Global air quality is deteriorating, posing a risk to public health, in what has become just the latest of a series of warnings from the World Health Organization on the state of the world's skies.

Air quality in most cities fails to meet the organization's health guidelines, according to the new report, and it looks set to increase the risk of developing or worsening respiratory illnesses and other health problems. The WHO's Urban Air Quality database covers 1,600 cities in over 91 countries and shows that health risks from air pollution have prompted more cities to monitor air quality.

But only 12 percent of the world's population lives in cities with acceptable air quality, while half of people living in urban areas are exposed to air pollution that is more than 2.5-times higher than recommended levels. Reasons for a drop in air quality include an over-reliance on cars and fossil fuels. Mexico City, Karachi, and Delhi are cities with some of the highest levels of air pollution. 

The WHO report comes one month after the organization found that air pollution is responsible for the deaths of every one in eight people around the world. This year has already seen extreme air pollution in Asia — and evidence that it's causing storms in the United States — as well as Paris and London, which both fell under clouds of thick smog this spring. While there are many types of air pollution, the WHO says that high concentrations of small and fine particulate pollution is linked to higher mortality rates from heart disease, stroke, and cancer.

A National Climate Assessment report released on Tuesday states that the U.S. is already seeing the effects of climate change, including “extreme precipitation.” It notes that in the Northwest, air pollution from increased wildfires could cause respiratory illnesses to be exacerbated.