What Time of Day Is PM2.5 the Lowest?

My intuition has always been that air pollution is lowest at night because there are fewer cars on the road and fewer factories humming. Apparently I’m not alone: 139 voted for their guess about what time of day has the lowest PM2.5, and night time came in first:

I also know people who arrange their schedules to work out in the morning to avoid the worst pollution. But how accurate are our intuitions?

To get to the bottom of it, I analyzed thousands of hours of PM2.5 data from the US Embassy in Beijing. When I finally got the answer, I was surprised:

Instead of during the quiet of night, it’s the afternoon–right around rush hour–that PM2.5 is the lowest. So if you’re planning a picnic or insist on exercising outside, you’re usually best off between noon and 6pm.

What about other cities?

Is that how PM2.5 generally works, or is it unique to Beijing’s activity or climate? Fortunately, US consulates in several other cities publish their historical data.

Shenyang

In terms of climate, Shenyang is pretty similar to Beijing, and its daily PM2.5 patterns are very similar (although night time isn’t quite as bad as Beijing):

Chengdu

The pattern in Chengdu is similar. PM2.5 is lowest in the afternoon and highest in the morning.

Shanghai

The picture starts changing when you get to the south. In Shanghai, PM2.5 is lowest in the afternoon (like the north), but there’s also a dip in the very early morning:

Guangzhou

Guangzhou’s pattern was virtually identical to Shanghai’s: a dip in the very early morning and a dip in the afternoon.

Bottom Line: When Is the Safest to Be Outside?

In all five cities, the afternoon had the lowest PM2.5 levels. And in contrast to many people’s intuition, the night time had the worst air in several cities. Thus, you’re usually best off organizing your picnic or Tai chi in the afternoon.

But keep in mind that the air is NEVER safe on average in any of the five cities at any time of day. So take “safest” with a grain of salt!

Smart Air

Open Data

All of the data is available from the US embassy and consulate websites. Thanks to Josh Malina for collecting and analyzing the consulate data.


How I Protect Myself

Smart Air is a  certified B Corp  committed to combating the myths big companies use to inflate the price of clean air.

Smart Air provides empirically backed, no-nonsense purifiers and masks, that remove the same particles as the big companies for a fraction of the cost. Only corporations benefit when clean air is a luxury.

Check out the Sqair!


Free Guide to Breathing Safe

Want to learn more about breathing clean air? Join thousands more and stay up to date on protecting your health.

Subscribe
Notify of

0 Comments
Inline Feedbacks
View all comments
Smart Air low cost purifiers

Smart Air is a social enterprise and certified B-Corp that offers simple, no-nonsense air purifiers and provides free education to protect people from the harms of air pollution. We are proud to be the only certified B-Corp dedicated to fighting air pollution.

Certified B-Corp air purifier company