Hundreds of doctors, nurses and other health workers are calling on the Prime Minister to take action to get polluting diesel vehicles off the UK's roads as soon as possible.
The recently formed Doctors Against Diesel group is campaigning for greater awareness of the health impacts of diesel emissions and for action to reduce the number of vehicles using the polluting fuel in towns and cities.
Air pollution from sources including factories and vehicles, particularly diesel engines, is linked to the early deaths of about 40,000 people a year in the UK - and causes problems such as heart and lung diseases and asthma.
London saw legal annual limits for pollution breached on some busy roads in the first week of January.
Nearly 300 health professionals have written to Theresa May, highlighting evidence of the impacts of pollutants including nitrogen dioxide and soot, particularly for children, and calling for a diesel reduction initiative.
Professor Jonathan Grigg of Queen Mary University of London and founding member of Doctors Against Diesel said: "There is overwhelming evidence that locally generated sooty particulate matter and nitrogen dioxide is harming children's health.
"In London, we know that diesel engines are a major and unnecessary cause of air pollution along our roads.
"Cutting diesel emissions would therefore have an immediate impact on children's personal exposure, and improve their long-term health."
Reducing pollution exposure for children and encouraging them to cycle and walk more would be a "major public health advance - and must be done as soon as possible", he said.
Professor John Middleton, president of the UK Faculty of Public Health said: "Diesel is the primary source of nitrogen dioxide in urban areas and is linked to health effects that begin before birth and extend throughout the life course, from childhood lung development and asthma, to increased risk of heart disease, stroke, lung cancer and dementia.
"It is time for diesel to be recognised as the health emergency that it is."
Figures from the Society of Motor Manufacturers and Traders show diesel vehicles continue to have a significant share of the market.
Some 1.29 million new diesel cars were registered last year, representing a market share of 47.7%, down from 48.5% in 2015.
The letter comes as research suggests commuters using public transport are being exposed to up to eight times more pollution than car users.
While motorists produce the most pollution per commuter, they are the least exposed to harmful particulate matter as they are sealed off from the outside, the study by the University of Surrey found.
Commuters on some underground lines are the most exposed to poor air quality, followed by those on buses, according to the research, published in the journal Environment International.
Campaigners, including environmental law firm ClientEarth which has repeatedly taken the Government to court over air pollution, are also calling for a new Clean Air Act to tackle the UK's pollution crisis.
The coalition calling for a new Clean Air Act includes the British Lung Foundation, the Royal College of Physicians, Medact, Greenpeace, Campaign for Better Transport, Sustrans, and Friends of the Earth.
The campaigners want new legislation to tackle air pollution sources such as diesel and speed up the shift to zero-emissions transport, enshrine the right to breathe clean air in law and make the UK a world leader in clean technology.
A new Act would come more than 60 years after the original Clean Air Act which was introduced in 1956 to tackle smog.
ClientEarth chief executive James Thornton said: "This is an urgent public health crisis over which the Prime Minister must take personal control.
"Instead of making the same arguments against taking action that were made 60 years ago, we need the Government to wake up to our air pollution crisis.
"Just as we did back then, we now need urgent action including a new Clean Air Act that is ambitious, fair and far-reaching enough to clean up our air across all of the United Kingdom."
Responding to the diesel phase-out call by health professionals, Friends of the Earth campaigner Sophie Neuburg said: "Road traffic is the biggest problem for UK air pollution, and diesel vehicles are worst of all.
"This is why we need a diesel scrappage scheme to get the worst polluting vehicles off our roads."