The leak, in the city of Visakhapatnam in Andhra Pradesh state, has been traced to the LG Polymers plant.
Doctors say “hundreds” of people have been taken to hospital – many complaining of a burning sensation in the eyes and difficulties breathing.
The incident, which took place around 03:00 local time (21:30 GMT), may have been due to negligence, officials say.
The leak occurred when the plant was being re-opened for the first time since 24 March when India went into lockdown to curb the spread of coronavirus.
The state Industries Minister Goutam Reddy told BBC Telugu that it looked as though proper procedures and guidelines were not followed when the plant was being re-opened.
As the gas spread, residents ran out of their homes in panic. Distressing visuals of people fainting and dropping unconscious on the streets are being shared on social media.
Some factory employees are believed to have been inside when the leak occurred, but officials say they have no information about them.
It is feared that the fumes have spread over a radius of about 3km (2 miles) and officials have been evacuating people from surrounding areas.
A senior district official said that initial attempts to control the gas leak were unsuccessful. However, local news agencies have reported that the situation is now under control.
Meanwhile, Rajendra Reddy, a senior official in the Andhra Pradesh Pollution Control Board, told the BBC that the leaked gas was styrene, which is usually refrigerated.
“We are trying to understand the long-term impact of the chemical on those who have inhaled it during the leak,” he said.
In the meantime, officials have asked people to protect themselves by covering their faces with a wet cloth.
India has a tragic history of gas leaks.
In 1984, a chemical leak in a plant in the central city of Bhopal killed thousands of people, in what is acknowledged to be the world’s worst industrial disaster.
More than 35 years later, victims say children are still being born with disabilities because of the effects of the spill.
Reporting by BBC Telugu’s Satish Balla and Deepthi Bathini
What is Styrene and how can exposure impact humans?
- Styrene gas is a colourless, or light yellow, flammable liquid primarily used in the production of polystyrene plastics and resins – it is used to manufacture plastics and rubbers which are used in containers for foodstuffs, packaging, synthetic marble, flooring, disposable tableware and moulded furniture
- Breathing air contaminated with styrene vapours can cause irritation of the nose and throat, coughing and wheezing, and create a build-up of fluid in the lungs
- Exposure to larger amounts can result in the onset of “styrene sickness”, the signs and symptoms of which include headache, nausea, vomiting, weakness, tiredness, dizziness, confusion and clumsy or unsteady motion (known collectively as central nervous system depression)
- In some cases exposure to styrene can also result in irregular heartbeats and even coma
- Several epidemiologic studies suggest there may be an association between styrene exposure and an increased risk of leukemia and lymphoma though the evidence is inconclusive
Information from the PHE Centre for Radiation, Chemical and Environmental Hazards and US Environment Protection Agency
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