Higher air pollution in the UK trigger hundreds more heart attacks, strokes and acute asthma attacks each year, research suggests.
A team at King's College London looked at data from London, Birmingham, Bristol, Derby, Liverpool, Manchester, Nottingham, Oxford and Southampton.
They calculated days with above average pollution levels would see an extra 124 cardiac arrests over the year.
NHS England boss Simon Stevens said it was evidence of "a health emergency".
The figure is based on ambulance call data and does not count heart attacks suffered by patients already in hospital.
It points to significant short-term health risks caused by air pollution, on top of contributing to almost 500,000 premature deaths in Europe every year.
On days with high pollution levels, across the nine cities in total, they calculated that there would be a total of 231 additional hospital admissions for stroke, with an extra 193 children and adults taken to hospital for asthma treatment.
Dr Heather Walton, of King's College London's Environmental Research Group, said air pollution reduction policies concentrated in the main on effects connected to life expectancy.
"However, health studies show clear links with a much wider range of health effects," she added.
In London, high-pollution days would see an extra 87 cardiac arrests per year, an extra 144 strokes, and 74 children and 33 adults ending up in hospital with asthma-related issues.
In Birmingham the figure would be 12 more out-of-hospital cardiac arrests, 27 additional admissions for stroke and 26 more for asthma.
Bristol, Liverpool, Manchester, Nottingham, Oxford and Southampton would see between two and six more out-of-hospital heart attacks and up to 14 extra hospital admissions for both stroke and asthma.
Only in Derby would there be no apparent increase.
Among the long-term risks associated with high pollution levels are stunted lung growth and low birth weight.
The King's College research also suggests cutting air pollution by a fifth would decrease incidents of lung cancer by between 5% and 7% across the nine cities surveyed.
Mr Stevens said: "It's clear that the climate emergency is in fact also a health emergency.
"Since these avoidable deaths are happening now - not in 2025 or 2050 - together we need to act now."
The figures were published ahead of Wednesday's International Clean Air Summit hosted by 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 for Environment, Food and Rural Affairs said it was "taking urgent action to improve air quality and tackle pollution" and that new legislation will "increase local powers to address key sources of air pollution".
"We are already working hard to reduce transport emissions and are investing £3.5 billion to clean up our air."
Analysis by Robert Cuffe, BBC Head of Statistics:
Researchers sometimes struggle to make their statistics human.
This team worries that "life years lost" are too abstract.
So they have looked at things that might hit closer to home like heart attacks or asthma attacks that hospitalise children.
Using already published studies, they have worked out how many more of these events to expect on days with above-average pollution.
The numbers might not knock your socks off: in London, for every 100 cardiac arrest ambulance callouts on low-pollution days, they would expect to see 102 on high-pollution days.
But the numbers add up and reinforce the case for further reductions in air pollution.