89]).
While it could be argued that the presence of erosive features indicates environmental aggressiveness, this could be less relevant from the ecological and human health perspectives. For example, water in its purest form can cause erosion of surfaces but this is not a matter of concern for humans and other organisms.
Another aspect related to the possible assessment of the effects of pollution is the distribution of decay features, considering both the regional distribution and frequency and also the distribution according to morphological characteristics of the built element. This last issue is especially relevant since it is essential to assess the heterogeneity of conditions for the development of decay features and hence the heterogeneity of recording conditions on the built environment.
Under favorable conditions (assuming homogeneous fixation conditions), patterns of distribution can also help to study patterns of circulation of pollutants both horizontally and vertically. However, the recording conditions on a given element can be highly heterogeneous depending on morphology, surface irregularity, moisture and orientation. Fixation of pollutants would be different on horizontal and vertical surfaces . Even in a straight-faced wall there would be differential patterns of alteration ( see also Figure 1). Temperature and relative humidity variations control formation by condensation of moisture films that promote reaction between the substrate and gases and the fixation of particulate matter [92,93]. Sheltering conditions can greatly affect the fixation of pollutants in outside portions as is illustrated in Figure 6. The characteristics of the location of the built elements can also affect distribution of pollutants. In canyon streets there are higher concentrations of pollutants from traffic of vehicles near the ground while pollutants related to more distant sources, such as industrial sources at the outskirts of the towns, are more concentrated at the upper level [94-96].
Built surfaces can be used as passive samplers that are permanently exposed to the pollution agents, allowing the study of the cumulative load up to the moment of sampling (see Figure 2), avoiding the problem of definition of time interval for collection of samples and that, depending on the processes of transport and fixation of pollutants would develop sequences of pollutants that could be studied by depth profiling (as has been classically done in the study of sedimentary sequences). Indoor and sheltered outdoor surfaces would be specially valued places where there would be cumulative absorption and dry deposition of pollutants without leaching. Sampling of products on surfaces of the built environment is generally possible, but sometimes restricted in terms of amounts (especially in the case of cultural heritage).
A general problem in the use of decay features is to define unequivocal relations between a given substance and a given source, including the distinction between contribution of exogenous agents and the substrate and distinction between different sources.
Regarding the characterization of the substances present in decay features, categorical and quantitative analytical studies can be performed. The expression “categorical analytical studies” is used here to represent all analytical studies whose result is placed in a given group, either as dichotomous presence/absence results, such as identification of compounds, or estimations that can be classified in ordered categories corresponding to different levels of abundance. Examples of dichotomous presence/absence studies are studies of identifications of substances by analytical methods such as x-ray diffraction, scanning electron microscopy and diverse spectroscopic techniques. The identified compounds can give information on the pollution sources and conditions using morphological, chemical and structural characteristics of particles [29,85,97] and the presence of specific substances such as organic compounds [98,99]. Isotopic signatures on living organisms  and mineral deposits [29,101] can also be used to characterize the surrounding pollution (for example, Klemm and Siedel  refer to a trend towards higher δ34S values with higher atmospheric pollution). A possible interesting application would be the fixation of radioactive isotopes on the substrates.
Spatial analysis of categorical studies, including the distribution of the frequency of decay features and identified substances, can help to assess regions with higher pollution load (concentration of occurrences of a given substance) on the presence of substances on decay features and might be useful in the screening and selection of locals for more detailed studies as well as definition of monitoring networks.
Quantitative analytical studies assess amounts of substances and could be used to compare samples of different regions, or different places in a built structure, or to compare the alteration features with the substrate [99,102,103]. One must be aware of the effect of differences in the length of exposure time and circumstances that might create diverse conditions of fixation, such as reactivity of substrates, environmental conditions and morphological aspects of buildings. To deal with these issues one can use normalization procedures such ratios to elements or ions of reference (using substances whose concentrations are considered to be unaffected by the presumed pollution sources) to compare contributions of possible pollution sources as well as the affected substrates [104,105]. Graphical procedures can also be used to compare elements or ions, considering possible mineralogical relations and signatures of pollution sources [106,107] and to classify atmospheric particles deposited on surfaces .
Besides the characterization of surfaces at any given time, longitudinal studies on cleaned materials can be of interest in the use of the built materials as records of pollution, following along the lines mentioned in the previous section. The study of the evolution of surfaces after cleaning might contribute to assess the persistence of pollution sources. In favorable conditions and depending on the time framework of interest, longitudinal studies of periodically cleaned surfaces might help to assess variations in time of the pollutants.
As has been shown, several issues related to interactions between pollutants and surfaces of the built environment, both at any given moment and from a longitudinal perspective, can have relevance for environmental options and policies concerning aspects such as choices of materials and forms, consumption of resources, production and transport of pollutants, restrictions on activities and products and the assessment of pollution situations. This overview has also indicated some possible uses of built surfaces for monitoring of pollutants.
Based on the aspects presented in the previous sections, some recommendations potentially relevant for environmental management programs are proposed:
To include assessments of the different impacts (substances and procedures in terms of consumption, release and restrictions) in the planning of interventions in the built environment (maintenance, conservation, restoration);
To specify choices of materials and forms that minimize environmental impacts (considering both durability and impacts of extraction);
To assess the performance of materials in the field in real structures and under real pollution conditions;
To evaluate the effects of restrictions on activities on the effects of pollutants in the built environment;
To develop criteria for acceptance of materials considering the risk of release of contaminants under the specific conditions of application of those materials;
To promote the existence of architectural sheltered built elements that would be useful for the monitoring of atmospheric pollution;
To consider a selection of surfaces that would be left unclean so as to be permanently available as records of pollution;
To promote citizens sensibility to monitoring of visual changes in new or cleaned surfaces and its possible significance in terms of the conservation of the built environment and as evidence of pollution characteristics.
The Centre of Geological Research, Management and Valorisation of Resources (CIG-R) is supported by the Fundação para a Ciência e Tecnologia (Portugal) Portuguese funds (pluriannual funding program for research units, PEst-OE/CTE/UI0697/2011). The collaboration between the authors in this subjected has benefited from Portuguese-Spanish collaboration Project “Ação Integrada E-141/10” (Fundação das Universidades Portuguesas)/“Acción Integrada PT2009-0077” (Ministerio de Ciencia e Innovación).
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