Spikes in air pollution in Liverpool and Manchester are triggering more heart attacks compared to days when the air is cleaner, according to new research.
A study by King's College London found there are significant short-term health risks caused by air pollution, as well as contributing to up to 36,000 deaths every year.
The study looked at data from nine English cities - London, Birmingham, Bristol, Derby, Liverpool, Manchester, Nottingham, Oxford and Southampton.
It found on high pollution days - days when pollutant levels were in the top half of the annual range - there were an extra 124 cardiac arrests on average.
The figure discounts cardiac arrests suffered by patients already in hospital and is based on ambulance call data.
The research also found there was an average of 231 additional hospital admissions for stroke, with an extra 193 children and adults hospitalised for asthma.
Mr Stevens called for immediate action adding that the NHS needed to radically reduce its own carbon footprint, as well as adapting its supply chain and transport to do its bit to cut pollutants.
The risk was found to be greatest in London, where high pollution days cause an extra 87 cardiac arrests on average. Birmingham saw the second highest risk, with 12 more out-of-hospital cardiac arrests.
Bristol, Liverpool, Manchester, Nottingham, Oxford and Southampton saw between two and six additional out-of-hospital heart attacks on high pollution days.
Among the long-term risks associated with high pollution levels are stunted lung growth and low birth weight.
The research also found cutting air pollution by a fifth would decrease incidents of lung cancer by between 5% and 7% across the nine cities surveyed.
The figures were published ahead of the International Clean Air Summit this Wednesday hosted by Mayor of London Sadiq Khan and the UK100 this week.
The UK100 is a network of local government leaders, who have pledged to help their communities shift to 100% clean energy by 2050.
The full report is due to be published in November.