RIO DE JANEIRO – Four workers were killed and three others injured when they were exposed to toxic fumes while cleaning tanks at a chemical plant in Sao Paulo, Brazil’s largest city, the fire department said Sunday.

The accident happened Saturday afternoon at the plant in Jardin Santo Amaro, a neighborhood in the southern section of Sao Paulo.

The workers were wearing masks, but they were overcome by toxic fumes from sulfuric and muriatic acid, which are used for routine cleaning at the nickel-processing plant.

Firefighters found one person dead at the scene and four others in serious condition.

One of the men died while being transported to the hospital, the fire department said.

Firefighters found two other victims inside the tank.

A worker cleaning the tank fainted and fell inside, and two co-workers who went to help him also died, plant employees said.

The toxic fumes spread, affecting several other workers who were outside the tank.

All workers assigned to clean tanks are required to wear safety equipment, including masks, gloves, boots and coveralls, company spokesmen said.

An investigation has been opened to determine the cause of the accident and who is responsible, police said.