In order to get the greatest yield from the crops he plants, a farmer will often use some kind of chemical to kill weeds or insects in his field. This is often a necessary action, but unfortunately the chemical don't stay just on the field where they were sprayed. When a hard rain comes, the ground in the field may run off into a nearby water supply. It was very effective for the farmer, but researchers found that when the DDT got into the water and the food chain, it was deadly. When the use of this chemical was banned, environmentalists saw an increase in certain species of animals that had almost been wiped out because of this dangerous agricultural chemical.
Factories also introduce dangerous chemicals into our environment.
This means there are things not used in the final product, but necessary to make the product. This waste cannot be used again. Many years ago our water supplies were seriously contaminated because these waste products were dumped into the water. There were also certain kinds of waste that were emitted into the air from exhaust pipes on cars, trucks, planes, buses and from factories. When these waste products are found to be harmful to humans and other animals, they are called toxic waste.