The incident took place late last night, they said.
"The process of mixing of three different gases was in progress at the unit, when around 11.30 pm some workers complained of nausea, eye irritation and suffocation, possibly due to leakage of hazardous gas. They were rushed to a hospital," a police officer said.
One worker, identified as Sanjay Sharma, died of suffocation and a case of accidental death was registered at Ulhasnagar city police station in this connection, police said.
Investigation into the case is on.