Everyone loves a breath of fresh air. Unfortunately, too often our air is anything but fresh.

While air quality varies dramatically from place to place and day to day, nearly the entire world — about 99% of the global population — is exposed to air at some point that doesn't meet the strict standards set by the World Health Organization. Polluted air, laden with noxious gasses or tiny, invisible particles that burrow into human bodies, kills 7 million people prematurely every year, the U.N. health agency estimates.

And for the millions living in some of the world’s smoggiest cities — many of them in Asia like New Delhi; Dhaka, Bangladesh; Bangkok; and Jakarta, Indonesia — bad air might seem inescapable.

But there are things that people can do, starting with understanding that the air isn’t only polluted when it looks smoggy, said Tanushree Ganguly of the Energy Policy Institute of Chicago in India.

“Blue skies can’t guarantee you clean air,” she said.

What are the most dangerous kinds of air pollutants and their sources?

Air pollutants often come from people burning things: Fuels such as coal, natural gas, diesel and gasoline for electricity and transportation; crops or trees for agricultural purposes; or as a result of wildfires.

Fine, inhalable particles, known as particulate matter, are among the most dangerous. The tiniest of these — known as PM 2.5 because they are less than 2.5 microns in diameter — can get deep into human lungs and are mostly created by burning fuels. Coarser particles, known as PM 10, are linked to agriculture, roadways, mining or the wind blowing eroded dust, according to the WHO.

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

The sources and intensity of air pollution varies in different cities and seasons. For instance, old motorbikes and industrial boilers are major contributors to bad air in Indonesian capital Jakarta while burning of agricultural waste is a major reason for air pollution spikes in cities in Thailand and India. Brick kilns that burn coal adds to pollution in Dhaka, Bangladesh's capital. And seasonal forest fires cause problems in Brazil and North America.

What health problems can air pollution cause?

Air pollution is the second-largest risk factor for early death globally, behind high blood pressure, according to a recent report by the Health Effects Institute.

Short-term exposure can trigger asthma attacks and increase the risk of heart attacks and stroke, especially in the elderly or people with medical problems. Long-term exposure can cause serious heart and lung problems that can lead to death, including heart disease, chronic obstructive pulmonary disease and lung infections.

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

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

What’s the best way to tell if air is safe?

Over 6,000 cities in 117 countries now monitor air quality, and many weather mobile apps include air quality information. But trying to gauge how bad the air is by looking at these numbers can be confusing.

To help people understand air quality levels more easily, many countries have adopted an air quality index or AQI — a numerical scale where larger numbers mean worse air. They are also often assigned different colors to show whether the air is clean or not.

But different countries have different air quality standards. For instance, India’s daily PM 2.5 limit is more than 1.5 times higher than that of Thailand and 4 times higher than WHO standards.

This means that countries calculate AQIs differently and the numbers aren’t comparable. This is also why sometimes AQI scores by private companies using stricter standards may be different from those calculated by national regulators.

What are the best ways to protect yourself from air pollution?

The goal, of course, is to limit exposure when air quality is bad, by staying inside or wearing a mask.

Staying inside however, isn't always possible, especially for people who must live or work outside, noted Danny Djarum, an air quality researcher at the World Resources Institute, an environmental advocacy group. “They can’t really afford not going out,” he said.

Pakaphol Asavakomolnant, an office worker in Bangkok, said that he wears a mask every day and avoids riding to work on a motorbike. “I get a sore throat when I come to work in the morning and I forget to wear a mask,” he said.

People also need to be aware of indoor air pollution which can often be caused by common household activities like cooking or even burning an incense stick.

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

Air purifiers can help reduce indoor air pollution, but they have their limitations. They work by pulling air from a room, pushing it through a filter that traps pollutants before circulating it back.

But they’re are most effective when used in small spaces and when people are nearby. Air purifiers can only clean a certain amount of air, said Rajasekhar Balasubramanian, who studies urban air quality at the National University of Singapore. “If we have a tiny air purifier in a large room, it won’t be effective,” he said.

Air purifiers are also too expensive for people in many developing countries.

“The majority of people who are affected by air pollution can’t really afford air purifiers,” said WRI's Djarum.