During the winter pollution season in Beijing, everyone people get worried about the air pollution. I heard about this so much that I decided to make a meme out of it. 

But I see people who seem to think pollution takes a vacation for the summer. Take as an example this Zhihu user who asked how to store a purifier for the summer when it’s not needed. 

What The Data Says

Are people right that winter air pollution is worse than the summer? And is it safe? We analyzed data on small particles in the air (PM2.5) from the US Embassy in Beijing. The data covers 2008-2017.

The good news: First, people’s intuition is right. The winter is worse. Beijing’s winter averages 104 micrograms (ug/m3) versus 81 in the summer. 

The bad news: “Better” is still far from “safe.” Beijing’s average is still 3 times above the World Health Organization’s 24-hour limit (25 micrograms) and 9 times the annual limit (10 micrograms).

The Midsummer Bump

There’s another weird trend in the data. See that bump in June and July? The summer is better, but Beijing consistently sees worse pollution in June and July.

We analyzed PM2.5 in 10 major cities across China, and the trend was the same–better in summer, but still unsafe.

This isn’t just a China story. We found the same pattern when we analyzed pollution levels in Delhi.

While the summer skies are bluer, it doesn’t mean the air is safe. Summers also see increases in ozone air pollution because ozone forms more when temperatures are high and sunlight is strong.

A lot of air-breathers put away their masks in the summer, but if you see me on the streets of Beijing this July, you’ll see me wearing a mask!