pollutants

Pollutants are substances suspended or mixed into the air that aren't a natural, integral part of it — they get added from outside sources like dust, smoke, and vehicle or industrial emissions.

• Common air pollutants include particulate matter (tiny dust and soot particles) and gases such as carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide.
• You can actually catch some pollutants in action — leave a clean black sheet of paper near an open window for a few hours, and fine dust particles will visibly settle onto it.
• Because pollutants vary so much by place and time, scientists use a single measure called the Air Quality Index (AQI) to summarise how clean or polluted the air is at a given moment.
• Unlike the gases that make up "real" air, pollutants don't belong there permanently — reducing emissions and dust at the source is what brings their levels back down.