Pollutant (noun)

1. A substance that pollutes air, water, or soil, especially a toxic chemical substance.
2. A substance or agent that makes something impure or unclean.

Origin:

Late 19th century: from french polluant, present participle of polluer "pollute".

Examples:

1. Carbon monoxide is a common air pollutant.
2. Industrial waste is the main pollutant of the river.
3. The pollutant caused a fish kill in the river.
4. The pollutant was identified as the cause of the water contamination.
5. The pollutant was banned by the government to protect the environment.