【答案解析】
Our environment is polluted by many pollutants . The gases emitted from factories and the exhaust pipes of automobiles pollute the air. Detergents and waste chemicals pollute rivers and other water supplies. Pesticides such as DDT leave poisonous residues in the tissues of organisms. Garbage cast out by people is left to accumulate in unsanitary heaps which attract flies and give off bad smells.
These pollutants endanger public health . Once they have entered the environment, they are hard to get rid of. As people have to breathe, eat and drink them, they directly or indirectly affect bodily functions and cause many infectious diseases.
Since we know where the pollutants come from and their harmfulness, we should take steps to clear away the sources of the pollution and establish a set of regulations to keep our environment free from pollution.