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 focused on tiny particles called PM2.5s that 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.
The results are published today in the European Heart Journal.
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.