This Is When Exercise In Air Pollution Does More Harm Than Good

I’m in Beijing right now, and I’m going to exercise today. So what can I do when the world outside my window looks like this? Would exercise in this air pollution do more harm than good? Is it better to not exercise at all?

1. The point where exercise becomes harmful

Researchers at Cambridge analyzed studies on the benefits of exercise and the harms of air pollution and tried to calculate how bad the air has to be for exercise to be harmful. There are lots of different types of pollution, but they focused on particle pollution (PM2.5), which the WHO says kills more people than any other pollutant. They used biking outdoors as the form of exercise—something I’ve experienced in Delhi.

Here’s the cost-benefit analysis in a city with 50 micrograms/m3 of PM2.5 pollution. That’s about the average level in Shanghai. For comparison, the WHO annual limit is 10 micrograms.

First check out the green circle, the tipping point. Biking outside is beneficial to lifespan (in other words, “not dying”) up to about 70 minutes.

Then check out the red circle, the breakeven point.

At about 300 minutes it actually starts to become harmful. But at that point you’ve been biking for almost five hours, so you need a break anyway.

Is exercise as good as pollution is bad?

Let’s say you do both. You exercise, but you breathe polluted air. How do those balance out? Are you back to the starting point?

For a sense of scale, I got statistics on the danger of different behaviors from this recent epidemiological study. They don’t have exercise, so I compared eating unhealthy foods. Here’s the years of life lost from PM2.5 versus eating unhealthy foods.

The effect of eating a bad diet is worse than air pollution. My guess is exercise is at least as potent as air pollution, if not stronger. That’s why exercise is still helpful in a polluted city.

But what about really bad days and really bad cities?

Hey, 50 micrograms, that’s child’s play!

Places like Beijing and Delhi have real man air. The World Health Organization found Delhi’s PM2.5 averages 153 micrograms. Beijing easily has days that bad. So how does the calculus change then?

Here’s a slightly different graph with the tipping point (red) and break-even point (blue). I drew a line for Delhi’s average PM2.5. In Delhi, biking starts to be harmful at just 30 minutes (the breakeven point).

The line for the tipping point doesn’t even extend up to Delhi’s PM2.5 levels, so we don’t know at what point it loses its beneficial effect. I tried extending the line up and came up with, maybe 2 minutes?

And keep in mind, this is only considering PM2.5 pollution. It does not take into account any gas pollutants like ozone or NO2. So the total effect of all air pollutants would be worse.

Bottom line: This estimate suggests outdoor exercise on bad air days or particularly bad cities becomes harmful after just a few minutes.

But, but…

Keep in mind this is an estimate. It is based on epidemiology research, but it uses lots of assumptions. There are lots of factors that could change the equation:

If you’re overweight, the benefit of exercise would probably be much higher.
If your body reacts strongly to air pollution, the harm probably starts earlier for you.
This estimate is only for death. It doesn’t calculate in not-fun, but not-death problems like lung function and asthma.

If all that sounds like a lot of assumptions and uncertainty, here are two ways to change that equation.

2. How I’m getting my exercise today in Beijing.

Method 1: When I’m in Beijing, I solve this problem by wearing a mask.

These simple masks are as cheap as 50 cents, and scientific studies conclusively show they reduce particle exposure by well over 90% (I run through the evidence here). Solid random-assignment studies have also found that wearing masks just like these prevent effects on blood pressure and heart rate variability (I explain those studies here).

I’ve fit-tested these masks on my very own face and found these masks easily score over 90%.

Here are results from several masks on my face and my colleague’s face:

So when I’m in Beijing and Delhi, I bike around all day getting good cardio exercise while breathing less particulate than people sitting indoors. And I don’t need to mess around with cost-benefit models!

Method 2: I work out indoors.

Here’s a trick: Simply by being indoors, you’re breathing less particulate pollution.

I tested air indoors and outdoors in Beijing and Shanghai (including at my gym!). The data showed that particulate levels indoors are only about 60% of outdoor levels, even in places that don’t use purifiers.