Mold (microscopic fungi that live on plant or animal matter) isn't a late 20th century plague. Mold has been ubiquitous since the beginnings of life itself. It is a critical part of the ecosystem, providing the enzymes that break down and recycle organic matter. The estimated number of species of fungi ranges from tens of thousands to more than 300,000. Fewer than 200 have been described as agents that can cause or exacerbate a human ailment.
Mold requires moisture, oxygen and a source of food to thrive. Mold growth is stimulated by warm, damp and humid conditions. Mold will feed on cellulose material such as wood products (including paper, wallpaper, boards, furniture) carpet, and even plants - especially decaying plant matter that drops in soil. Greenhouses, for example, are ideal breeding places for mold. Mold spreads and reproduces by making small, lightweight spores that are able to travel through the air and are capable of surviving a long time through adverse environmental conditions.
The presence of mold doesn't automatically mean there's a human health problem, especially when the levels of mold in the indoor air are less than levels of the same mold type in the outdoor air. Mold is always found in outside air; the majority of fungi in the indoor environment come from the outdoors. And as such, outdoor air sampling must always accompany indoor air sampling. Even then, test counts only suggest the types and relative quantities of mold present at a certain time and place. The weather is constantly changing. The number and types of spores actually present in the mold count can significantly change within a 24-hour period because weather and spore dispersal are directly related.
For example, some common molds have dry spore types (spores are released in dry, windy weather). Other molds need high humidity, fog or dew to release spores. Rain can wash larger spores out of the air and cause smaller spores to be released. Spore populations also differ in day and night time (dry spores favor day time; wet spore types thrive at night).
Mold spores themselves are not usually the source of irritation to humans. Allergies and ailments may arise from elevated levels of organic compounds emitted by spores. Known irritants are also called mycotoxins. However, objectivity in determining bodily injury is almost impossible since these compounds affect different people in different ways at different times and circumstances.