They asked the volunteers to walk for two hours midday in one of two locations in London.
Commenting on the findings, Dr Mireille Toledano, from the School of Public Health at Imperial College London and senior author of the research, said: "Our study has shown that a small but significant proportion of babies born underweight in London are directly attributable to exposure to air pollution, particularly to small particles produced by road traffic".
They argue the study should "increase awareness that prenatal exposure to small particle air pollution is detrimental to the unborn child", but stress that increasing awareness without solutions "may serve only to increase maternal anxiety and guilt".
"If people can not find a green place or a park to exercise, I think they probably should exercise indoors", Chung said.
Previous BHF research has shown that long-term exposure to air pollution leads to inflammation in the blood vessels, including those supplying the heart, and promotes the buildup of fatty plaques in the linings of blood vessels, which can eventually lead to a heart attack or stroke.
Volunteers who took a walk in Hyde Park experienced a decrease in the stiffness of their arteries, a benefit normally seen after exercise. They also reported more coughing, shortness of breath, sputum and wheezing. One was a quieter area in Hyde Park, while the other was along a busier section of Oxford Street, where pollution usually exceeds the air quality limits set by the World Health Organization.
"For many people, such as the elderly or those with chronic disease, the only exercise they very often can do is to walk", Chung said. A study in younger people should be done, he said.
However, recent studies across a range of age groups have found physical activity to be beneficial, even in the face of high pollution levels.
"Exercise is crucial in reducing the risk of a heart attack or stroke, but it seems unsafe levels of air pollution could be erasing these benefits in older adults".
"Our model indicates that in London, health benefits of active travel always outweigh the risk from pollution", lead author of the research Marko Tainio said when it was published in 2016. "With the annual number of births projected to continue increasing in London, the absolute health burden will increase at the population level, unless air quality in London improves". Also, Professor Chung and colleagues noticed that health benefits of walking were attenuated, not completely negated, among the healthy participants. "We should provide them with spaces to enable that instead of giving them no choice but to walk and cycle through busy, polluted streets". "But for people commuting or shopping, even in a city as polluted as London, we would still encourage walking and cycling".
Ian Colbeck, professor of environmental science at Essex University, said the paper highlighted the risks to health by walking along polluted roads, for the over-60s with specific pre-existing medical conditions. Air pollution occurs when harmful substances including particulates and biological molecules are in- troduced into Earth's atmosphere.
"Combined with evidence from other recent studies, our findings underscore that we can't really tolerate the levels of air pollution that we now find on our busy streets", said Fan Chung, professor of respiratory medicine and head of experimental studies medicine at Imperial College's National Heart & Lung Institute. People like outdoor exercise. "Is it possible that the differences in response were caused by something other than the differences in air pollution and noise? For example, the planned removal of buses and taxis from Oxford Street should help to achieve this", he said.