Molds are ubiquitous in nature with several hundred thousand individual species, and mould spores are a common component of household and workplace dust. However, when mould spores are present in large quantities, they can present a serious health hazard to humans, potentially causing allergic reactions and respiratory problems.
Some moulds also produce mycotoxins that can pose serious health risks to humans and animals. Some studies claim that exposure to high levels of mycotoxins can lead to neurological problems and in some cases death. Prolonged exposure, e.g. daily home exposure, may be particularly harmful.
The term "toxic mould" refers to moulds that produce mycotoxins, such as Stachybotrys chartarum, that are especially harmful to human health, there are about 16 different species of "toxic mould" that are of considerable danger to humans and animals including:-
Aspergillus is another common mould. It can look grey, brown, yellow, green, white, or black. There are many species in the genus, and they generally fall into Hazard Classes A or B. Some can cause infection in people with weak immune systems, and some of them can make toxins in certain circumstances. Others will only cause allergic reactions. Aspergillus can grow on walls, insulation, paper products, soil, clothing, and many other places.
In indoor environments Fusarium species are generally found under very wet conditions. They are commonly isolated from carpet and mattress dust, damp walls, wallpaper, polyester polyurethane foam, humidifier pans and areas where stagnant water occurs in HVAC systems. Some species cause keratitis in humans, and infect eyes and finger nails. Fusarium species are also an inhalation hazard.Fusarium also produce a number of different mycotoxins which include trichothecenes (T-2 toxin, HT-2 toxin, deoxynivalenol (DON) and nivalenol), zearalenone and fumonisins. The Fusarium species are probably the most prevalent toxin-producing fungi in the northern temperate regions
Paecilomyces species are common environmental moulds. They are widespread in soils, composts, and food products. In indoor environment, Paecilomyces have been isolated from air, damp walls, wet plaster work, carpet dust and HVAC fans. Paecilomyces species are rarely associated with human infections but some species such as P. variotii, P. marquandii and P. lilacinus are emerging as causative agents of mycoses in immuno-compromised patients.
Penicillium is a name that often strikes people as familiar, and that is because modern antibiotics were discovered thanks to a species of Penicillium long ago. However, that doesn’t mean that the genus can’t be hazardous. It can look blue, green, or white, and its species are generally classified as B or C. It can be found on foods, such as cheese and fruit, or in the walls, the insulation, and other places.
Stachybotrys is the infamous "black mould" that made the news in association with ill health effects many years ago. It needs a very damp area to grow, and is considered a Hazard Class A mould, as it can create toxins. It looks black on surfaces.
In an indoor environment, Trichoderma spp are commonly found on gypsum board and water saturated wood, wallpaper, carpet and mattress dust, paint, and air-conditioning filters. Generally, Trichoderma species require relatively higher water activity than some other indoor moulds such as Penicillium or Aspergillus. Human infection by species of Trichoderma is limited to individuals with severely weakened immune systems. However, some species such as T. harzianum and T. viride are producers of potent mycotoxins.
Please bear in mind that not all moulds are toxic and not all toxic mould is black which is a common misconception - some species of so called "black mould" pose no threat to human health whatsoever.