I just analyzed Bangkok’s PM2.5 data (), and I found it varies systematically by season and time of day.
Bangkok Average PM2.5
First, some basics. In 2018,. That puts Bangkok just under three times the .
Bangkok’s Air Pollution: The Bad News
But that average obscures more serious pollution in the winter. For example, January 2018 reached PM2.5 levels that were more than double Bangkok’s average—and nearly 6 times the WHO annual limit.
Reasons Why Bangkok’s Pollution is Worse in Winter
1. In the summer, air mixes to higher up in the atmosphere, so pollution is less concentrated. But in the winter, the air stays closer to the earth, so we stew in our own waste.
2. Winter is more likely to bring “inversions.” Inversions happen when a layer of , underneath a layer of warmer air. This keeps our pollution near us, preventing it from scattering away.
3. In Thailand, the winter also brings .
Bangkok PM2.5 Tends to Be Worse at Night
I also analyzed the data to see what time of day PM2.5 pollution tends to be the worst in Bangkok. Here’s what I found.
Bangkok air pollution was worse at night and best in the late afternoon. There’s also a spike around morning rush hour.
Now, these aren’t huge differences. From top to bottom, it’s a difference of about 4 micrograms. However, studies have found significant health effects with, and adjusting around these patterns can be an easy way to reduce exposure.
Bangkok’s trends are similar to the daily PM2.5. From Asia to Europe to America, nighttime PM2.5 is the worst, while late afternoon air tends to be the best.
Also, wind tends to pick up around midday and (the brown line below).
Bottom line: Climate factors help make Bangkok’s PM2.5 pollution worse at night and in the winter on average.
Here are a few simple (inexpensive)when I’m in Delhi and Beijing.
Breathe safe, Bangkok!
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.