Studies have shown that people who are atopic (sensitive), already have allergies, asthma, or compromised immune systems and occupy damp or moldy buildings are at an increased risk of health problems such as inflammatory responses to mold spores, metabolites such as mycotoxins, and other components. Other problems are respiratory and/or immune system responses including respiratory symptoms, respiratory infections, exacerbation of asthma, and rarely hypersensitivity pneumonitis, allergic alveolitis, chronic rhinosinusitis and allergic fungal sinusitis. A person's reaction to mold depends on their sensitivity and other health conditions, the amount of mold present, length of exposure, and the type of mold or mold products.
Some molds also produce mycotoxins, which, in sufficient quantity, can pose serious health risks to humans and animals. The colloquial term "toxic mold" (or more accurately, toxigenic mold) refers to molds that produce mycotoxins known to harm humans, not to all molds.  Exposure to high levels of mycotoxins can lead to neurological problems and, in some cases, death. Prolonged exposure, e.g., daily workplace exposure, can be particularly harmful.
The five most common genera of indoor molds are Cladosporium, Penicillium, Aspergillus, Alternaria, and Trichoderma.
Damp environments that allow mold to grow can also allow the proliferation of bacteria and release volatile organic compounds.