Over the years, people used to think that the best time for exercise is in the morning, followed by the evening, because the air is freshest at that time. However, due to urban air pollution's sake, the best exercise time has changed. 

Studies have shown that in general there are two peak periods each day in air pollution. One is before sunrise, the other for the evening. Especially in winter, the upper temperature is high and surface temperature low. Atmospheric convection almost stopped. Therefore, harmful pollutants on the ground cannot be spread to the upper atmosphere but remain in the lower breathing zone. Concentrated in the industrial or high rise residential and car sped off the road on both sides, this phenomenon is particularly typical. At this time, the harmful gas to be higher than 2-3 times the normal case.