Final answer:
A contaminant is a substance that can be harmful when present in the wrong location or concentration, and it may have various health effects.
Step-by-step explanation:
A chemical that causes undesirable effects by being in the wrong place and/or in the wrong concentration is called a contaminant. Contaminants can be harmful to humans or other forms of life when they are released into the environment. They often result from residential, commercial, or industrial sources and may cause various adverse effects on health, including those affecting the nervous system or endocrine system. Toxic heavy metals like mercury and lead are common examples of contaminants that can accumulate in organisms and cause serious health issues.