Mould Spores

mould spores

Mould spores are microscopic reproductive cells produce by moulds and other fungi and serving  a similar purpose to that of seeds in the plant world. Many mould spores and fragments contain allergenic compounds which can trigger a wide range of respiratory irritation symptoms.


A number of fungal spore types share similar allergens which means that those who are allergic to moulds are likely to be sensitised to multiple types. Because of the fact different moulds vary in the dispersal times of their spores (month and daytime variations), some people can be adversely affected for significant part of the year.

Mould Spores in Indoor Environment

In the indoor environment the mould spore release is independent of the outdoor environmental conditions and time of year or day. The factors governing the release of spores into the indoor air are all related to human occupancy and activity. If materials contaminated by moulds are disturbed by maintenance activities, cleaning, vacuuming or simply by air current, large quantities of spores can be released in to the air.

Not all mould species present in the outdoor environment would find the indoor conditions suitable for growth but some of the mould such as Penicillium, Mucor,  Aspergillus, Cladosporium, Alternaria and many others will positively thrive in the indoor environment. Availability of moisture is usually the main factor influencing whether the settled mould spores will germinate and produce unsightly mould growths on walls, fabrics or other materials.

Mould Spores in Outdoor Environment

In the outdoor environment mould spores are released and distributed in a variety of ways and have specific climatic requirements. Some mould types favour warm, dry weather and are wind-dispersed and the numbers of these can be very high in the outdoor air. Examples of these ‘dry weather’ spores include Cladosporium, Alternaria and Epicoccum. Other types have evolved in such a way that they need to cross a certain, often species specific, moisture and temperature thresholds before they are released. A combination of humid and warm periods often results in high spore risk. Examples of ‘wet weather’ spores include Sporobolomyces, Didymella and Tilletiopsis which are often abundant during humid and temperate summer nights.

Mould spores are mainly seasonal in their release but many types can be found in the air all year round typically peaking in the summer or late autumn. In spring the mould spores are generally low in number but the risk can rise to moderate in the late spring if conditions are favourable. In summer and early autumn the spore risk starts to rise in mid-June with the increase in temperatures and in late June/early July the risk starts to peak and continues until late September. Typically, during this season, there will be the usually ‘dry-weather’ mould spore types by day and then the ‘wet-weather’ mould spores types during the night. In mid autumn many of the main allergenic types are going into decline but there are still plenty of spores around and the risk can be moderate or even high in October on humid and warm days. Towards the end of the year, November, the risk of significant mould spores exposure decreases to low by the end of the month. In winter the spore counts are generally very low but some sensitive individuals can still be sensitised by mould spores such as Penicillium and Aspergillus.