People aged over 60 should choose quiet, green areas to exercise, says a study by Imperial College London and Duke University in the US, which suggests that the benefits to the heart and lungs of a two-hour walk are quickly neutralized if the physical activity is performed in a polluted area (The Lancet, December 5). The researchers recruited 119 men and women over 60: one-third of them healthy and two-thirds with a history of heart or lung problems whose illness is under control. The volunteers walked for two hours in a quiet area of Hyde Park, a large green space in London, and then performed the same task on another day in the polluted and busy environment of the city center, on a commercial stretch of Oxford Street. The analyses showed that all the participants who walked in the park gained significant pulmonary benefits and reduced stiffness of the arteries one hour after finishing the exercise. In many cases, the positive effects lasted for up to 24 hours. The tests, however, found that walking in the most polluted area of the city provided almost no health gains, and any resulting benefits lasted for just two hours.