A pollutant is any substance that contaminates the air, water, or soil and causes harm to living things or the environment. Pollutants come in many forms: smoke from factories that makes the air hard to breathe, chemicals dumped into rivers that poison fish, or plastic waste that doesn't break down and clutters oceans and landscapes.

Some pollutants are invisible but dangerous, like carbon monoxide from car exhaust. Others you can see and smell, like the smog that hangs over some cities on hot days. Noise can even be a pollutant when it's loud enough to disturb wildlife or harm people's hearing.

What makes something a pollutant is that it causes actual damage to living things or the environment. A little mud in a stream isn't a pollutant, but toxic chemicals from a factory are. Scientists measure pollutant levels to understand how much harm they cause and what needs to be cleaned up or prevented. Understanding pollutants helps us protect the places where we live, the water we drink, and the air we breathe.