On September 21, 2001, an explosion occurred in an ammonium nitrate storage warehouse at the AZF plant in Toulouse, located within an industrial site classified as a Seveso site in 1982. The blast, equivalent to a 3.4 magnitude earthquake on the Richter scale, devastated the site. The shockwave spread through the city, damaging numerous homes and public buildings. A toxic cloud drifted over the metropolitan area within a few hours. The Haute-Garonne Prefecture immediately activated a red alert. The white alert, specific to healthcare facilities, was also triggered to manage the massive influx of injured people. Containment orders were issued to the public but were quickly lifted by the prefect of Haute-Garonne, who reported “no traces of toxicity” two hours after the explosion. The initial toll from the disaster was 30 deaths and more than 3,000 injured. Twenty-seven thousand homes were completely or partially damaged. Further chemical releases occurred in the weeks that followed before the complete removal of the stockpiles present on the site. The day after the disaster, the French Institute for Public Health Surveillance (InVS) and the Midi-Pyrénées Regional Epidemiology Response Unit (CIRE) conducted an initial assessment mission on-site, with the following objectives: - to identify the nature of the risks faced by the population; - assess the need for technical support to local health authorities in the areas of health surveillance and management strategies; - examine the relevance and feasibility of an epidemiological study of the health consequences of the explosion.