My intuition has always been that there is least air pollution 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 least air pollution (lowest PM 2.5), 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 PM 2.5 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 PM 2.5 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 PM 2.5 generally works, or is it unique to Beijing’s activity or climate? Fortunately, US consulates in several other cities publish their historical data.
In terms of climate, Shenyang is pretty similar to Beijing, and its daily PM 2.5 patterns are very similar (although night time isn’t quite as bad as Beijing):
The pattern in Chengdu is similar. PM 2.5 is lowest in the afternoon and highest in the morning.
The picture starts changing when you get to the south. In Shanghai, PM 2.5 is lowest in the afternoon (like the north), but there’s also a dip in the very early morning:
But also note that the scale is really zoomed in for Shanghai and Guangzhou (below). The difference between the lowest and the highest averages for Shanghai is only about 4 micrograms, as opposed to about 25 micrograms in Beijing.
Guangzhou’s pattern was virtually identical to Shanghai’s: a dip in the very early morning and a dip in the afternoon.
When is it safest to be outside?
In all five cities, the afternoon had the lowest PM 2.5 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!
Read more about why pollution is worse at night and in different seasons here.
All of the data is available from the US embassy and consulate websites. Thanks to Josh Malina for collecting and analyzing the consulate data.
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.