The World Health Organization (WHO) announced Thursday that outdoor air pollution is now classified as “carcinogenic to humans.”

The designation comes after the International Agency for Research on Cancer (IARC) performed an extensive review of existing studies and found sufficient evidence to say with certainty that air pollution causes lung cancer and increases the risk of bladder cancer.

Particulate matter—the mixture of small particles, including acids, metals, and soil, found in air pollution—is also considered carcinogenic, the WHO announced.

“The air we breathe has become polluted with a mixture of cancer-causing substances. We now know that outdoor air pollution is not only a major risk to health in general, but also a leading environmental cause of cancer deaths,” Dr. Kurt Straif, Head of the IARC Monographs Section, said in a statement.

The IARC’s Monographs Section is the “encyclopedia of carcinogens” and has long worked to determine what's in air pollution, from dust to fossil fuel exhaust.

“Our task was to evaluate the air everyone breathes rather than focus on specific air pollutants,” Deputy Head of Monographs Dr. Dana Loomis said. “The results from the reviewed studies point in the same direction: the risk of developing lung cancer is significantly increased in people exposed to air pollution.”

Researchers say air pollution is predominantly caused by transportation, stationary power generation, industrial and agricultural emissions, residential heating and cooking, as well as some natural sources.

While experts have identified other ills of air pollution, including an increased risk of asthma and heart disease, this is the first global declaration that air pollution is a primary cause of some cancers. The WHO estimates that air pollution causes 1.34 million premature deaths each year.

Earlier this year, University of California, Los Angeles researchers found that a child’s exposure to air pollution in the womb and the first year of life increases his or her risk of developing certain rare cancers.

Read More: Air Pollution Raises Risk of Rare Childhood Cancers »

Air Pollution in the U.S. and Worldwide

The five worst cities for air pollution, according to the WHO, are:

  1. Ahwaz, Iran
  2. Ulan Bator, Mongolia
  3. Sanadaj, Iran
  4. Ludhiana, India
  5. Quetta, Pakistan

According to the American Lung Association, the areas in the U.S. with the most polluted air, based on measures of year-round particle pollution, are:

  1. Bakersfield-Delano, Calif.
  2. Merced, Calif.
  3. Fresno-Madera, Calif.
  4. Hanford-Corcoran, Calif. & the greater Los Angeles area

While areas of California have the highest levels of particle matter, other areas of the state, such as the San Francisco Bay Area, have “Spare the Air” days when pollution levels are high. On those days, commuters are encouraged to use public transportation or bike to reduce air pollution.

How to Reduce the Effects of Air Pollution

Protecting against the effects of air pollution is difficult on an individual basis. It requires action by public authorities at the national, regional, and international levels, the WHO says.

Those living in heavily polluted areas can wear dust masks while outside, or avoid going outside on days of heavy pollution. Rural areas tend to have less air pollution, so living outside of densely populated cities may reduce your risk of harm from air pollution.

However, indoor air pollution—which accounts for about two million premature deaths every year, mostly in developing countries—is controllable. Using air filters and certain air-purifying plants can help reduce levels of air pollution in your home.