Dampness, Mould, House Dust Mites and Allergy

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The risk of asthma, allergic rhinitis, and atopic (hereditary) dermatitis is higher in damp buildings than non-damp buildings.

However, dampness itself may have no or little direct effect to allergy development. Dampness only favours proliferation of biological and increased emission of chemical pollutants. A strong association has been found between exposure to visible mould and asthma and common colds.

Studies have also shown that the risk of asthma in damp homes is highest among individuals with hereditary allergy.

Causes of dampness in a building may be complex. Possible causes may include:

  • Penetrating damp due to defects in the roof, walls or plumbing
  • Rising damp
  • Condensation
  • The activities of the building’s occupants such as cooking, dishwashing, and cleaning.

How Does Dampness Contribute to Increased Asthma and Allergy Risks?

Mould on ceiling wood due to leaking pipes

Mould on ceiling wood due to leaking pipes

Dampness contributes indirectly to asthma and allergy risks. Increase in humidity in the building encourages proliferation of moulds, bacteria, house dust mites, and other micro-organisms some of which are toxin producers or allergens. Dampness also results in increased emission of chemical pollutants from degrading building materials.

Mould spores are generally everywhere. Nutrients are also available in form of organic dust or cellulose found on most building surfaces. In most cases the limiting factor to mould growth in a building is moisture. Moulds have a minimum level of dampness (relative humidity) below which growth is limited. Generally relative humidity below 65% restricts mould growth.

Dampness is also essential for the survival of house dust mites. Since house dust mites do not drink free water they absorb it from the air and the environment. House dust mites feed on human skin scales (flakes), pollen, moulds, bacteria, animal dander, and skin scales of birds. These food sources have to be moist for mites to utilize them.

Therefore, the food consumption of mites increases (and hence their population) at high relative humidity. It has been estimated that human beings shed dead skin at a daily rate of 0.5-1.0g per person and several thousand mites can survive for months on just 0.25g, so moisture and not food would be the limiting factor to mite proliferation. House dust mites survive best at relative humidity of 70-85% and temperatures of 24-27 °C.

The Interaction Between Moulds, Other Micro-organisms and Mites

Mites help in dispersal of bacteria, yeasts, moulds and other saprophytic microorganisms. Their populations tend to flourish in the presence of moulds since mites also feed on moulds and the moulds help to break down (pre-digest) and soften the skin scales.

For example, it has been suggested that the growth of the house dust mite, Dermatophagoides pteronyssimus, in carpets, mattresses and dust may be stimulated by growth of xerotolerant (drought-tolerant) species of Aspergillus such as Aspergillus glaucus on human skin scales and other dry household organic particulates.

However, with rising relative humidity (RH) mould activity increases rapidly and above 85% RH mite populations decline as the food supply becomes contaminated by mould and other microbial metabolites.

Moulds, Mites and Allergy

Moulds and mites allergens trigger allergic reactions in sensitive individuals. Mould spores are recognized allergens. Some of the allergies associated with mould exposure include:

  • Immediate type hypersensitivity (Type 1 allergies).
  • Delayed type hypersensitivity (Type 3 allergies) also referred to as allergic alveolitis. These types of allergies are characterized by wheeze, shortness of breath, cough, chest tightness, and in some prolonged cases, pulmonary fibrosis.
  • Allergic bronchopulmonary aspergillosis (ABPA). In this case the bronchial passages are colonized by a species of Aspergillus, usually Aspergillus fumigatus, Aspergillus flavus or Aspergillus niger.
  • Allergic mycotic sinusitis. This type of allergy involves infection of mucus adhering to the sinus walls.

The house dust mites (Dermatophagoides pteronyssinus, D. farinae, D. microceras, and Euroglyphus maynei) are the most important sources of allergens in house dust. At least 14 groups of allergens have been identified from D. farinae and D. pteronyssinus. These allergens have been implicated as causes of atopic asthma, perennial rhinitis, and atopic dermatitis among other allergic conditions.

Reducing the risk of Allergy and other respiratory problems

Reducing exposure to mould spores, mites and chemical pollutants reduces the risk of allergy and other respiratory problems. Since dampness encourages proliferation of moulds and mites and increases emission of chemical pollutants, reducing dampness in buildings would reduce exposure to these pollutants. Dampness in buildings can be reduced by:

  • Mechanical ventilation.
  • Vapour barriers.
  • Channelling ground water away from foundations.
  • Sealing below-ground walls.
  • Protecting ground-level concrete slabs from moisture intrusion.
  • Venting showers and other moisture-generating sources to the outside.
  • Using air conditioners and de-humidifiers.
  • Using exhaust fans whenever cooking, dishwashing, and cleaning.

Dampness in homes and workplaces should be controlled in order to reduce the risk of occupants developing respiratory problems.

If you have questions about this article, or would like to inquire about our bacteria and mold testing services, contact us by telephone or email.

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Dr Jackson Kung'u
Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at www.drjacksonkungu.com.

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