I think the answer is a pollutant. but im nor quit sure. the question is. A substance that enters Earth's cycles and does harm to living things is?

A substance that enters Earth's cycles and does harm to living things is called a pollutant. It is a substance that makes land, water, air unsuitable and unsafe for use.

There are three factors that determine how severe a pollutant is namely the chemical nature of the pollutant, its concentration and its persistence.

Some pollutants are biodegradable and will thus not persist long term in the environment. Air pollution, water pollution, littering and noise pollution make up the major ways through which the environment is polluted.