Molds grow readily indoors in the presence of dampness. Their visibility enables their effects to be investigated by means of questionnaire surveys, although these are subject to imprecision and potential bias. Exposure to airborne mold particles can be measured in various ways that also have disadvantages and limitations. Many surveys have been conducted on the health effects of molds; most have examined the association between molds and symptoms, although some studies have used lung function tests and other objective health indices. Most surveys suggest that indoor mold growth is associated with ill health, particularly of the respiratory tract. Knowing how important mold exposure really is in health terms is difficult, owing to the tendency for mold growth to be associated with other factors that are prejudicial to health.