May 19, 2017 07:59 PM

Caixin Explains: Five Things You Need to Know About Ozone Pollution

(Beijing) – A nasty bout of ozone that blanketed northern China on Thursday and Friday was the latest air-quality woe to hit millions of residents and bedevil government officials. Here are five things you need to know about this less-talked-about but hazardous pollutant that often chokes parts of China, usually between April and October.

What is ozone?

Ozone, or trioxygen, is a pale blue gas with a distinctive pungent smell. Inhaling very small concentrations of ozone can leave you feeling refreshed, similar to breathing the air after a heavy thunderstorm. But a higher concentration of ozone has a sharp odor similar of chlorine.

Ozone occurs both in the Earth’s upper atmosphere and at ground level and it can be good or bad, depending on where it is found. The good variety, known as stratospheric ozone, occurs naturally in the upper atmosphere, where it forms a protective layer that shields us from the sun’s harmful ultraviolet rays. It should not be confused with ground-level ozone, which is a hazardous air pollutant.

How is ground level ozone formed?

Ground-level ozone is formed from chemical reactions between nitrogen oxides and volatile organic compounds under high temperatures and strong sunlight. Emissions from automobiles, power plants, burning coal, petroleum refineries and chemical plants are the main source of these two substances. And remember, ground-level ozone is a layman’s term for tropospheric ozone, which extends from the Earth’s surface to 12 to 20 kilometers above sea level.

How does ozone pollution affect your health?

Ground-level ozone is a major substance in photochemical smog, which reduces daytime visibility and causes respiratory problems. It can also cause headaches, irritate the throat, and leave you with a burning sensation in the eyes.

Several hours of exposure to ozone levels exceeding 240 micrograms per cubic meter, which is nearly 2.5 times the “guideline level” recommended by the World Health Organization (WHO), will lead to a “significant reduction in lung functioning,” inflammation of airways and a rise in the number of deaths that can be attributed to ozone pollution.

The WHO recommends an ozone concentration of 100 micrograms per cubic meter of air as a safe guideline level, where healthy members of the public will not feel any adverse effects. China has set its national standard at 160 micrograms per cubic meter of air.

Ozone can also damage crops by stunting their growth and making leaves turn yellow or wither prematurely.

Why is ozone pollution getting worse in China?

Ozone pollution in China has gotten worse in recent years. In Beijing, for example, the annual average rose from 183.4 micrograms per cubic meter of air in 2013 to 197.2 in 2014 and increased to 226 in 2015. As the forming of ozone is closely related to industrial and urban emissions, which have been on the rise in recent years, ozone has become a major pollutant contributing to the bad air quality in parts of the country.

The government has realized the seriousness of the issue, and taken serious steps to cut industrial emissions, assist traditional polluters to upgrade their technology and control the number of cars allowed on streets in major cities. We have to wait and see how effective these measures are at curbing air pollution.

How can you protect yourself on days with severe ozone pollution?

An average face mask can help you reduce the amount of fine particulate matter like PM2.5 that you inhale on a smoggy day, but it cannot protect you from ozone. Usually it will take a mask with a filter that includes a layer of activated carbon that can absorb the harmful gases.

A better way to protect yourself is to keep an eye on the weather forecast and air pollution readings, and refrain from outdoor activities on smoggy days.

Contact reporter Wu Gang (

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