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Air quality differs from place to place and day to day. But about 99 percent of the world’s population breathes air at some point that does not meet pollution guidelines set by the World Health Organization (WHO).

Different forms of air pollution kill about 7 million people each year, the U.N. health agency estimates. But health experts say there are some actions individuals can take to protect themselves, even in the world’s most polluted cities.

The first step is to start with the understanding that air is not only polluted when it looks smoky or smoggy, said Tanushree Ganguly. She is an air quality expert with the Energy Policy Institute of Chicago’s India program.

“Blue skies can’t guarantee you clean air,” Ganguly told The Associated Press.

Pollutants¹ and their sources

Air pollutants often come from people burning things. Examples include fuels as well as clearing operations involving crops or trees. Pollutants can also be released from wildfires.

Among the most dangerous particles we can breathe in are called particulate² matter. The smallest of these are known as PM 2.5. This is because they are less than 2.5 microns in diameter. These particles can travel deep inside human lungs and are mostly created by burning fuels. Larger particles, known as PM 10, are linked to agriculture, roadways, mining or wind-blown dust.

Other dangerous pollutants include gases like nitrogen dioxide or sulfur³ dioxide, which are also produced from burning fuels. This information comes from Anumita Roychowdhury, an air pollution expert at the Center for Science and Environment in New Delhi.

Health effects

The American-based Health Effects Institute says air pollution is the second-largest risk factor for early death worldwide, behind high blood pressure.

Short-term exposure can cause asthma⁴ attacks and increase the risk of heart attacks and stroke, especially in older people and those with existing medical problems. Long-term exposure can cause serious heart and lung problems that can lead to death.

A recent study by the U.N. children’s agency UNICEF found that more than 500 million children in East Asia and Pacific countries breathe unhealthy air. The pollution is linked to the deaths of 100 children under age 5 every day. June Kunugi, UNICEF’s Regional Director for East Asia, said the polluted air limits growth, harms lungs and affects thinking abilities.

“Every breath matters, but for too many children every breath can bring harm,” she said.

How to tell if air is safe?

Over 6,000 cities in 117 countries now record and follow air quality levels. Many mobile apps include air quality information as well. But it can be difficult to know how bad the air is by looking at these numbers.

To help people better understand air quality levels, many countries have established an air quality index, or AQI. Larger numbers on this index mean worse air.

Different countries have different air quality standards. For example, India’s daily PM 2.5 limit is more than 1.5 times higher than in Thailand and 4 times higher than WHO standards.

How can you protect yourself?

For individuals, experts say the goal should be to limit exposure when air quality is bad, by staying indoors or wearing a mask. This is not always possible, though. Danny Djarum is with the World Resources Institute. He told the AP there are many people who live or work outside and “can’t really afford” to stay indoors.

People also need to watch indoor air pollution that can be caused by common household activities like cooking or burning incense⁵.

What are the benefits — and limitations — of air purifiers?

While air purifiers can help reduce indoor air pollution, experts say they do have limitations. The devices work by pulling air from a room and then pushing it through a filter that traps pollutants. The filtered air is then sent back into the room.

Air purifiers work best when used in small spaces and when people are nearby. This is because they can only clean a certain amount of air, said Rajasekhar Balasubramanian of the National University of Singapore. He added, “If we have a tiny air purifier in a large room, it won’t be effective.”

Words in This Story

smog - n. a kind of air pollution that can consist of smoke, gases, chemicals, etc.

factor - n. something that contributes to the production of a result

expose – v. to subject to risk from a harmful condition

asthma – n. a kind of a chronic⁶ lung disorder⁷

standard – n. something established by authority as a rule for the measure of quality

afford – v. to be able to pay for

incense – n. a kind of material used to produce a pleasant smell when burned

filter – n. to pass a liquid or gas through a piece of equipment in order to remove solid pieces or other substances