Biological agents are a heterogeneous group of particles or chemicals of microbial, animal, or plant origin. They emerge from numerous sources, some of which can be part of the everyday use of the building, are clearly harmful and therefore need to be specifically controlled ,are always present in the indoor environment.Indoor air can contain a wide variety of biological contaminants such as bacteria, viruses, fungi, allergens from insects and insect parts, animal dander, and toxic components such as endotoxins and mycotoxins.For example, bacteria that belong to the normal microflora of the human skin are continuously emitted into the indoor environment.Being living organisms, biological agents are able to increase their concentrations indoors even if their original source would be outdoors. They thus can create an indoor source of their spores, and metabolic and other products' diversity in indoor environments is large. The relative importance to health of individual biological agents as sources of allergens in indoor environments is still to be assessed except for house dust mites or hair from pets. While many of these are nonpathogenic, others induce disease by infection of the respiratory tract or by immunologic means, such as allergy.Influence of dampness and moisture. Dampness and moisture conditions regulate the biological contamination of indoor environments.