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 occurs when harmful substances accumulate in the environment to levels that are detrimental to living organisms and the ecosystem. Just having a harmful substance detected (in small amounts) may not be pollution. Similarly, "too much of any substance" - if the substance is not harmful - doesn't cause pollution. And the number of people handling substances (if the substances are not harmful or not accumulated to harmful levels) is not the cause of pollution.

Answer:

harmful substances build to unhealthy levels.