PM 2.5 starts to become a major health problem when there is more than 35.5 micrograms (µg) of PM 2.5 per cubic meter of air, according to the US Environmental Protection Agency. But the WHO recommends keeping yearly average PM 2.5 levels three times lower than that. The most harmful pollutant to human health is called PM 2.5, particulate matter smaller than 2.5 microns in diameter that’s found in soot, smoke, and dust. PM 2.5 is especially dangerous because it can get lodged in the lungs and cause long-term health problems like asthma and chronic lung disease.

Key trends from 2008-2013:

• Global urban air pollution levels increased by 8%, despite improvements in some regions.
• In general, urban air pollution levels were lowest in high-income countries, with lower levels most prevalent in Europe, the Americas, and the Western Pacific Region.
• The highest urban air pollution levels were experienced in low-and middle-income countries in WHO’s Eastern Mediterranean and South-East Asia Regions, with annual mean levels often exceeding 5-10 times WHO limits, followed by low-income cities in the Western Pacific Region.
• In the Eastern Mediterranean and South-East Asia Regions and low-income countries in the Western Pacific Region, levels of urban air pollution has increased by more than 5% in more than two-thirds of the cities.
• In the African Region urban air pollution data remains very sparse, however available data revealed particulate matter (PM) levels above the median. The database now contains PM measurements for more than twice as many cities than previous versions.

Here’s a map of the annual median concentration of PM 2.5 — green areas are within the levels that’s considered healthy, according to the WHO’s standards.

Source: WHO, http://www.who.int/phe/health_topics/outdoorair/databases/cities/en/

The WHO links 3 million deaths a year to air pollution. Most of those deaths occur in low- and middle-income countries.