A group of 11 organisations are urging the Mayor to make London a diesel-free city by 2025.
It's thought more than 9000 people die here each year as a result of air pollution.
Last month Sadiq Khan said he planned to phase out diesel buses as part of efforts to tackle the issue. But campaigners believe he can go further.
Areeba Hamid is from Greenpeace - one of the groups lobbying City Hall.