Sudden spikes of air pollution in the UK encourage hundreds of heart attacks, strokes and acute attacks on these days, research suggests.
A team at King's College London looked at data from London, Birmingham, Bristol, Derby, Liverpool, Manchester, Nottingham, Oxford and Southampton.
During the days of pollutant levels in the upper half of the annual range, there were on average 124 additional cardiac holdings.
Simon Stevens, England's England boss, said it was evidence of a "health emergency".
The figure is based on ambulance call data and does not include heart attacks that patients already have in hospital.
It focuses on significant short-term health risks arising from air pollution, leading to almost 500,000 premature deaths in Europe each year.
On days with high levels of pollution, overall in the nine city cities, there were an average of 231 hospital admissions for stroke, and 193 children and additional adults were taken to hospital for asthma treatment.
Dr Heather Walton, of King's College Environmental Research Group in London, said air pollution reduction policies focused primarily on the effects of life expectancy.
"However, health studies show clear links to a wider range of health effects," she said.
In London, high pollution days resulted in 87 additional cardiac holdings, at least 144 strokes, and 74 children and 33 adults in hospital with asthma related issues.
Birmingham saw 12 cardiac holdings outside the hospital anymore, 27 more stroke admissions and 26 more asthma.
Bristol, Liverpool, Manchester, Nottingham, Oxford and Southampton felt more than an out-of-hospital heart attack and up to 14 additional hospital admissions for stroke and asthma on high-pollution days.
There was no apparent increase in Derryby.
Long-term risks associated with high levels of pollution include lung growth and low birth weight.
King's College research also recommends that air pollution by one-fifth of lung cancer incidents would reduce between 5% and 7% across the nine cities surveyed.
Mr Stevens said: "The climate crisis is also clearly a health emergency.
"As these avoidable deaths are now happening – not in 2025 or 2050 – together we must act now."
The figures were published before Wednesday's International Air Summit hosted by the Mayor of London, Sadiq Khan and the UK100 network of local government leaders.
UK100 director Polly Billington said: "Local government needs additional powers and resources to address this public health crisis."
The Department of the Environment, Food and Rural Affairs said it was "taking urgent action to improve air quality and tackle pollution" and that new legislation "local powers to address key sources of air pollution" will increase. .