(CNN) Going for a long walk on polluted, traffic-laden streets in an attempt to get fit has negligible benefits on your health, a new study has found.

UK researchers explored the benefits of walking in people over the age of 60 and compared the impact on their health when they walked along polluted urban streets versus in the open spaces of a park.

It perhaps comes as no surprise that people walking in the park fared better. The surprise was that benefits of walking were negligible, in terms of boosting heart and respiratory health, when walking along polluted streets.

The findings, published Tuesday in the journal The Lancet, suggest that short-term exposure to traffic pollution prevents the cardiorespiratory -- heart and lung -- benefits of physical activity during that time.

"When you walk, your airways open up ... and your blood vessels dilate, or open up... and these effects can last for a few days. When you do this in a polluted place, these effects are much smaller, so you've lost the benefits of exercise," said Dr. Fan Chung, professor of respiratory medicine at Imperial College London, who led the study.

"When you exercise in polluted areas, you breathe in more, and you get more of the particles and gases getting to your lungs," he said.

Pollution versus park

Chung's team set out find out the effects of pollution on people with heart and lung disease, most of whom are over the age of 60, he said.

For a fair comparison, a healthy control group was included, but to the researchers' surprise, they saw a significant impact from pollution on everyone.

The team recruited 119 adults over the age of 60 and divided them into three groups, based on whether they were healthy or had heart disease or chronic obstructive pulmonary disease, known as COPD.

Participants were randomly assigned to walk for two hours on London's Oxford Street, a major road and shopping district in the city, or in the open spaces of the 350-acre Hyde Park, just a mile away. A few weeks later, they walked in the other location.

Traffic along Oxford Street is restricted to allow mainly buses and taxis, which typically run on diesel fuel. Overall, London breached air pollution limits for 2017 just five days into the new year.

A general view of Oxford Street, London.

For all the participants, walking in Hyde Park led to improvements in lung capacity and function as well as reduced stiffness of the arteries, which is otherwise a risk factor for cardiovascular disease, for up to 26 hours afterward.

However, when people walked along Oxford Street, the researchers found a minor increase in lung capacity and a rise in arterial stiffness, which could be attributed to exposure to black carbon soot and ultrafine particles from diesel exhaust, they said.

"In a polluted place, the (positive) effects are much smaller," Chung said.

People with COPD fared the worst along Oxford Street, with both their airways narrowing and their arteries stiffening. They also reported more coughing, shortness of breath, sputum and wheezing.

The team highlighted that some of the benefits from walking in the park could be impacted by more pleasant surroundings and reduced stress, but they believe this does not account for the significant difference they saw.

"If people cannot find a green place or a park to exercise, I think they probably should exercise indoors," Chung said.

People take a walk in Hyde Park during the last days of autumn in October.

To walk or not to walk

"We're not talking about very high levels of pollution that you see in India or China. We're talking about pollution you get on an ordinary day walking up and down the high street," Chung said. "At that level, we are seeing effects that are negating the benefits of walking."

He believes it's most important for people with heart and lung disease avoid these areas and adds that despite being conducted in London, the study has global relevance.

"These would apply to European cities and North American cities where the pollution levels are more similar," Chung said.

Asian pollution is one or two orders of magnitude greater, and similar studies should be done at that level of pollution, 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.

One piece of research, by teams at the Centre for Diet and Activity Research at the University of Cambridge, found that the benefits of walking and cycling outweigh the negative effects of air pollution -- the converse of this new study.

"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. "Even in Delhi, one of the most polluted cities in the world -- with pollution levels 10 times those in London -- people would need to cycle over five hours per week before the pollution risks outweigh the health benefits."

In response to the new findings, Tainio said, "it is also important to notice that this study looked at the short-term impacts. ... These findings need to be confirmed with empirical long-term studies examining tradeoffs over months and years. Also, Professor Chung and colleagues noticed that health benefits of walking were attenuated, not completely negated, among the healthy participants.

"The authors suggest that people should avoid walking in busy streets and should instead walk in parks or in green space. We agree that this is good advice for recreational walking for people who can make that choice," he added. "But for people commuting or shopping, even in a city as polluted as London, we would still encourage walking and cycling."

"This paper highlights the risks to health by walking along polluted roads for the over-60s with specific pre-existing medical conditions," added Ian Colbeck, professor of environmental science at the University of Essex. "However, we know from other research that for the vast majority of the population, the benefits of any physical activity far outweigh any harm caused by air pollution, except for the most extreme air pollution concentrations. It's important to that people continue to exercise."

Professor Stephen Holgate, special adviser on air quality to the Royal College of Physicians in the UK, believes that "we can be quite confident from this (new) study that it is the pollution that is the factor responsible for changes in lung function."

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Holgate, who was not involved in the new research, added that the sample sizes were small but that "the study deliberately selected COPD and ischemic heart disease at-risk patients, and ... overall, the findings add to evidence of the importance of pollutant effects in vulnerable groups and have implications for pollution in general from vehicles (diesel, petrol, brakes and tires) as sources of pollutants."

Tainio highlighted that encouraging people to exercise could in turn reduce pollution levels.

"The authors are right to highlight that policies should aim to reduce air pollution and noise levels in streets to protect vulnerable population from potential harm. For example, the planned removal of buses and taxis from Oxford Street should help to achieve this," he said.

"However, it is important to remember the role that walking and cycling can play in helping to reduce air pollution and noise by removing motorized transport from the streets."