Dr Penny Woods, chief executive of the British Lung Foundation, said: 'How very sad that our streets are so traffic-polluted that older people trying to keep themselves healthy are in fact doing themselves harm.
Findings of the study suggested that pregnant mothers exposed to air pollution from air pollution in the capital are more likely to give birth to babies that are underweight or smaller than they should be.
Environmental measurements were also collected, to track pollution levels and volunteers' exposure.
They also call for more green spaces in urban environments. Two-thirds of the volunteers had been diagnosed with either heart disease or chronic obstructive pulmonary disease (COPD), while the others were healthy (no pre-existing heart or lung condition). They were randomly allotted to walk either in the west end of Oxford Street which is restricted to taxis and buses emitting diesel fumes and which has frequently breached air quality limits set by the World Health Organisation, or a traffic-free area of Hyde Park.
Walking is the sort of low impact exercise recommended by the NHS for older people to improve their cardiovascular fitness, but the study found that the impact of air pollution negated this. However, it is still a notorious area for dirty air, with high levels of black carbon, nitrogen dioxide, and fine particulate matter.
In comparison, when they walked in Hyde Park "all participants, irrespective of their disease status..."
On the other hand, those on Oxford Street experienced a smaller increase, and their results did not last for the rest of the day.
Significant differences in arterial stiffness were also observed.
For the research, the team recruited 119 volunteers over the age of 60 who were healthy, had stable COPD, or had stable heart disease.
"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", coauthor Fan Chung said in a statement.
Interestingly, the study found that the volunteers with heart disease who were being treated with medication were less negatively affected by the pollution. The results may or may not be applicable to people living elsewhere. A study in younger people should be done, he said. It can't tell us, therefore, what the long-term benefits (or non-benefits) of exercise are in relation to pollution. The negative effect may well be the same in younger people, say the authors, and it reinforces the urgency of reducing emissions in city streets.
Still, the findings point to how hard it is for many people to personally improve their health when the built environments of our communities do not support - or even undermine - those efforts.
Professor Fan Chung, who led the research, said, "these findings are important as for many people, such as the elderly or those with chronic disease, very often the only exercise they can do is to walk".
"Our research suggests that we might advise older adults to walk in green spaces, away from built-up areas and pollution from traffic".
"For people living in the inner city it may be hard to find areas where they can go and walk, away from pollution".
"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", he said. Nevertheless, our studies found that even these relatively low levels were associated with increased risk of adverse adult lung health outcomes such as asthma and poor lung function.