Three men who were cleaning a sump inside a Bantala Leather Complex tannery died on Monday apparently after inhaling toxic gases.

Another man who had entered the sump is critical and is being treated at Calcutta National Medical College and Hospital.

Wastewater generated during the processing of animal hide accumulates in the sump that the deceased were trying to clean. Sulphuric acid, sodium sulphate, formic acid, calcium and sodium are some of the chemicals used in the process, a tannery owner in the leather complex said.

According to witnesses, the recommended safety measures - wearing gas masks and using fans to blow away toxic gases from the sump - had been ignored during the clean-up operation.

Pervez Ahmed, the owner of Labbaik International, the tannery, said he was on vacation and did not know the details of what had happened.

"I cannot say much since I am on vacation. There is no cover on the sump, then how could someone die of inhaling gas?" Ahmed told Metro.

Six men had gone to clean the sump at the tannery early on Monday. Three of them entered the 7ft deep sump while the rest stayed above.

It is not clear whether the sump was filled with wastewater.

Mrityunjoy Mondal, 65, one of the men who did not enter the sump, said the three deceased fell ill after working in the sump for about 15 minutes.

"The fourth person tried to rescue the three but he too collapsed in the sump. The two of us who had not entered the sump and some security guards managed to haul the four up. With the help of a few others who had gathered, we took them to hospital," Mondal said.

The four were first taken to a nursing home adjacent to the leather complex and then to CNMC, where Ashok Mondal, 32, Chandidas Pramanick, 52, and Amiya Pramanick, 46, were declared dead. Khokhon Mondal, 37, has been admitted to the hospital.

"He is in the intensive therapy unit. He is suffering from gas poisoning and is critical. We have kept a ventilator ready," said Pit Baran Chakraborty, the medical superintendent cum vice principal of CNMC.

Ashok and Chandidas were residents of Pratapnagar in Sonarpur, from where Khokhon also hails. Amiya lived in Tardaha, about 4km from the leather complex.

Witnesses and leather complex sources said none of the deceased or the injured was wearing oxygen mask and there was none to keep an eye on them. The only safety measures cleaners usually adopt before cleaning a sump are removing the water and scraping out the slush from the bottom, said source.

The gates of Labbaik International were closed after the accident occurred at about 6am.

An official of the Calcutta Leather Complex Tanneries Association told Metro that they provide specific safety guidelines to tannery owners. "We ask the owners to ensure that everyone cleaning the sump wears an oxygen mask and to install fans near the sump to blow away toxic gases," said A.K. Ghosh, senior general manager of the common effluent treatment plant run by the association.

The association cannot ensure that tanneries adhere to the guidelines, an official said.

Workers from other tanneries complained that they were forced to work under hazardous conditions.

The owner of a tannery usually hires a contractor to clean the sump. The contractor hires labourers for about Rs 400 per day.

Police have started unnatural death cases in connection with the incident.

Arijit Sinha, the additional superintendent of police, South 24-Parganas, said a forensic team would visit the tannery to ascertain the cause of the deaths.