If you live in a city and walk for exercise, try to do it in a park rather than along busy streets where traffic-related air pollution is likely to be elevated.
In a study in the Lancet in January 2018, 119 people over age 60 took long midday walks in London—along busy Oxford Street and, several weeks later, in leafy Hyde Park. Testing done before and after the walks showed that while the park walk led to cardiorespiratory benefits (in blood pressure, lung volume, and arterial stiffness), there were little or no improvements after the street walks. This was true in healthy participants as well as those who had heart disease or chronic obstructive pulmonary disease (COPD).
The researchers suggested that walking along heavily trafficked streets may be less beneficial not only because of air pollution but also because of stress related to noise and crowds.
Also see Better Walking Workouts.