A major new study finds that 95 percent of the world’s population breathes dangerous air. The report by the Health Effects Institute reinforces data from the World Health Organization, which says that air pollution in many of the world’s largest cities is killing millions of people. In 2016, the U.N. reported that 3.3 million premature deaths occur every year due to unsafe air.

In the U.S., more than two of every five Americans live in areas with unhealthy levels of smog and air pollution. According to Janice Nolen of the American Lung Association, climate change and global warming were big contributing factors. Ozone levels spiked in 2016, which was the nation’s second hottest year on record.

The American Lung Association says eight of America’s 10 most polluted cities are in California.

Nolen worries about renewed efforts by the Trump administration to undermine the Clean Air Act. The White House announced in January that it plans to throw out many air emissions policies that had been opposed by fossil fuel companies. The Environmental Protection Agency said it planned to withdraw its “once in, always in” policy, which determines how to regulate major sources of hazardous air pollutants. The administration also plans to repeal regulations that reduce carbon pollution from power plants and remove limits on emission from oil and gas operations.

The Guardian reports:

Cities are home to an increasing majority of the world’s people, exposing billions to unsafe air, particularly in developing countries, but in rural areas the risk of indoor air pollution is often caused by burning solid fuels. One in three people worldwide faces the double whammy of unsafe air both indoors and out. The report by the Health Effects Institute used new findings such as satellite data and better monitoring to estimate the numbers of people exposed to air polluted above the levels deemed safe by the World Health Organisation. This exposure has made air pollution the fourth highest cause of death globally, after high blood pressure, diet and smoking, and the greatest environmental health risk. Experts estimate that exposure to air pollution contributed to more than 6 [million] deaths worldwide last year, playing a role in increasing the risk of stroke, heart attack, lung cancer and chronic lung disease. China and India accounted for more than half of the death toll. Burning solid fuel such as coal or biomass in their homes for cooking or heating exposed 2.6 billion people to indoor air pollution in 2016, the report found. Indoor air pollution can also affect air quality in the surrounding area, with this effect contributing to one in four pollution deaths in India and nearly one in five in China.

Some positives revealed in the report by the Health Effects Institute, a nonprofit research organization, are the aggressive moves made by China and India to cut the use of coal and ramp up controls on indoor air pollution. In the 1990s, the number of people exposed to indoor air pollution was estimated at 3.6 billion; today that figure stands at 2.4 billion.