Many national governments — though by no means all — measure air pollution and set official standards, and they often use different scales and breakpoints to determine what’s healthy to breathe and what’s not. In communicating that information to the public, one common approach is to create an index combining measurements for some of the most worrying pollutants, including those mentioned above. Some indexes aggregate and average these pollutants over a specific time period — say 24 hours, or a year — to produce an overall air quality “score.” Others indicate a score based on the riskiest pollutant at a given time — and very often that is PM2.5. Many indexes use colors to make it easier for citizens to understand the overall quality of outside air, and to issue advisories when it is considered unsafe. In the chart below, the upper band shows a commonly used scoring system for air quality. A score below 50 is considered good — and green. Scores above 400 are downright dangerous.
The bottom scale shows the U.S. Environmental Protection Agency’s current benchmarks specifically for PM2.5, which is measured in micrograms per cubic meter of air — sometimes rendered as µg/m³. The higher the mass of fine particulates in the air, the more dangerous it is to breathe. The very narrow range for what’s acceptable here reflects PM2.5’s potency as a killer.
The 2018 “State of Global Air” report — a collaboration between the research nonprofit Health Effects Institute in Boston and the Institute for Health Metrics and Evaluation at the University of Washington in Seattle — makes clear the impacts of fine particulate pollution. “PM2.5 was responsible for a substantially larger number of attributable deaths than other more well-known risk factors (such as alcohol use … or high sodium intake),” the report noted, “and for an equivalent number of attributable deaths as high cholesterol and high body mass index.”
An ancillary problem, particularly in the developing world, is indoor air pollution arising from the burning of wood, coal, and other fuels for heat and cooking. As yet, there is no networked system for measuring PM2.5 inside households, but the Boston and Seattle researchers note that “numerous individual studies indicate that household air pollution concentrations often far exceed those observed in ambient air.”
In many places, winters are particularly bad for air quality, as warmer air in the upper atmosphere traps ambient pollution near the earth’s surface. Above, a mix of desert dust and human-caused pollution can often be seen hanging over large sections of the Middle East and North Africa, and a noxious band of air — tied closely to industry and car exhaust — is often visible stretching across great swaths of India, China, and parts of Southeast Asia. Meanwhile, clouds of polluted air can often be spotted lingering over parts of the American West Coast — even outside of wildfire season — as well as South America, and Europe.