Why do some people get sick from mould and others do not? It is because of genetic and physiological differences. Sensitivity to mould in about 20 % of the human population is believed to be hereditary. Other factors that influence individual's resistance to mould include age and general health status. For example, infants, the elderly and those recovering from diseases may be at risk of mould exposure even at levels that would otherwise be considered "normal".

Susceptibility differences among individuals and the variability of moulds in their ability to cause health problems make it difficult to determine and set permissible exposure levels for indoor moulds.