Ozone Pollution: Often Invisible, Always Dangerous

Thomas Talhelm
June 11, 2018

June 11, 2018
Thomas Talhelm

When summer rolls around, enticing blue skies often draw people outside. Throw on top the fact that most cities in China have less PM2.5 pollution in the summer, what’s stopping us from heading outside?

Ozone Pollution on Rise Across China

Ozone has been on a steady increase across China in recent years. To assess China’s ozone situation, Smart Air analyzed 2019 ozone levels for 12 of China’s major cities. We found that during summer months (May-August) in 2019, 83% of the cities analyzed had ozone levels exceeding the WHO standard. Only one city was below the WHO limit: Shenzhen.

12 china's major cities ozone level summer vs. winter

Averaging across the entire year, 25% of cities exceeded the WHO limit (Tianjin, Wuhan, Guangzhou). However, three other cities were not far off the limit (Beijing, Changsha, and Shanghai).

China’s ozone performance is still far better than its particulate pollution performance. For PM2.5, 100% of cities failed to meet the WHO annual limit. Thus, is ozone really such a big risk? Turns out, ozone turns up just at the worst time.

How Does Ozone Pollution Vary Throughout The Year?

Using Beijing as an example, ozone is highest from May to August. Ozone reaches almost two times the WHO limit of 100µgm/m³ in June.

Shanghai shows the same monthly pattern, although it’s less pronounced. Shanghai’s ozone reaches 30% above the WHO limit from May to July.

Where Does Ozone Pollution Come From?

Ozone Pollution = Exhaust + Sunlight

Ozone tends to peak during the summer because two important ingredients – heat and sunlight – are more common in the summer. When sunlight hits pollutants from cars, other vehicles, and power plants, it reacts to form ozone.

Ozone typically peaks in the hottest, sunniest part of the day. In most places, that’s around noon or early afternoon. For example, in Rio de Janeiro, Brazil, ozone peaks at 2:30pm.

Ozone pollution daily hourly pattern worst

Ozone Turns up Just at the Worst Time

The health effects of ozone can be pretty serious. Ozone causes coughing, asthma, and long-term damage that increases the chance of heart attacks and cancer. What’s more, on sunny and warm days, we often spend more time outside, which means we’re more exposed to more ozone in the air.

Health Effects of Ozone: Children More 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. Pound-for-pound, children breathe 50% more air than adults.

Children are also more likely to have asthma. Thus, ozone harms children even more than adults.

What Are The Official Ozone Pollution Limits?

The World Health Organization’s limit for ozone is 100μg/m³. Beijing’s ozone often surpasses that limit, such as the day before this article was written, when levels reached 1.4 times the WHO ozone limit.

Two apps give easy access to local ozone data. The AirVisual App and Blue (Weilan) Map App both report ozone readings.

Is Ozone a Problem Indoors?

Since the source of most ozone is from vehicles and power plants, most ozone comes from outdoors, not indoors. What’s more, fortunately, ozone is highly unstable. When ozone comes into contact with indoor surfaces, it gets removed from the air.

Indoor ozone pollution not worse than outdoor

In homes that have air conditioning and windows closed, studies have found that ozone is typically less than 10% of outdoor ozone levels. But if you have a central air system that brings in outdoor air, ozone levels will be higher. And if 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. 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.

DIY activated carbon filters can remove ozone pollution

Another challenge is that most carbon filters need to be changed more frequently than particle filters.

What Should I Do When Ozone Is High?

The United States Environmental Protection Agency recommends that people:

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.

Breathe safe!

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