Résumé : Background: A train which was transporting chemicals derailed and exploded on the 4th of May, 2013 in Wetteren, Belgium. Objective: To describe the trends of air distribution and concentration of acrylonitrile (ACN) in order to determine environmental contamination in the areas exposed to ACN. Methods: Ambient air monitoring was used to describe the exposure of ACN. Samples of ACN air concentrations from indoor and outdoor locations were collected during the three weeks following the train accident. A series of maps showing the distribution and concentration, and thus exposure hotspots, were produced with ArcGIS. Statistical hypothesis tests were used to establish whether differences existed between the ACN air concentration samples collected from these places. Potential risk levels were defined according to the “Intervention Values for Emergency Response” (French and Dutch limit values were available and used in 2013). Results: Of the 3006 geo-referenced samples, four areas presented high and alarming levels of ACN concentrations in the air (> 90 ppm) namely, near the train accident, in the sewers and nearby the Waste Water Treatment Plant (WWTP). Polluted environments which were categorised as having an immediate risk level were in sewers leading from the site of the train accident to the WWTP through the city (330 ppm), directly above manhole covers (196 ppm) and in private bathrooms and lavatories (98 ppm). Findings showed peaks of ACN concentration up to seventeen days after the release of the chemical as well as at a distance from the train accident. Discussion and conclusion: The data description analysis provides further information about the demarcation of risk areas and the routes of ACN distribution. Besides air contamination, water was a significant pathway for ACN and water must therefore considered during the process of exposure assessment. The distribution of ACN concentrations collected in this study, which was based on environmental monitoring, were in line with previous studies conducted on human biomonitoring. The results are able to determine an anticipatory approach directly focusing on the identification of environmental areas at risk during a chemical exposure and to show that individuals were exposed to high levels of concentration in various places.