a contaminant is a substance present in nature at a level higher than typical levels or that would not otherwise be there. this may be due to human activity. the term contaminant is often used interchangeably with "pollutant", which is a substance that hurts the surrounding environment. while a contaminant is sometimes defined as a substance present in the environment as a result of human activity, but without harmful effects, it is sometimes the case that toxic or harmful effects from contamination only become apparent at a later date.