Almost any natural material will support a population of molds. They are found on plants, foods, dry leaves, and other organic matter. Molds have an important role in breaking down organic material. Molds are essential to what biologists call nutrient cycling. Nutrient cycling is the process where nutrients get used again and again by living organisms.
Molds commonly release spores in the air that can cause allergic reactions in many people.
For some people, a relatively small number of spores can cause illness. However, for other people a larger number of spores must be present to cause illness. The common symptoms include wheezing, burning eyes, dry cough, nose and eye irritation, sore throat, and skin rashes. Many molds produce toxins that the mold uses to prevent the growth of other organisms. These toxins may suppress the immune system, cause skin rashes, eye and nose irritation, fever, and in infants, rare cases of bleeding in the lungs.