House Dustmites

Up to 80 percent of asthma in New Zealand is associated with allergy. One of the most common allergens (substances that provoke an allergic reaction) is produced by the house dust mite.

What are house dust mites?

House dust mites are microscopic creatures, less than half a millimetre long. They live in most human environments and thrive in warm, moist places such as bedding and carpets. They mainly feed on flaked-off dead skin. They are sensitive to dry conditions, changes in temperature and ultra-violet light. Their faeces, which are small and light, get into the air easily and can provoke a strong allergic response when inhaled. If you suspect house dust mite waste may affect your asthma, you can ask your doctor for a skin prick test which can help indicate the likelihood of a substance as a trigger.

What can we do about house dust mite allergen?

Get bedding covers which provide a barrier. Most manufacturers of pillows, mattresses, bed bases and duvet inners use material which allows dust mite faeces through into the air. However, you can buy special barrier covers for these items. Suppliers include some local Asthma Societies, and Air Flow Products (ph 0800-247-3569). Look for covers which retain 99.9% of dust mite waste (the retailer should be able to give you written evidence of this); have strong sewn seams; totally enclose the bedding item; can be damp-dusted, rather than having to be removed frequently to be washed; and allow mositure out - check to see that they have a water vapour permeability index of at least 40 percent. If your asthma is triggered by house dust mite faeces, and you take only one step towards reducing exposure to them, this should be the one - it's the single most effective measure you can take.

Consider removing as much fitted carpet as possible. Especially in bedrooms.

Check out your vacuuming. Vacuum cleaners won't eliminate dust mites, but can reduce the level of dust mite faeces. Vacuum at least once a week, or - if you have asthma which is triggered by dust mite faeces - have someone else do it for you. Use vacuum cleaners with a micro-filter, an "S-class" filter, or an HEPA ("High Efficiency Particulate Air") filter system. Machines without these are more likely to recirculate the fine dust mite faeces through their exhaust vent, back into the air. Avoid using vacuum cleaners which expose you to the contents when you empty them. After vacuuming, leave the house and allow the dust to settle for a half hour or so before re-entering.

Damp dust weekly. Avoid using feather dusters. The barrier covers on mattresses and bedding (see 1 above) should be damp dusted once a week, but not removed.

When you do the washing, hang things out in the sun to dry. Alternatively, if you use a drier, run it an extra half hour on warm after the contents are dry.

Air blankets weekly, where possible in natural sunlight for a whole day. Do the same with any loose rugs.

Minimise the number of things you keep under the bed, or on top of wardrobes.
In addition, with any young children:
· put soft toys in the deep freeze for 24 hours every 3 weeks. Use washable soft toys, and avoid fluffy toys where possible;
· avoid using sheepskins (and especially avoid using them as infant bedding);
· do not put young children on a bottom bunk;
· remember that early exposure to allergens may be of significance in the development of asthma.

Bear in mind that:

Wet and steam cleaning removes dust mites from carpets more readily than vacuuming alone, but research suggests that it only reduces their levels for a short period, and the residual water may promote fungal growth.
A hot wash (using water hotter than 55°C) will kill dust mites in sheets, pillow cases and quilts. However, there are practical difficulties associated with hot washing in New Zealand. For domestic hot water, the maximum recommended temperature to prevent the risk of hot water burns is 50°C. Most domestic hot water cylinders do not have highly accurate thermostats, so it is difficult to set temperatures precisely - in addition to which there is often a temperature difference between the water in the cylinder, and the water in the tap. If you decide to use a hot wash, the most practical steps would be to purchase a thermometer, and to supplement water from the tap with hotter water from a jug or pan, to ensure that the wash temperature is greater than 55°C. Take care to avoid burns.

A five-minute wash at 25°C using detergent washing powders is sufficient to remove most mite allergen from clothing and sheets, pillowcases, etc. (This will not, however, kill mites living in the clothes or bedding.)
Measures which are not currently recommended, as there isn't evidence to indicate that they are effective in reducing house dust mite allergen in New Zealand homes, include air filters, chemicals, and dehumidifiers.
Synthetic fill bedding is no longer recommended over feather fill items. It used to be thought that less house dust mite allergen would be released from synthetic fill, but this in not the case - possbily because the material around synthetic fill has a looser weave. Effective barrier covers are needed for both types of fill.
On average, we spend a third of our time (25 years of our lives) in bed. Measures to minimise exposure to the allergen are therefore especially important in the bedroom.