The term “asbestos
” is used to refer to a group of fibrous silicate minerals. Regulation is limited to six of these minerals, which only partially account for the existing, naturally-occurring fibers [1
]. Among these regulated minerals, chrysotile is part of the serpentines, while actinolite, amosite, anthopyllite, crocidolite and tremolite are part of the amphiboles. All of them are classified as “carcinogenic to humans
” by the International Agency for Research on Cancer.
The hazard of asbestos is due to the fibers that are released into the environment and therefore inhalable. In case of asbestos cement (AC) roofs, the release is mainly due to weathering (i.e.
, rain, ice and wind). Several studies demonstrated that exposure to asbestos fibers causes mesothelioma and lung as well as larynx and ovarian cancers [2
], and concluded that no safe level of exposure is predictable [6
Between the 1970s and the 1990s in Europe, asbestos was largely used in both industrial and domestic contexts, due to its physical and chemical resistance as well as its heat and acoustical insulation properties. Its use was very common for pipes, floor tiles, textiles, and also roofing material in the form of AC. Asbestos is still present worldwide, although an increasing number of countries have already banned it. At the European level the use and marketing of asbestos have been banned since 1 January 2005. In Italy, its production and use were prohibited with the national law “257/92”; however, older buildings containing asbestos remain widespread and constitute a continuous source of asbestos emission, unless it is removed or encapsulated. In Lombardia Region—where the study area is located—the national law was implemented with the regional law “17/2003”, which led to the development of the 2006 regional plan (i.e.
, PRAL) for removing asbestos by 2016. For the first time, the PRAL required municipalities to perform the census of asbestos materials with the aid of citizen self-declarations. Among the required information (e.g., building type, year the roof was laid, location, quantity, planned restoration interventions) the year the roof was laid plays an important role for the roof deterioration assessment, since older roofs are more likely to be weathered. As a matter of fact, in 2008, the year the roof was laid was introduced in the regional law as variable for the definition of the AC roof conservation status (i.e.
, Decreto Direzione Generale Sanità “13237/2008”). However, the census is affected by a high degree of uncertainty, due to inspection difficulties by the local authorities and/or omissions in self-declarations. Remote sensing can help in overcoming these omissions by providing maps of AC used as roofing material. Aerial remote sensing techniques have been successfully applied in the recent past for asbestos mapping purposes [7
]. To the best of our knowledge, only one study [12
] deals with the assessment of AC deterioration through spectral diagnostic bands. Diagnostic bands related to the percentage of surfacing fibers are located in the shortwave infrared region at 2.32 µm and in the thermal infrared at 9.44 µm. However, a signal to noise ratio (SNR) of 277 is recommended to discriminate the 2.32 µm asbestos absorption feature. This SNR can be achieved in laboratory using field spectrometers, but these requirements are not met by the MIVIS (Multispectral Infrared and Visible Imaging Spectrometer) sensor used in the mentioned study [12
]. Moreover, they suggested that the thermal infrared is promising for the asbestos deterioration assessment, although the separation of emissivity and temperature implies some challenging issues. In our study we propose a novel spectral indicator (i.e.
, ISD, Index of Surface Deterioration) that is not directly related to the surfacing fibers, but rather related to the abundance of mosses and lichens that are reported to be more developed on older and weathered substrata [13
]. In particular, we used the absorption feature in the red wavelengths around 0.68 µm that was found to be related to the presence of photosynthetic pigments of mosses and lichens colonizing the roof.
With the ISD calculated using visible and near infrared spectral regions, we attempted to provide a novel and more operative remotely sensed index for defining the roof weathering status, and thus the priorities for AC removal. Although the presence of mosses and lichens may be a barrier to the fiber detachment [13
], we assume that the oldest roofs have a more friable matrix that is more likely to be colonized by the vegetation, and to release fibers from the un-colonized roof portions.
The research project was conducted in five municipalities in Northern Italy (i.e.
, Monza, Muggiò, Lissone, Seregno, Biassono), spanning an area of about 65 km2
with a population of about 248,000 inhabitants. An airborne overpass was performed with the MIVIS sensor. Ground measurements were also acquired with a portable field spectroradiometer on selected roofs, having different weathering status. The main objectives of the study can be summarized as follows:
the mapping of AC roofs;
the association of the AC presence to the cadastre in order to provide the cadastral building references to the municipalities;
the assessment of the weathering status of each AC roof through the definition of a novel operative spectral index (i.e., ISD), which is related to the vegetation component that colonize weathered roofs.