14. July 2020
Carpets promote growth of mould spores

A recent study in the US has demonstrated that the number of mould spores in a carpet mainly depends on the humidity.

“Mould in the home is a very relevant issue. According to estimates, mould accounts for 16 per cent of all costs associated with asthma, acute bronchitis or hay fever”, says Karen Dannemiller, a specialist in environmental chemistry and biochemistry. The floor is a major collection point for house dust and the microorganisms it contains. Carpets are the real culprit when it comes to collecting dust. A team at Ohio State University has now investigated how well fungal spores grow in carpets (Nastasi 2020). A mould allergy can be triggered when fungal spores are inhaled.
The study results were presented by Karen Dannemiller of Ohio State University at the 2020 Annual Meeting of the American Academy of Allergy Asthma and Immunology (AAAI).

How the researchers proceeded

Nicholas Nastasi and his team examined the factors that contribute to the proliferation of fungal spores on carpets. To do this, they observed the growth of the fungi under different conditions by varying the dust concentration and humidity. They also tested the effects on different carpet materials:

  • polypropylene,
  • nylon (polyamide) and
  • wool (natural fibre).

The results

“Moisture is the most important trigger of fungal growth”, Dannemiller concluded. Fungal spores grew especially well at a relative humidity of 85 per cent or higher. However, they hardly grew at all at levels below 50 per cent. The growth of fungi was therefore 1,000 times higher at 100 per cent humidity than at 50 per cent humidity.


House dust also influenced the growth of fungi, but to a lesser degree than moisture. When examined under a microscope, the concentration of fungi on slides with house dust was 100 times higher than samples without house dust.

Moreover, it has long been known that the humidity in carpets can be even higher than in the ambient air (Cunningham 1998), Dannemiller added.


On the other hand, the carpet material was not a significant factor. The growth of fungi depends mainly on the dust concentration: At low concentrations, the growth of fungi increases, especially in wool carpets. If house dust is present at high levels, the fungal spores multiply mainly on nylon fibres.

Everyday measures

The relative humidity in closed rooms should be between 30 and 50 per cent, advises Dannemiller. “But you will exceed that level when taking a shower, cooking, or opening the windows in humid weather.


If it is more humid outdoors than indoors, use shock ventilation to dehumidify the rooms several times a day. If it is more humid outdoors than indoors, keep the windows closed. Dehumidifiers can help in these situations. Placing a bowl of coarse salt in the room also helps remove moisture from the air.

Another tip: It is better to hang wet laundry to dry in the basement or outside, since it contains a lot of moisture which is released into the surroundings as it dries.

If you have an allergy, you might want get rid of your carpets altogether, at least in the bathroom.



Nastasi N et al. Morphology and quantification of fungal growth in residential dust and carpets. Building and Environment. 2020;174:106774.



Cunningham MJ. Direct Measurements of Temperature and Humidity in Dust Mite Microhabitats. Clin Exp Allergy. 1998;28(9):1104-12.