Some people think that doing exercise in the evening is better. From the perspective of cleanliness of air, exercise time shall not be early but be appropriately late. Experts believe that the worst quality of the air is in the morning, which is due to early morning low surface temperature, air pollutants such as carbon dioxide and dust’s distribution is not easy. After 3 hours, when the air is the cleanest, choosing the evening exercise and breathing the "fresh" air benefits your health.