Microbial pollution is one of the key constituents of indoor air pollution. It consists of hundreds
of species of bacteria and fungi, and in particular filamentous fungi (moulds) growing indoors
when sufficient moisture is available. Health problems associated with moisture and biological
agents include increased prevalence of respiratory symptoms, allergies, and asthma as well as
perturbation of the immunological system. Based on the extensive review of the scientific
evidence, this WHO working group identified the main health risks due to excess moisture,
associated with microbial growth and contamination of indoor spaces. It also formulated WHO
guidelines for protecting public health, recommending that persistent dampness and microbial
growth on interior surfaces and in building structures should be prevented (or minimized) as they
may lead to adverse health effects.