Mould aka Mold
According to Health Canada,
"Mould is the common word for any fungus that grows on food or damp materials. Mould can be black, white or almost any colour. It often looks like a stain or smudge and it may smell musty.
In order to grow, mould needs moisture and a material it can live on. It then releases "spores" into the air which are small enough that people can actually breathe them in. Breathing in large amounts of these spores and the by-products they produce can negatively impact your health."
According to the World Health Organization's (WHO) 228 page research collaboration / report, "Guidelines for Indoor Air Quality: Dampness and Mould,"
Excess moisture on almost all indoor materials leads to growth of microbes, such as mould, fungi and bacteria, which subsequently emit spores, cells, fragments and volatile organic compounds into indoor air. Moreover, dampness initiates chemical or biological degradation of materials, which also pollutes indoor air. Dampness has therefore been suggested to be a strong, consistent indicator of risk of asthma and respiratory symptoms (e.g. cough and wheeze)"