Air pollution increases the chances of heart attack survivors dying early, research has shown.
Higher levels of tiny sooty particles in the air led to more deaths among patients after they had left hospital.
Experts monitored more than 154,000 patients treated for heart attacks and angina for an average period of 3.7 years.
Exposure to man-made pollutants increased death rates among survivors of acute coronary syndrome (ACS) by 12%, the scientists found. This meant that over the study period there were almost 5,000 extra deaths.
The research, published in the European Heart Journal, focused on tiny particles called PM2.5s which measure no more than 2.5 micrometres across - 30 times smaller than the width of a human hair. Road vehicles, factories and power stations are the main man-made sources of the particles, which lodge deep in the lungs.
Author Dr Cathryn Tonne, from the London School of Hygiene and Tropical Medicine, said: "We found that for every 10 microgram per cubic metre increase in PM2.5 there was a 20% increase in the death rate. For example, over one year of follow-up after patients had been admitted to hospital with ACS, there would be 20% more deaths among patients exposed to PM2.5 levels of 20 micrograms per cubic metre compared to patients exposed to PM2.5 levels of 10 micrograms per cubic metre."
The research, conducted in England and Wales, found that London had the highest estimated concentration of the particles - an average of 14.1 micrograms per cubic metre of air. People living in the north-east of England had the lowest exposures.
Previous studies have linked exposure to pollution particles and an increased risk of heart disease and early death. But evidence of the impact of air pollution on survival after heart attacks has been inconsistent. Patients from poorer backgrounds often live in areas with higher air pollution and tend to do less well after a diagnosis of heart problems.
"This raises the possibility that exposure to air pollution may explain, in part, the differences in prognosis among heart attack patients from different backgrounds," said Dr Tonne. "Our findings confirm an association between PM2.5 and increased rates of death in survivors of ACS."
The research indicated that other factors also played an important role in reducing the life expectancy of patients in deprived areas.