Reducing exposure to air pollution could help those with heart disease.
University researchers found that wearing a facemask reduced the effects of traffic fumes in heart disease patients.
Nearly 100 volunteers with heart disease walked both with and without a facemask - designed for occupational use in factories - for two hours along a busy route in Beijing.
They carried equipment measuring humidity, temperature and air pollution as well as blood pressure and heart monitors.
When participants wore facemasks, they experienced lower blood pressure, healthier patterns of heart activity and a better heart oxygen supply.
We have shown that reducing exposure to particulate air pollution can have direct and rapid beneficial effects on the cardiovascular system, can improve cardiovascular health, and, if sustained, could have the potential to reduce heart attacks.
The researchers looked at the concentration of air pollution particles that come mostly from burning fuels, and are especially prevalent in traffic fumes.
The particles are only one fortieth of the width of a human hair.
They are thought to penetrate deep into lungs where they can affect blood vessels and circulating blood cells.
Beijing is one of the top 20 most polluted cities in the world.
During the British Heart Foundation-funded study, the air contained 60 to 90 micrograms of particles per cubic metre.
By comparison in London the average particle concentration is only 20 to 30 micrograms.
The facemasks were highly effective and reduced the amount of particles inhaled by the volunteers by 97 per cent.
The study was presented at the World Congress of Cardiology.
For most people, the benefits of exercising outdoors outweigh the risk from air pollution. We would advise people who have heart disease to try and avoid spending long periods outdoors in places where air pollution is high, as it could worsen their condition.