(Hsu et al/The Atlantic) View full-screen map

By Angel Hsu and Alisa Zomer. This post initially appeared on The Atlantic.

In March, the World Health Organization estimated that air pollution was responsible for 7 million premature deaths in 2012. That’s one out of every eight total deaths in the world. Air quality has gotten worse over the last decade, and for more people.

The 2014 Environmental Performance Index (EPI), a biennial global ranking that compares countries on high-priority environmental issues, shows that over the last decade, the number of people breathing unsafe air has risen by 606 million and now totals 1.78 billion. That’s one quarter of the global population.

But where are people suffering the worst air pollution, and how do we know? To help answer this question, we’ve compiled a map that shows national and city-level exposures to air pollutants that have the greatest effect on human health, fine particulate matter (PM2.5). PM2.5 originates from combustion of fuels from both mobile sources like vehicles and stationary sources, including power plants, industry, and household biomass burning. Although invisible to the human eye, PM2.5 contributes to acute lower respiratory infections and other diseases such as cancer. It can penetrate human lung and blood tissue, leading to higher incidences of cardiovascular and lung disease.

The map overlays two sources of data on air pollution: for cities, the WHO’s recent release of its Ambient (outdoor) air pollution in cities database 2014; for countries, ground-level exposures to PM2.5 are derived from satellite measures in the 2014 EPI. For the first time, both national and city-level data can be compared to see how air pollution is distributed globally. Here are some observations we made using the map.

There are differences in where air pollution is being monitored.

Many countries still don’t have ground-based monitors to measure fine particulate matter (PM2.5). Nearly 70 percent of the air monitoring stations are disproportionately located in wealthy countries. Especially in Africa, Southeast Asia, and South America, there are major gaps in monitoring. These are also areas that are industrializing pretty rapidly. Scientists predict that nearly half of the projected growth in urban areas by 2030 will be in China and India alone, while urban areas in Africa are expected to grow a whopping 590 percent from 2000 levels.

Many of these countries that are industrializing and urbanizing so quickly lack ground monitors and tools to communicate to citizens the dangers of different levels of exposure. India, for example, still lacks any index like the U.S.’s Air Quality Index that communicates daily health warnings of various levels of air pollution to the public.

Poor air quality affects all countries, regardless of wealth.

From the country-level data in the map, it’s easy to see that most countries in Europe have annual averaged exposures to PM2.5 that exceed levels the WHO deems as “safe” (i.e., 10 micrograms/cubic meter). Paris in March of this year had to impose major restrictions on motor vehicles when air pollution hit dangerously high levels. Salt Lake City in the United States regularly experiences severe smog in the winter time, leading to public health and economic concerns among residents there. What this troubling trend demonstrates is that there isn’t an “Environmental Kuznets Curve” necessarily between wealth and air pollution: As countries get richer, we’re not seeing a decrease in air pollution, suggesting its severity as a global problem.

Cities have it worst.

Comparing the range of observed values between the country-level and city-level data, it’s easy to see that air pollution is worse in cities than when averaged across a whole country. New Delhi, the capital of India, for instance, is the city with the worst air pollution. Many other cities in South Asia, including in Pakistan, Iran, and Bangladesh, all suffer pollution levels more than 10 times higher than the safe threshold. These data further corroborate reports made earlier this year that suggested India, and in particular New Delhi, has severe air pollution that is often overlooked in lieu of China’s air pollution problem (see The New York TimesThe Wall Street Journal, and Time).

While India’s air comes out worst among cities, China’s air quality at a national scale is the worst globally. Compared to India, part of this difference in results is due to the fact that China has more people living in cities (more than half) than India (around one-third). More people living in cities where air pollution is worse means more people exposed, which is reflected in the national aggregate average exposure numbers. In May 2014, China announced that it will remove 5 million older, polluting cars from roads, over half just in Beijing, to address air pollution problems, but the question remains as to whether this number is enough. Statistics for China’s vehicle ownership have grown exponentially over the last decade and topped 240 million at the end of 2012.

While this map provides a baseline understanding of air pollution in your city or country, it’s important to keep in mind that these data represent annual averages or longer term trends. Checking local readings of air pollution levels where available, particularly for groups sensitive to air pollution, such as those who suffer from asthma, can help one decide whether to limit outdoor activities on high-pollution days. Others have taken matters into their own hands, including a proliferation of smartphone apps that compile air quality data (like this one). A DIY home air pollution filter movement has also become popular in major cities in China.

Interactive map produced by Pedro Cunha, Dmitry PavlukAnna Young, and Diego Torres Quintanilla.