The inherent safety of substances in incidental scenarios: a procedure for the assessment of hazards due to decomposition products

The undesired formation of decomposition products in accidental scenarios involving the release of chemicals is a core issue for process safety assessment. As a matter of fact, the undesired reactions that may occur in accidental conditions may result in the generation of significant quantities of substances different from the original ones and, in particular, more dangerous for humans or for the ecosystem. The negative consequences of these events are, on one hand, confirmed by the experience (e.g. in the Seveso accident) and, on the other hand, considered by the legislation concerning the control of major accident hazard. Nevertheless, no robust and widely accepted assessment method exists for the comparison between the hazards of the original substance and those of its decomposition products. This issue is of particular concern in the preliminary identification of the hazards due to this category of accidents. In the present work, an approach to the assessment of this hazard was developed in the more general framework of the analysis of the inherent safety of processes. The toxicological, physical and chemical parameters mostly influencing the hazardous behaviour of a released substance are identified by the analysis of specific environmental fate models and used in the evaluation of hazard footprints. From these data four hazard indexes are determined, that express in a quantitative way the overall behaviour of a substance with respect to specific categories of hazards (acute toxicity, chronic toxicity and carcinogenicity for humans, ecotoxicity for animals, plants and algae). Both footprints and indexes allow a comprehensive representation of the hazard profile of a substance. Since for many substances the data used for the evaluation of the different parameters appeared to be scant, a simplified estimation procedure was developed, based on the identification of structure activity relationships. The combination of the presented assessment procedure for substances and of an array of experimental protocols for the quali/quantitative determination of the decomposition products (i.e. protocols based on calorimetric and analytical techniques) result in an effective tool for the comparison of the hazards derived by undesired substance decompositions in accidental scenarios. This is confirmed by the application of the described approach to several case studies on different accidental scenarios involving various substances and considering different categories of hazards and of targets.