Heating, Cooling and HVAC

One creature that has had considerable publicity in recent years is the house dust mite (dermatagophydes pteronyssinus).

Often described as microscopic, these mites, (which are related to spiders), are approximately 0.3mm long when adult. This is about the size of a full stop on a printed page, but because of their lack of colour they are often classified as invisible. Just as well!

Most humans naturally shed up to 3 grams of dead surface skin a day, some males make an extra contribution of 0.3 g/d in the form of dust from electric shaving. Although it may seem contradictory, it has been found that frequent showering increases skin cell shedding. Other research indicates that the majority of dust in a domestic bathroom is dead shed skin cells, especially after energetic towelling. This also releases the other main dust constituent, towel lint (cotton is a natural, vegetable cellulose product), Likewise, talcum powder is another contributor. Although inert, it’s function is to absorb moisture from the air.

The dead skin cells indirectly form the diet for the mites. In high humidity conditions, a microscopic fungus grows on the skin cells, and it is this fungus that breaks the skin cells down for the mites to eat. The 20 or so faecal pellets the mites excrete are microscopic (about 5 microns) are very dry and easily broken into even smaller pieces. These can remain continuously suspended in the air and it is these pellets that can cause respiratory problems when inhaled.

Studies by Dr John Maunder and Tin Htut at Cambridge University indicate that allergens from mite faecal pellets are a major contribution to the increase in respiratory problems, including Asthma.

House Dust Mites need high humidity levels to provide their food and ideal living and breeding conditions. Controlling humidity has been found to limit mite population. Diligent domestic hygiene, including use of vacuum cleaners with high efficiency filters which prevent the mites and their faeces being blown back into the room air will help. More recently, fine weave bedding, sheets and pillowcases, that prevent mite transfer has been introduced, this has also been shown to reduce numbers. Bedding, furnishing and carpets form the ideal living areas for mites, so a reversal of the trend for wall to wall carpeting to hard, easy to clean floor surfaces, and washable rugs, may be required where sufferers live. It has been estimated that thousands share your pillow at night, so vigorous “plumping up” will release mites and faecal pellets into the air, just where your pillow, your moist breath, perspiration and skin cells provide ideal accommodation. A two year old pillow can harbour enough mites, dead and alive, and faecal pellets to make a quarter of its weight.

Once children have been exposed to these allergens they can remain sensitised. Continuous exposure in later years maximises and aggravates the condition.

Dr Jill Warner of Southampton University Hospital has found that the mites life cycle spans 10 weeks from egg to death. During their reproductive period, females will produce 40-80 eggs. Mites cannot regulate their own body temperature, therefore at low temperatures, and the associated lower humidity, egg production and population growth is reduced and the death rate increases. This is one reason why Asthma Clinics are in the cold, dry, high altitude atmosphere of the Alps.

Conversely, the best conditions for mite reproduction and population growth is 20°C and 80% Relative Humidity; fairly normal in a centrally heated but possibly inadequately ventilated modern home. Loss of body moisture and heat, plus the food supply from the human body, provides ideal conditions for the House Dust Mite.

Put into perspective, some of the statistics are frightening. It is estimated that 3.4 million people in the UK suffer from Asthma and similar allergies. Over 100 thousand people are hospitalized every year. There is a death every 20 minutes. Lastly, £900 million are spent annually in Asthma and Respiratory problems.

It would be arrogant of the ventilation industry to claim that fans can cure Asthma, but what can be done is to reduce humidity to a level that is still perfectly acceptable to humans, but not to the House Dust Mite.

This, together with some of the other measures mentioned earlier will certainly reduce Mite population and hence the allergens in their droppings, thus producing a healthier indoor environment.

Source:The Vent Axia-Ventilation Hand Book