Spilled chemical powder at a Suffolk factory is reacting with rain to produce toxic fumes.
Residents in Haverhill are being urged to stay indoors and close windows as the fume cloud could be an irritant.
Homes on Chalkstone Estate, Roman Way and Wilsey Estate in Haverhill are affected, Suffolk fire service said.
Firefighters were called to International Fragrances and Flavours on Duddery Hill at lunchtime after a tonne of powder blew out from a tanker.
A lot of the powder has been covered with salvage sheets but some of it is reacting with rain, firefighters said.
Crews wearing chemical protection suits are at the scene dealing with the incident.