Take a quick look at environment pollution. 

Environmental pollution is the effect of undesirable changes in our surroundings that have harmful effects on plants, animals and human beings.

A substance, which causes pollution, is known as pollutant. Pollutants can be solid, liquid or gaseous substances present in greater concentration than in natural abundance and are produced due to human activities or due to natural happenings.

Some pollutants can be degredable, like discarded vegetables. But some pollutants which are slowly degradable remain in the environment in an unchanged form for many decades. For examples, plastic materials, heavy metals, nuclear wastes, $DDT$ and other many chemicals. These types of substanes once released into the environment are difficult ot remove. These pollutants cannot be degraded by natural processes. So they are harmful to living organisms. In the process of environmental pollution, pollutants originate from a source and get transported by air or water or are dumped into the soil by human beings.