Population growth is the change in population over time and can be quantified as the change in the number of individuals in a population using “per unit time” for measurement. Overpopulation has a negative impact on the environment due to pollution. The more people there are the more resources they use and the more pollution that results. This pollution may include air pollution due to increased fossil fuel emission from vehicles or land or water pollution due to increased amounts of waste. Population increases as people are born to immigrate into a country and decrease as people die or emigrate. Rates of population growth usually expressed as a percentage vary greatly.