Many different biological factors affect the size of human populations. Ever since the 19th century Reverend Thomas Malthus' famous Malthusian equation which discussed the relationship between human population and food supply, economists and population scientists have been concerned about what is known as "carrying capacity", the ability of a given area of land to provide food sufficient for a given population. Many biological factors, such as climate, crop yields, diseases, etc. affect carrying capacity, as do such issues as whether food is cultivated in the form of plant or animal food. For example, it takes 10-20 more times as many natural resources to create a pound of animal protein as it does to create a pound of vegetable protein.
Temperture, humidity, air polution are just some factors that can affect human health, affecting human population. Depending on conditions, the human population can be determined