EXPERT REACTION: Traffic-polluted streets no walk in the park

Media Release

The Lancet: Traffic-polluted streets may negate the cardiorespiratory benefits of walking in older adults

Study analysed health impact of a walk through the traffic-polluted Oxford Street in London, compared to Hyde Park.

Even short-term (2 hour) exposure to tiny particles of soot or dust found in traffic fumes on busy roads appears to thwart the benefits of walking on the heart and lungs among older adults (aged 60 or over), according to a study comparing the health effects of walking along a traffic-polluted road versus walking through a park. The effect was particularly marked in people with existing respiratory illness.

The study, published in The Lancet, suggests that short term exposure to pollution is associated with stiffening of the arteries and impaired lung function, and strengthens the case to reduce vehicle emissions so that everyone can enjoy the health benefits of physical activity.

“Our findings indicate that in traffic congested streets, like London’s Oxford Street, the health benefits of walking do not always outweigh the risk from traffic pollution. However, this should not be seen as a barrier to many older people for whom walking is the only exercise they do. We suggest that, where possible, older adults walk in parks or other green spaces away from busy roads”, says senior author Professor Fan Chung from the National Heart & Lung Institute at Imperial College London, UK. [1]

Air pollution is responsible for around 5.5 million premature deaths worldwide every year. In the UK, polluted air contributes to 40,000 deaths each year, nearly a quarter of them in London. Earlier research shows that exposure to fine particulate matter (with a diameter of 2.5 micrometers or less; PM2.5) found in diesel exhaust fumes increases the risk of cardiovascular disease and death, and can reduce lung function, particularly in the elderly and those with chronic obstructive pulmonary disease (COPD).

The study was done in 119 older adults (aged 60 or over) including 40 healthy volunteers, 40 individuals with stable COPD, and 39 with stable ischaemic heart disease. Participants were randomly assigned to spend 2 hours walking along the western end of London’s Oxford Street where traffic is restricted to diesel-powered buses and cabs, or through a traffic-free section of Hyde Park, London. 3 to 8 weeks later, participants did the other walk. All participants had abstained from smoking for the past 12 months, and continued any medications as usual throughout the study.

Levels of traffic-related air pollutants (black carbon, particulate matter, ultrafine particles, and nitrogen dioxide) and measures of lung function and cardiovascular responses were taken before and during each walk.

In healthy participants, walking in Hyde Park led to an improvement in measures of lung capacity (average 7.5% increase in FEV1 [the maximum amount of air that can be exhaled in one second] at 5 hours after the start of the walk) and arterial stiffness (5% decrease in pulse wave velocity on average after 3 hours) that persisted for up to 26 hours. In contrast, walking on Oxford Street led to only a small transient increase in lung capacity, and substantial worsening of arterial stiffness (average 7% increase in pulse wave velocity) associated with greater exposure to black carbon soot and ultrafine particles from diesel exhaust.

The detrimental health effects of pollution were particularly marked in participants with COPD who experienced a narrowing (obstruction) of the small airways—reporting more respiratory symptoms including cough, sputum production, shortness of breath, and wheeze—and increased arterial stiffness after walking in Oxford Street compared with Hyde Park. These harmful effects were associated with increasing exposure to black carbon soot and ultrafine particles, supporting the view that fossil fuel combustion particles are particularly toxic to individuals with cardiovascular and lung disease.

Interestingly, further analyses found that pollution levels on Oxford Street resulted in worsening arterial stiffness in participants with ischaemic heart disease not using cardiovascular drugs, but had little effect on those taking medication, suggesting that these drugs might have protective effects [2]. More research is needed to confirm this finding.

According to Professor Chung: “Our data indicates that taking medications which improve arterial stiffness such as statins, ACE inhibitors, and calcium channel blockers may well reduce the adverse effects of air pollution in individuals with ischaemic heart disease.”[1]

The authors note that the study did not include a resting control group, so they cannot be certain that walking contributed to the changes in lung function and arterial stiffness, although previous studies have shown that walking improves arterial stiffness. They add that the study looked at the short-term beneficial effects of exercise in relation to pollution, and further research is needed on the long-term effects of regular walking in polluted environments.

They conclude, “Our study provides a clear message to improve the quality of the air we all share. In London, the introduction of the low emission zone has had little impact on particulate matter levels. More radical solutions, such as recently announced to phase out diesel-powered black cabs and replace them with battery powered electric alternatives, are needed.”[1]

Writing in a linked comment, Professor George Thurston and Dr Jonathan Newman from the New York University School of Medicine, New York, USA say: “The changes in arterial stiffness reported in the study…are biologically consistent with the air pollution and CVD health associations found in the population-based studies of hospital admissions and mortality, further strengthening the consensus that the association between particulate matter and CVD is causal. Although more studies are needed on the respective health effects of all the individual constituents and sources of PM2.5, the results of this and other recent urban studies already indicate that policy makers and health professionals should make a reduction in public exposures to diesel particulate matter a high priority in PM2.5 air pollution control and patient avoidance strategies.”