A 50m cordon has been set up around a truck by Essex Police after a 1,000-litre container of highly toxic cresylic acid developed a leak.
The container was on a truck making a delivery to the Blackwater Trading Estate in Maldon, Essex.
Fumes from the material are hazardous and the liquid can burn the skin, so firefighters have advised businesses on the trading estate to take extra care.
A spokesman said specialist chemical cleaners had been called in.
Station manager John Sherrington said: "The container had developed a three-inch split before it was loaded onto the lorry.
"We have established a cordon around the truck and have told all the businesses on the estate what is going on.
"The only premises affected is the one which the delivery was going to."