Striding down a polluted street is no walk in the park when it comes to the benefits of light exercise.

According to a study published in The Lancet, healthy participants who walked for two hours in an urban park demonstrated better lung function and a decrease in arterial stiffness up to 26 hours after their walk. Those benefits were weakened or even reversed when the same patients walked on a commercial strip in London frequented by diesel-powered buses and taxi cabs. Lung function was measured by forced expiratory volume in the first second (FEV₁) and forced vital capacity (FVC), while arterial stiffness was measured by pulse wave velocity (PWV) and augmentation index.

Participants with chronic obstructive pulmonary disease (COPD) and ischemic heart disease saw smaller increases in lung function and smaller decreases in arterial stiffness when compared to healthy individuals after the walk through the park, but they were more likely to retain those benefits after walking down the smoggy street. Lead study author Rudy Sinharay, MBBS, and colleagues said that observation may be explained by the higher likelihood ischemic heart disease patients are on protective medications that improve arterial stiffness such as statins, angiotensin-converting enzyme inhibitors and calcium channel blockers.

“Our findings suggest that healthy people, as well as those with chronic cardiorespiratory disorders, should minimize walking on streets with high levels of pollution because this curtails or even reverses the cardiorespiratory benefits of exercise,” Siharay et al. wrote. “Instead, walking exercise should be enjoyed in urban green space areas away from high density traffic. Patients with chronic cardiorespiratory disorders should be encouraged to use appropriate medication to mitigate the adverse effects of air pollution. Current ambient levels of air pollution along busy streets are unacceptable and need to be controlled.”

The study included 40 healthy volunteers, 40 with COPD and 39 with ischemic heart disease, all over 60 years old.

Participants took a two-hour walk at each location—separated by multiple weeks—and covered an average of five kilometers each time. During each walk session, black carbon, particulate matter (PM) concentrations, ultrafine particles and nitrogen dioxide concentrations were measured.

COPD participants reported four times the severity of wheezing, three times the severity of sputum and about twice the burden of shortness of breath and coughing during the street walk. Healthy volunteers and those with ischemic heart disease didn’t experience a noticeable difference in these symptoms, except for the ischemic heart participants having a 4.13-fold coughing severity at the more highly polluted location.

In a related editorial, George Thurston, ScD, and Jonathan Newman, MD, MPH—both with the New York University School of Medicine—suggested larger studies could look at biomarkers of oxidative stress and implications of longer-term air pollution exposure on arterial stiffness. They echoed the study authors’ assertion that policymakers should strive to reduce public exposure to diesel particulate matter.

“The changes in arterial stiffness reported in the study by Sinharay and colleagues are biologically consistent with the air pollution and CVD (cardiovascular disease) 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,” Thurston and Newman wrote. “Although more studies are needed on the respective health effects of all the individual constituents and sources of PM_2.5, the results of this and other recent urban studies already indicate that policy makers and health professionals should make the reduction in public exposures to diesel particulate matter a high priority in PM_2.5 air pollution control and patient avoidance strategies.”