Asthma has many causes, and sufferers should be tested to identify their particular allergy.

New Zealand, has over 600,000 asthmatics, and the House Dust Mite (HDM) is attributed to be related to nearly 90% of asthma problems.  In Australia the percentage of the population who are asthma reactive is much less than in NZ, with the majority being affected by pollens, rather than dust mites.

Of all asthma allergens (Pollens, Moulds, Fungi etc), in nearly every case, fresh air ventilation (with a pollen air filter as may be appropriate), with an HRV has been proven to improve the condition. The following is a guide only.  See our reference links for websites and publications of interest.

The common dust mite as seen through an electron microscope.

The Dust Mite

Dust mites are tiny eight legged creatures belonging to the spider family.  They are almost invisible without the aid of a magnifying glass or microscope.  There are millions of dust mites in most NZ homes.  Over 18,000 dust mites have been counted in one gram of ordinary household dust.  They thrive in warm damp places that are or have been, occupied by humans and animals — bedding, carpets, and clothing.

What is not appreciated is that the dust mite itself does no harm unless its tiny body is ingested into our lungs.  The asthma allergic reaction is to enzymes that originate from the digestive function of the dust mite.  Dust mites carry this digestive enzyme in their stomach, and saliva.

It is this digestive enzyme which is the asthma allergen.

The stomach of dust mites performs a digestive process which encapsulates excreta “pellets” with the digestive enzyme.  Every dust mite excretes these pellets approximately 20 times a day.  Each pellet is invisible to the eye, and so light that it remains suspended in still air (like dust) inside a home for hours.  They also cover partly eaten food particles with saliva that contains the enzyme.

When this enzyme is ingested (by the simple act of breathing) by humans who are allergic to the dust mite enzyme, it can trigger  an asthma attack.  The greater the number of enzymes in the air that is breathed, (dust mite excreta pellets floating as dust in the air of under ventilated homes), the greater the potential for an asthma attack.

What About Insecticides to Kill Dust Mites ?

If all the dust mites in your home were eliminated today, you and your family will bring home each day, dust mites that have been picked up (on clothing) from other persons and places.  When dust mites are low in numbers, the food supply (human skin scales) increases, and so, when dust mites arrive to re-populate, their numbers explode (like a plague of rabbits or mice), until the food supply reduces to normal levels.

If the HRV Changes Air and Dehumidifies Will it Kill Dust Mites ?

The most cost effective means to dehumidify a whole house is an HRV, but HRVs only dehumidify when outdoor air is colder than indoor air, ie winter and colder months.  Tests in New Zealand have proven that while there is a noticeable improvement, the low humidity level required is not sustainable for long enough to continuously control dust mites :-

The moisture produced by cooking an evening meal, taking a shower, or 2 occupants sleeping in a bedroom  is sufficient to enable dust mites to survive.

With all the information and devices known today, it is as impossible to rid NZ homes of dust mites as it is to rid the West Coast of sandflies, and the native forests of opossums.

However, the HRV can control dust mite allergens.   While it is desirable to be rid of house dust mites, the dust mite itself is not a major harm to humans, it is the digestive enzyme found in their excrement and partially eaten food which is the allergen.   The HRV can lower humidity making it more difficult for the dust mite to proliferate and, if designed and installed properly, it will definitely reduce allergen concentration.

In winter, the incoming fresh air from the HRV is preheated and dehumidified.  In a whole house system the entire home has the benefit of fresh dry air.  The rate of dehumidification may not be sufficient to rid the home of dust mites but it will make their life more unpleasant, and the effects of whole house dehumidification and ventilation are a significant benefit to all occupants.

How Does the HRV Help to Reduce Dust Mite Related Asthma Attacks ?

By diluting away air that is contaminated with dust mite allergens, and replacing the foul air with fresh outdoor air. This is known as ventilation by dilution.  The ventilation must be continuous, and the ventilation rate is calculated to suit  the internal volume of the ventilated space.

In tightly sealed, under ventilated homes, concentrations of dust mite excreta pellets float in the air in colossal numbers, but because they are unseen they are unrecognised as a problem.

These allergens are ingested by the normal breathing of home occupants.  If the occupants respiratory system is sensitised, (allergic) to the dust mite enzyme, the chances of an asthma attack are high.

The HRV exhaust air flow collects the allergens and exhausts them outside.  Simultaneously the HRV replaces the allergen laden air with fresh, warm, dry air.  The result is that the allergen laden air is continuously diluted with fresh, warm, dry air that contains no dust mite allergens.  Thus the occupants of the home breathe fresh, dry air, and so the chance of an asthma attack is dramatically reduced — It really is this simple.

Opening doors and windows will produce the same beneficial results, but also expose the home to winter cold and insecurity.

This does not excuse the homeowner from the need to implement other practical measures to reduce exposure to dust mite or other allergens.

There are special types of mattress and pillow covers which are proven to be an effective means to reduce exposure.

