In many areas of the world, taking a deep breath of air can be unhealthy and dangerous.

The WHO estimates that air pollution is responsible for about 6.5 million premature -- or early -- deaths every year. It estimates that nearly 600,000 of those who die are children under the age of five.

Most of these deaths, WHO officials say, happen in developing countries.

With all this pollution in the air, many people try to protect themselves and reduce their exposure. So, some people use face masks. You may have seen pictures of people wearing such masks in big cities like Beijing and Tokyo.

But do they work?

One expert tells VOA Learning English that some work much better than others.

"We see lots of air pollution that just passes right through these cloth materials. And for us, it's an important public health issue because people worldwide are choosing to use these kinds of masks to reduce their exposure."

Richard Peltier is a professor at the University of Massachusetts, Amherst, in the Department of Public Health and Health Sciences.

He studies the effects of exposure to air pollution.

Recently, he was the lead author of a study on how well inexpensive face masks work.

Peltier led a team that tested several types of inexpensive face masks they bought on the streets of Kathmandu, Nepal. These masks are commonly used in other polluted parts of the world.

The N95 is generally considered to be the gold standard, or the best. The U.S. government rates the N95 face mask for people who might work around pollutants. So, air quality experts like Peltier know exactly what it can do and what it cannot do.

Peltier calls the N95 mask a more advanced version of the inexpensive cloth masks that he studied. The researchers used the N95 mask in their study for comparison.

However, Peltier says, N95 masks are either not available or too expensive in many places where pollution is severe. Also, he adds, it is made of paper. So, it can't be washed and reused like cloth face masks.

"But what we find in many parts of the world are the unavailability of N95 masks. You can't buy these. Or if they are available, they are too expensive. So, people chose to substitute the N95 masks for inexpensive pieces of cloth."

The face masks that Peltier and his team studied can be bought on city streets in places like China, Nepal and India. He explains that these face masks have some effect -- but not enough.

"So, the masks that we studied do have some effect. It's not a complete effect and it's nowhere near comparable to an N95 mask."

The researchers found that the most popular type of mask -- an inexpensive cloth rectangle that you can wash -- provided little protection against the smallest particles. These particles of pollution are less than 2.5 micrometers and can penetrate deep into the lungs.

He adds that there are other face masks on the market that meet the expectation of an N95 mask. But most of these masks are too expensive. So, they are not an option for people in the developing world.

"But there are other technologies out there -- commercial products – that do sort of meet the expectation of an N95 mask. They do work very, very well. However, one of the issues that we find, is that a lot of these masks are too expensive. They're not financially attainable by many people in the developing world."

Peltier says if you have a job that pays you $25 a week, you cannot afford a $25-$35 face mask.

So, among the cheaper cloth face masks -- is there a difference in quality?

Peltier says yes. He explains that cloth masks with exhalation valves performed better in testing.

However, it is not the exhalation valve itself that makes the mask more effective. Face masks with these valves are usually thicker. The thicker material is what makes the mask better at keeping out pollutants.

"The cloth masks that had exhalation valves were a little thicker in material. There was more material to the masks itself. And we think that's actually what causes the pollution to be captured by the mask and reducing the exposure. It's not the exhalation valve itself. That valve is merely for a user's comfort. We think that the feature of the mask that makes it most useful is how thick the material is. How much material is between the air pollution and you."

Also, surgical masks made out of paper performed surprisingly well. In an interview with the New York Times, Peltier explained that these paper masks didn't fit as snugly to the face. But rather they stuck against the wearer's wet mouth.

Peltier and other air pollution and health experts agree on one important thing: No matter how much you pay for a mask, they only work if they fit snugly on your face.

But, if they are made out of cloth, too tight is not good either. During his study, Peltier found that if a person pulled their cloth mask too tightly, it lost the ability to filter anything.

You might think that wearing any type of face mask is better than wearing nothing at all.

"Wearing a face mask – an inexpensive, non-working face mask made out of cloth – does give a false sense of security because it makes you feel like you're being protected when in fact you're not. You feel like you're doing something good for your body to protect yourself from air pollution. But in reality it's not doing anything at all or it's doing very little."

Peltier adds that this false sense of security might prevent you from taking other measures to protect yourself, such as avoiding highly polluted areas or avoiding outside exercise during times of very high pollution.

Health officials around the world warn that air pollution is only getting worse. And not just in poorer countries.

"Air pollution is a global problem and it is growing in magnitude across the planet. And individuals can make informed decisions about reducing their exposure to air pollution, whether it's through wearing appropriate masks or avoiding an exposure that will, in fact, have health benefits to that individual."

And that's the Health & Lifestyle report.

I'm Anna Matteo.