Pollution.

Pollution has gotten worse over the years. If you look up pollution in the dictionary, you will find the addition of any substance or form of energy (e.g., heat, sound, radioactivity) to the environment at a rate faster than the environment can accommodate it by dispersion, breakdown, recycling, or storage in some harmless form. Many things are considered pollution that you would not think to be. For example, carbon dioxide is found in all animal tissues, but in a concentrated form, it can also kill them. Pollution is the result of mankind congregating in one place and remaining there for a long time. Pollution was never a problem as long as there was enough space avaliable for every person or group of people. When ares start to get densely populated, various things began polluting the city or area they lived in. A big contributer to the pollution of heavily populated areas is car exhaust. A lot of people means a lot of cars, and every single one of those cars releases exhaust, which heavily pollutes the air. There are other types of pollution like sewage, cigarette smoke, and radioactivity.