More than 90% of the world’s population was exposed to unhealthy air in 2017. That same year, air pollution was the 5th highest mortality risk factor globally.
These estimates use a key measure for outdoor (ambient) air pollution called PM2.5—a particulate matter measuring less than 2.5 micrometers in aerodynamic diameter—which is 28 times thinner than one strand of human hair. These particles are capable of penetrating deep into the respiratory tract and can cause severe health damage. According to the World Health Organization’s (WHO) Air Quality Guideline, long-term exposure to a PM2.5 concentration of more than 10 μg/m3 (micrograms per cubic meter) annually puts an individual at higher risk of cardiopulmonary disease and lung cancer.