The ban affects thousands of taxis, mini-buses and auto-rickshaws.
A ban on commercial vehicles more than 15 years old has come into force in the Indian city of Calcutta, as part of efforts to reduce pollution levels.
The restriction affects thousands of buses, taxis and rickshaws. Transport operators have criticised the decision and threatened to go on strike.
A recent study showed 70% of residents suffered respiratory problems, including lung cancer and asthma.
Many in Calcutta welcome the move but worry about how they will get to work.
Mudar Patherya, who is part of a campaign to clean up the city, said the ban on old vehicles - imposed by the government of West Bengal - was vital.
"I think it is life critical, because there has been a marked increase in the incidence of bronchial and respiratory issues.
"And I would say, if you do not remove the buses, we do not live. Period."
The state transport minister Subhas Chakrabarty said last week he estimated that the ban would affect nearly 3,000 buses and mini-buses, and 6,500 taxis.
A study in 2007 by a leading Indian cancer institute showed that 70% of people in Calcutta, India's third most populous city, suffered from respiratory disorders caused by air pollution.
Much of that comes from the old buses and taxis.
Transport unions, representing private bus, mini bus and taxi operators, on Friday called off a strike against the ban. But they are now threatening another one.
The operators say they are keen to switch to green fuels, or to buy new vehicles, but would need loans and easy repayment terms to help them make the move.