The most common types of mold are generally not hazardous to healthy individuals. However, people who have asthma, hay fever, or other allergies or have weakened immune systems are more likely to react to mold. The most common symptoms are running nose, eye irritation, cough, congestion, and aggravation of asthma. A small percentage of the population can develop more serious effects - such as fevers and breathing difficulties - but these effects are uncommon. Some types of mold can cause more serious health problems, but this is much more rare.

The presence of fungi on building materials as identified by a visual assessment or by bulk/surface sampling results does not necessitate that people will be exposed or exhibit health effects. In order for humans to be exposed indoors, fungal spores, fragments, or metabolites must be released into the air and inhaled, physically contacted (dermal exposure), or ingested. Whether or not symptoms develop in people exposed to fungi depends on the nature of the fungal material (e.g., allergenic, toxic, or infectious), the amount of exposure, and the susceptibility of exposed persons. Susceptibility varies with the genetic predisposition (e.g., allergic reactions do not always occur in all individuals), age, state of health, and concurrent exposures. For these reasons, and because measurements of exposure are not standardized and biological markers of exposure to fungi are largely unknown, it is not possible to determine "safe" or "unsafe" levels of exposure for people in general.

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