Physical activity is important in preventing heart and blood vessel disease in young people so long as they don't undertake very strenuous activity on days when air pollution levels are high, according to a nationwide study of nearly 1.5 million people.
The study was published today (Tuesday) in the European Heart Journal.
Here's what the study has to say
Until now, little has been known about the trade-offs between the health benefits of physical activity taking place outdoors and the potentially harmful effects of air pollution.