Moulds are a form of fungi, as are yeast and mushrooms. And, depending upon circumstances, the same mould can be either useful or a health hazard. In most non-contaminated workplaces the possible mould exposure would not be expected to present a health hazard except to very susceptible individuals. In contaminated situations the risk from exposure to mould increases.
Reactions are varied and complex depending upon many factors.
Human factors include personal susceptibility, route of exposure, age and state of health. Mould related factors include amount and length of time of exposure, virility and viability of the organism, and whether the effect is infection, allergenic, toxigenic or some combination of these. Example of mould growth on the inside of a wall after a flood. Some of these products can be extremely hazardous to people. Infection and toxic reactions tend to be quite uncommon.