He said he understood that people forced to buy a new car by the scheme would think that is a “lot of money for me to pay”.
He also argued that there was a scrappage scheme, though, it is restricted.
Mayor Sadiq Khan has been hit with a backlash against expanding the Ultra Low Emission Zone to the whole of Greater London from 29 August.
Critics say motorists have not been given enough time before the wider scheme comes into force.
But Labour leader Sir Keir told of his shock on a visit to the Crick Institute laboratories in north London last Friday.
“They blew up for me an incredibly detailed photograph of a lung with very dark marks on it which were all the air pollution from our roads which were causing cancer in that and many other patients,” he told LBC Radio.
“It’s worth us all just asking ourselves...if we are not prepared to do these sort of schemes what are we going to do.
“If increasing numbers of people, and young people as well, are getting cancer...I have to say..intake of breath when I saw the photgraph, they said there are the dark areas that are lung cancer because of it (air pollution). We can’t just sit that out.”
But Tory MPs have stepped up a campaign against the expansion, highlighting how people, in-cluding on low incomes, are being forced to sell their cars which are not ULEZ compliant.
Sir Keir was also forced to defend proposals to allow EU and other people from overseas who settle in the UK to vote at General Elections, as well as 16 and 17-year-olds.
He argued that denying the former the vote “feels wrong and something ought to be done about it” and that allowing more teenagers to vote is also “not such an outlandish idea”, though critics say both moves aim to boost support for Labour.
Sir Keir added: “These are some of the ideas that are going into the mix, but they’re not policy - we’re just looking at them.”