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Particulate matter less than or equal to 2.5 µm (micrometers) in aerodynamic diameter. PM2.5 is measured in units of micrograms per cubic meter (µg/m3).

Population-weighted annual average

Instead of calculating average air pollution levels where all areas receive equal weight, as is typically done, population-weighted averages give weight to the areas in proportion to their population, so that greater weight is given to exposures in areas where the most people live.


Any chemical that contributes to the formation of another compound or species (e.g. ozone or PM2.5). For example, nitrogen oxides (NOx), sulfur oxides (SOx), volatile organic compounds (VOCs), and ammonia (NH3) are all precursors for the formation of PM2.5 in the atmosphere.

Premature, or early, mortality

Dying earlier than expected when compared to a full life expectancy. It may be characterized in numbers of deaths or as years of life lost.