Mould growth in buildings can occur when mould spores, which are always present in buildings, are exposed to wet or humid conditions for a sufficient length of time and on an appropriate surface to support growth. Water sources can include plumbing leaks, spills, foundation seepage, roof or wall leaks, condensation on cool surfaces such as inside ductwork and on windowsills, etc. In the presence of wet or highly humid conditions, mould spores can germinate within 48 to 72 hours (about 3 days).