Several workers of a cold drink manufacturing unit located in Kamalgazi area in the southern fringes of Calcutta fell ill following leakage of toxic gas on Monday, police said.

Two firefighters, who went there to rescue the workers also fell ill, a senior police officer, said.

"We evacuated the workers of the factory and people of the nearby locality and trying to spot what the problem is. Going by the pungent smell of the gas it seems to be Ammonia. We have a team of doctors with us," the officer told PTI.

Ammonia, a basic building block for ammonium nitrate fertilizer, is irritating and corrosive. Exposure to high concentrations of ammonia in the air causes immediate burning of the nose, throat and respiratory tract. This can cause bronchiolar and alveolar oedema, and airway destruction resulting in respiratory distress.