The Best Time to Exercise in a City with High Air Pollution

When it comes to exercise, the important thing is that you actually do it in order to avoid health complaints such as cardiovascular disease. Exercise can improve your fitness, your health and your mood. Even so, it can be difficult to exercise in a polluted city with unhealthy air.

Nobody wants to be breathing in pollution when they are doing their best to stay healthy so when is the best time to exercise in a city with high air pollution levels? We decided to find out.

The best time to exercise in a highly polluted city

When do you think would be the best time to exercise in a city with high pollution levels? If you are anything like us, you might think that the best time to exercise would be when the city streets are at their quietest; the dead of night when there are fewer vehicles on the roads. Fewer people means less emissions and particle pollution, right? That should mean that the best time to engage in physical activity is in the early hours of the morning but that isn’t the case!

By analysing several years’ worth of PM 2.5 data in Beijing it is possible to see that pollution levels actually spike at night with the highest pollution levels occurring between the hours of 10pm and 4am. The lowest pollution levels are found to be between midday and 6pm, a time when there are a lot of people and vehicles about, and when there is a lot of energy being used.

Roughly the same result was found to be true for citizens in Shenyang (midday-6pm), Chengdu (2pm-7pm) and Delhi (11am-7pm). Hyderabad, Mumbai and Kolkata also showed a similar result.

This may change depending on which polluted country you are in but the best time to exercise in a city with high pollution levels is in the early to mid-afternoon.

Why is the afternoon the best time to exercise in a highly polluted city?

PM2.5 is lowest in the afternoon but why is this the case? Most would think that PM2.5 would be at its highest during the day with so many cars on the roads. The answer is a mixture of a number of different variables.

Wind is stronger in the afternoon

Wind plays a large role in dispersing pollution. Interestingly, there tends to be only a light wind in the morning with most wind picking up in the afternoon. Although it is unwise to assume, it is highly likely that wind being more common in the afternoon contributes to this time of day being the best time to exercise in highly polluted areas.

The mixing heights are lower at night

Mixing heights refers to the height above a surface at which a pollutant can be dispersed as it mixes with the air. The mixing height tends to be higher during the day as a result of numerous meteorological factors including wind speed and direction, temperature, humidity, solar radiation, and rainfall.

During a surface temperature inversion (which means temperature increases with height), pollution tends to be lower to the surface of the Earth and more difficult to disperse. The most common time for a surface temperature inversion is at nighttime on a clear day. So don’t go exercising in a highly polluted city after a lovely sunny day as pollutant levels will be more concentrated closer to the ground and you will be at higher risk!

Rule breaking at night in a highly polluted city

A purely theoretical third reason may very well be that more laws are broken at night time than during the day because there are fewer regulatory enforcers. Highly polluted cities are looking to curb their environmental impact and they do so by enacting strict laws on pollution and emissions.

If someone wishes to break these laws and emit pollutants into the atmosphere, they are more likely to do so at night when police are less likely to be watching.

Does the best time to exercise depend on the city?

For the purposes of this article, we’ve chosen to focus on PM2.5, which is one of the most common forms of pollution. It is particularly prevalent in cities across China and India in the form of smog and vehicular emissions, which is the most commonly thought of pollutant when people are asked to imagine a polluted city.

The best time to exercise in a city with a different form of pollution such as ozone (particularly common in the US), may find that there is a better time to exercise than the mid-afternoon.

Likewise the trends that have been uncovered throughout this research may not read across to all other cities which may carry different health risks. If you are wanting to research your own city, it’s best to look into the research yourself. Many towns and cities have data on their air quality that can be read online.

Can exercise do more harm than good in a highly polluted city?

Air pollution research at Cambridge University has tried to look into how bad air quality has to be for exercising outdoors to do more harm than good with a particular focus on the concentration of PM 2.5 in the air. As of this writing, the air quality of Kolkata, India shows that PM 2.5 concentration is at 161.6 micrograms, which is 32.3 times above the WHO annual air quality guidelines.

The researchers chose to focus on cities with a PM 2.5 concentration of 50 micrograms, which is similar to the average air quality in Shanghai, China. They found that someone would have to exercise for 5 hours in a highly polluted city before the exercise would be harmful to their health. Increasing the PM 2.5 concentration to 150 micrograms though, shows that anything more than 30 minutes of exercise - whether a run or a bike ride - will be detrimental to an athlete’s health.

With Kolkata residents having to exercise outdoors in 161.6 micrograms of PM 2.5, exercise outdoors should almost certainly be less than 30 minutes unless a specialist mask is worn. In general, exercising indoors accompanied by clean air solutions (such as an air purifier) would be far more beneficial to health.