The world's first ultra low emission zone has come into force across central London heralded by environmental campaigners as a ground breaking step in the fight against air pollution.
It's been heavily criticised by business leaders over the impact they say it will have on firms.
From today some lorries face an extra £100 a day to drive into the area, while cars could be charged £12.50. This coming on top of the current congestion charge.
Sadiq Khan was at the Evelina London Children's Hospital discussing the impact of pollution on childrens health.