Every year as summer approaches, levels of ozone – the invisible pollutant – tend to rise. In fact, ozone is becoming such a serious pollutant in China that the high levels during summer can make it more dangerous than PM2.5 at times. Based on data analyzed by Smart Air, Beijing’s ozone levels are near peak in June–and nearly double the World Health Organization limit.
Beijing Ozone Levels
Beijing’s ozone is near peak in June
Shanghai Ozone Levelss
Shanghai’s ozone data shows a similar pattern, with ozone peaking at 150% of the limit.
Shanghai’s ozone also peaks in summer at 150% the limit, analysis shows
Where does ozone come from?
Ozone = Exhaust + Sunlight
Ozone levels tend to peak during he summer, because two of the the important ingredients – heat and sunlight – are more common in the summer. When sunlight hits pollutants emitted by automobiles and power plants, it reacts to form ozone.
Ozone peaks in the hottest, sunniest part of the day. Ozone causes coughing, asthma, and long-term damage that increase the chance of heart attacks and cancer.
How Much Is Too Much?
The World Health Organization’s limit is 100μg/m³. Beijing often surpasses that limit, such as this day that reached three times the limit.
Beijing’s ozone reaches three times the WHO limit, July, 11, 2017.
Is Ozone a Problem Indoors?
Most ozone comes from outdoors, not indoors. And fortunately ozone is highly unstable. As it comes into contacts with indoor surfaces, it gets removed from the air.
Studies have found that air conditioned homes with doors and windows closed have indoor ozone levels that are typically less than 10% of outdoor levels. But if you have a central air system that brings in outdoor air, ozone levels will be higher. With the doors and windows open, studies have found indoor ozone is about 50% to 75% of outdoor levels.
Can I Remove Ozone From My Home?
Studies have shown that activated carbon filters can reduce ozone. However, it is harder to remove than particulate. For one, most carbon filters need to be changed more frequently than particle filters.
Activated carbon can reduce ozone indoors
And compared to HEPA filters, most carbon filters are nowhere near as effective at capturing ozone. HEPA filters can remove 99% of particles from a single pass of air. For ozone, researchers estimate the average carbon filter removes about 60% of ozone in a single pass.
How Can I Find Out How Bad the Ozone Is Near Me?
How can we know if ozone is above the limit? Here’s how to find it on two AQI apps popular at Smart Air:
#1 蔚蓝地图 Weilan Ditu
What Should I Do When Ozone Is High?
The United States Environmental Protection Agency recommends:
- Reduce the amount of time spent outdoors.
- Avoid high-intensity sports, such as football and basketball.
- If possible, shift outdoor activities to the morning or evening when ozone concentrations are lower.
Children Particularly At Risk
Children breathe more rapidly and have more lung surface area for their body size compared to adults, which means they take in more air per minute and inhale more air for their size. In fact, pound-for-pound, children breathe 50 percent more air than do adults.
As a result, children inhale a greater percentage of pollution. Ozone can cause breathing difficulties or asthma attacks in children.
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.