A pollutant is the dirty stuff itself — smoke, a chemical, tiny bits of plastic — that makes air, water, or soil unsafe. Car exhaust is a pollutant. Waste from a factory poured into a river is a pollutant. The word names the substance, not the mess it leaves behind: the river is polluted; the chemical in it is the pollutant. Scientists talk about air pollutants and water pollutants, and news uses the word in the same careful way. It is a school-and-news word, not a word for a messy room. Though actually, some pollutants you can see, like black smoke, and some you cannot, like gases that still make the air dangerous to breathe.