Most of them are not harmful to men and play a major role in element cycling, which is crucial in sustaining life on planet Earth. Legionella pneumophyla is a bacterium that causes respiratory infection and is often cited as an example of microbial dissemination from sources within buildings. There are also bacteria that are not pathogenic but may cause allergies. In our homes, they live in cooling coils and in air ducts of air-conditioning systems.
More research is needed to understand their effect on human health.
Molds are microorganisms that live everywhere, especially in humid and warm environments. They form filaments called hyphae, which penetrate the surface on which they grow, or form aerial mycelia that bears reproductive structures - spores. Mold spores are easily detached and made airborne by air flow, vacuuming, walking on a carpet or sitting on a couch. In indoor environments they grow in air-conditioning ducts, carpets, pots of house plants, etc. They produce and release millions of spores which are small enough to stay airborne threatening to invade the human respiratory system.