Air pollution is causing the early deaths of up to 50,000 people a year and making thousands more ill - but the government is failing to take enough action to tackle the problem, MPs have said.
The UK should be "ashamed" of its poor air quality and the harm it is causing to people's health and the environment, a report by the Commons Environmental Audit Committee said.
The report said pollutants such as ozone, nitrogen oxides and "particulate matter" from transport and power stations contributed to conditions such as asthma, heart disease and cancer.
The failure to reduce levels of pollution has "enormous" costs for the NHS, and puts the UK at risk of multimillion-pound fines from Brussels for missing air quality targets, the committee said.
It called for the government to make air quality a much higher priority, to raise awareness and educate the public on the dangers and how to minimise their exposure to pollution.
And the MPs said major changes were needed to policies on transport, which accounts for up to 70% of pollution in towns and cities.
Tim Yeo, Chairman of the Committee, said: "Air pollution probably causes more deaths than passive smoking, traffic accidents or obesity, yet it receives very little attention from government or the media.
"In the worst affected areas this invisible killer could be taking years off the lives of people most at risk, such as those with asthma."