The heart is directly affected by air pollution

"Could it be that their blood vessels also have enhanced constriction in response to air pollution?"

The researchers focused on ozone and fine particulate matter.

Fine particles are those with a diameter of less than 2.5 micrometers. They are emitted from burning fossil fuels, mostly from car engine exhaust, power generation and many industrial processes.

Ozone and additional particulate materials are created when the sun shines on these emissions.

In contrast to larger particles, which are trapped in the upper airways when inhaled, the fine particles travel down to the alveoli, tiny air sacs at the base of the lungs, where they can affect the rest of the cardiovascular system.

It is possible that the particles may even directly enter the blood.

Rush hour pollution

The volunteers breathed in a level of pollution similar to that found in urban areas during peak air pollution times such as rush-hour traffic.

The degree of blood vessel constriction produced by exposure to pollutants is unlikely to produce significant problems in healthy indviduals.

However, Dr Brook said that it could conceivably trigger cardiac events in people who are at risk of heart disease.

He said: "Our results are a clear demonstration that environmentally relevant concentrations of common air pollutants that can occur in urban settings adversely affect the blood vessels of healthy people."

A spokesperson for the British Heart Foundation said that air pollution was known to increase the risk of heart disease in those at risk.

However, she said that the study focused on too few people to draw firm conclusions.

The research is published in Circulation: Journal of the American Heart Association.