That’s according to a new comprehensive study from the Health Effects Institute and the Institute for Health Metrics and Evaluation, with expert input from the University of British Columbia. The Institute used satellite data and improved monitoring tools to conduct their research, and the results are bleak. Not only is the vast majority of the world’s population breathing unsafe air, but also the gap between wealthy countries and poor countries are increasing. There is now an 11-fold gap between the most- and least-polluted areas compared to a six-fold gap in 1990, according to Bob O’Keefe, the vice president of the Institute.

Exposure to pollution is associated with the increasing population of cities, but people who live in rural areas are also at risk. Burning solid fuels indoors makes the air inside a home dangerous to breathe. Globally, one in three people face unsafe air both indoors and outside. Air pollution is now the fourth leading cause of death worldwide, weighing in just below smoking, and the top environmental health risk. Pollution contributed to an estimated 6 million deaths in 2017.

Still, O’Keefe cited some cause for optimism. The number of people worldwide exposed to solid fuels indoors has fallen by over a billion since 1990, despite an increase in population, and China and India—two of the worst offenders when it comes to pollution—are improving their pollution controls.

But developing countries aren’t the only cause for concern. Between 2010 and 2016 there was an uptick in U.S. deaths where pollution was a contributing factor. Pollution-related deaths had previously been on a downward trend since 1990. In London, studies have shown that every area of the capital has pollution levels higher than global standards recommend. Cleaning up London’s air has become one of Mayor Sadiq Khan’s major initiatives.