Question

pollution occurs when
there is too much of any substance.
a harmful substance is detected.
harmful substances build to unhealthy levels.
too many people handle the substances.

Explanation:

Pollution is defined as the presence or introduction into the environment of substances or factors that cause harm or discomfort. It is not just about the presence of a harmful substance (a single detection may not be pollution if at low levels) or any substance in large quantity (many substances are harmless in nature even in large amounts like oxygen in normal cases). Also, the number of people handling substances is not the core definition. Pollution occurs when harmful substances accumulate to levels that are unhealthy for the environment, living organisms etc.

Answer:

harmful substances build to unhealthy levels.