Outside, mold is a natural part of the environment; it breaks down dead biological matter, such as dead trees or fallen leaves. However, mold is not a natural part of our environment indoors and should always be avoided. The reproduction of mold occurs by means of tiny spores that float through outdoor and indoor air. These spores are very small and invisible to the naked eye. When mold spores land on wet surfaces, the growth of mold may begin. Although there are many different types of mold, none of them are able to grow and reproduce without moisture or water.