Most vacuum cleaners have air filters, but few have air filters that can capture dust mite allergens. The dust mite allergens pass right through a conventional vacuum cleaner bag, and the air turbulence caused by the use of the vacuum cleaner simply makes the air within the rooms more contaminated, by agitating dormant dust mite allergens causing them to become airborne.

The best vacuum cleaners are those with a High Efficiency Particulate Air Cleaner, (HEPA filter) or better still is a whole house vacuum cleaner that removes the dust mite allergens (with the vacuumed air) from the room as it vacuums, with the dust collecting machinery installed outside the living space (so that dust mite faeces and other allergens are positively removed from the living space).

Numerous tests and millions of taxpayers dollars have failed to find an economical solution to eliminate dust mites in New Zealand homes, but the simple solution is to concentrate on the allergens, and the best method is ventilation by dilution.

There are practical measures than can be taken to reduce dust mites and allergen exposure …

To survive, dust mites need three basics …

  1. A comfortable temperature – Temperatures between 10°C and 40°C are comfortable to a dust mite, freezing kills dust mites (put pillows in the freezer for a day) and hot water washing (60°C +) also kills them.
  1. Food – Dust mites thrive on human skin scales, pet dander, flour (biscuit and bread crumbs), mould and fungi (as in mouldy shoes, clothes, carpets drapes and wallpaper).
  1. Water – Dust Mites cannot drink.   They absorb moisture from humid air through crystalline glands located under their armpits.  For sufficient moisture, dust mites like the humidity of the air to be above 60%.  At lower humidity they reduce their activities.  Dust mites cannot survive in humidity below 40%, but they have been known to survive for up to 30 days at 40%.  This is why claims of dehumidifiers that control dust mites are incorrect:

Dehumidifiers do not ventilate, they simply recirculate over and over, the same indoor air.  Recirculating indoor air allows allergens to multiply and make matters worse.

Dehumidifiers do not replace stale air (laden with dust mite enzymes and other pollutants), with fresh air.  A dehumidifier removes moisture in the process of recirculating allergen laden air.  Unless the dehumidifier can maintain the humidity in the entire home room including the carpet, furnishings and bedding below 40% RH continuously, then it will not keep dust mites down !!!

In NZ homes it is economically impossible to continuously maintain humidity at a level low enough, to eliminate dust mites.

Most dehumidifier instructions call for the room to be closed up so it can dehumidify.   Asthma enzymes from dust mites actually increase in a room that is closed up.

Worse still, the cold plates and water storage parts of a dehumidifier (mostly inaccessible for cleaning) support colonies of mould and fungi, which are a source of Indoor Air Pollution in the form of fungal spores.

Visit  http://www.dspinspections.com/moldin.htm#air-conditioners   for more information on the build up of mould in these appliances.

If indoor humidity could be kept below 40% RH continuously, as in dry Middle Eastern Countries, Central Australia, and Arizona, there would be no dust mites in NZ.

In controlled laboratory tests in Holland to identify survival rates, dust mites survived for to 30 days at 45% humidity, and when returned live to normal humidity (50-60%RH), they recovered and were breeding again within 3 days.

A Roof Space Ventilator forces air from the roof space, which is often allergen laden and moist into inaccessible nooks and crannies, and can aggravate problems by causing accumulations of dust mite allergens in wall spaces and cavities, and by contaminating cavities with humid air..

Dust Mites Themselves Do No Harm To Humans.

It is the enzyme from the stomach of the dust mite that is the asthma allergen.  This enzyme is contained in the excrement of the dust mite, the pellets of which are so small that they literally float in the indoor air, where they are easily and continually ingested by the normal breathing of the occupants.

A proven practical means to reduce exposure to dust mite allergens is ventilation.

You can open doors, windows, use fans – whatever.  Any ventilation, with outdoor air, will dilute away indoor pollutants and allergens.

Ventilation of the living space exhausts polluted indoor air to outside, replaces it with fresh outdoor air.

This is best described as Ventilation by Dilution.

Because it is costly (in terms of heat energy lost) and often impractical (security, noise etc.) to ventilate sufficiently simply by using fans and opening doors and windows, the HRV is the best option to continually dilute dust mite allergens with warmed, dried, fresh Outdoor Air because…

  1. It is continuous
  2. It dehumidifies
  3. It recovers energy

The HRV collects the allergens and expels them outside.   Polluted air is replaced with fresh outdoor air and the level of allergens that the occupiers are exposed to is reduced to a safe, healthy level.

The logic cannot be argued against — it really is this simple to ensure that your home has a healthy indoor environment – continuously.


0800 DRY AIR  (0800 379 247) 

Email: info@avonelectric.co.nz

Products are New Zealand made by Avon Electric Ltd.


Avon Electric Ltd
25 Taurus Place, Bromley
P.O Box 19748, Christchurch 8030 
Ph: 03 381-5595  - Fax: 03 381-5596