Thirty-five fire tenders rushed to the unit on the first floor of a commercial building under Madhyamgram police station limits in North 24 Parganas district.
Ten fire engines were still at work though it "has been controlled", Fire Brigade officials said.
Four firemen fell sick after inhaling the fumes emanating from the pesticide unit which caught fire, the officials said.
People ran out of their houses after pungent odour of gases filled the air.
The manager of the pesticide unit has fled, the police said.
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