Air pollution is not worse on the Tube network. Tube dust consists of metal particles, organic matter and mineral dusts, which are not currently known to have any significant health impacts.
Pollution on the Underground is very different to pollution above ground, both in the particle size distribution and in the chemical composition of particulate air pollution.
Outdoor air pollution is made up of pollutants such as nitrogen dioxide and diesel exhaust particulate emissions. There's a strong body of evidence showing the links between exposure to this outdoor pollution from exhaust emissions and health. The International Agency for Research on Cancer has classified outdoor air pollution, especially particulate matter in outdoor air pollution, as carcinogenic to humans and causing lung cancer.
The Mayor and TfL takes action on air quality on the Tube Network as well as above ground on outdoor air pollution.
The Ultra Low Emission Zone (ULEZ) was expanded across all London boroughs on 29 August 2023 because poor air quality is impacting the health of Londoners, and polluting vehicles are a key source of pollution.