The study is an analysis of what the WHO says is the world's most comprehensive database on ambient air pollution. The organization collected the data from more than 4,300 cities and 108 countries.
"I'm afraid what is dramatic is that air pollution levels still remain at dangerously high levels in many parts of the world," Dr. Maria Neira, director of the WHO's Department of Public Health, Environmental and Social Determinants of Health, said of the study published Tuesday. "No doubt that air pollution represents today not only the biggest environmental risk for health, but I will clearly say that this is a major, major challenge for public health at the moment and probably one of the biggest ones we are contemplating."
, a mix of solid and liquid droplets in the air, can get sucked into and embedded deep in your lungs when you breathe. That can lead to health conditions including asthma, lung cancer, heart disease
and chronic obstructive pulmonary disorder, or COPD. These outdoor particulates -- including sulfate, nitrates and black carbon -- are largely
created by car and truck traffic, manufacturing, power plants and farming. In total, air pollution caused about 4.2 million deaths in 2016, according to the WHO.
"Many of the world's megacities exceed WHO's guideline levels for air quality by more than five times, representing a major risk to people's health," Neira said. This is "a very dramatic problem that we are facing now."
People in Asia and Africa face the biggest problems. More than 90% of air pollution
-related deaths happen there, but cities in the Americas, Europe and the Eastern Mediterranean also have air pollution levels that are beyond what the WHO considers healthy.
Because the data are collected from various sources, it is difficult to rank cities. However, the new WHO data show that US cities on the more polluted side of the list include Los Angeles, Bakersfield and Fresno, California; Indianapolis; and the Elkhart-Goshen area of Indiana. Also on the list: Gary, Indiana; Mira Loma, Calexico and Napa, California; Louisville, Kentucky; and St. Louis.
But that can't compare to cities like Peshawar and Rawalpindi in Pakistan, which have some of the highest particulate air pollution levels in the database. Varanasi and Kanpur in India; Cairo; and Al Jubail, Saudi Arabia, also show higher levels.
If you want cleaner air, try somewhere like Wenden, Arizona (population 2,882), or Cheyenne, Wyoming (population 64,019). The Eureka-Arcata-Fortuna area of California; Battlement Mesa, Colorado; Wasilla, Alaska; Gillette, Wyoming; and Kapaa, Hawaii, are all on the cleaner-air list.
One of the bigger US cities with cleaner air is Honolulu, according to the WHO data.
The other large source of air pollution, a problem mostly in developing regions, is in people's homes
. More than 40% of the world's population does not have access to clean cooking technology
or lighting, the WHO says. Families use wood, dung or charcoal in cook stoves or open fires to make meals and heat their homes, creating airborne particulates indoors. Technological improvements haven't kept up with population growth, the WHO said, resulting in about 3.8 million deaths from household pollution alone in 2016. Women and children share a disproportionate part of the burden with greater exposure to this indoor pollution.
The good news is that many cities are monitoring air pollution, Neira said. And good data can inform political leaders to help them clean up the air.
There are also things you can do at a local level to reduce air pollution. Experts suggest replacing driving with walking, biking or taking public transportation. To protect yourself, stay inside when air pollution levels are high, especially if you have heart problems or are older. Installing filtration equipment in your home's ventilation system can reduce exposure.
The new study is "generally an impressive piece of work and demonstrated clearly the huge global impact of air pollution," said Kevin McConway, an emeritus professor of applied statistics at the Open University. "While we do still need to continue to take action on air pollution in richer Western cities like London, the position is far worse in lower- and middle-income countries and in many other parts of the world."
Dr. Anthony Frew, who specializes in allergy and respiratory medicine at Royal Sussex County Hospital, agrees but wants Westerners to be mindful that while they breathe relatively cleaner air, their lifestyle is a burden to the environment.
"This report is a timely reminder that we in the West need to remember that we are lucky to live where we do, but our prosperity is built, in part, on polluting industries elsewhere in the world, which impact on other people's health."