On the afternoon of 10 July 1976, there was an explosion at the ICMESA chemical factory near Seveso, in northern Italy.
A cloud containing a poisonous gas called dioxin spread over the town.
For days, residents didn't realise they were in danger.
They later developed skin conditions like chloracne and thousands of animals died or had to be slaughtered.
Rachel Naylor speaks to Giuseppe Cassina, former Mayor of Seveso.
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(Photo: Rescuer working during the dioxin contamination in Seveso, August 1976. Credit: Francois Lochon / Gamma-Rapho via Getty Images)