The emission appears to have occurred in conjunction with the flames reaching a storage building for pool cleaning products.
Around 160,000 residents are being urged to stay indoors in an area along the coast.
"If you are within the affected zone, you should not leave your home or workplace", the civil defense agency writes on social media.
Several roads and train stations in the area have been closed off to prevent people from approaching the risk area.
There is no information that anyone has been injured in connection with the fire or emission. The authorities expect the toxic cloud to dissipate once the fire has been extinguished.
