Air pollution is the introduction of particulates, biological molecules, or other harmful materials into Earth's atmosphere, causing diseases, death to humans, damage to other living organisms such as animals and food crops, or the natural or built environment. Air pollution may come from anthropogenic or natural sources.

The NowCast is a weighted average of hourly air monitoring data used by the United States Environmental Protection Agency for real-time reporting of the Air Quality Index (AQI) for PM (PM₁₀ or PM_2.5) data.