How can dust mites affect asthma?

House dust mites produce faeces and other waste products which can get into the air. When breathed in, this waste can cause an allergic response in people with asthma, leading to wheezing and other asthma symptoms. To minimise this effect, regular vacuuming, using barrier bedding covers, and drying washing in direct sunlight are among the measures recommended.

What are house dust mites?

House dust mites are microscopic creatures (too small to see without magnification), which are present in most human environments. They thrive in warm, moist places such as mattresses, bedding and carpets, and feed mainly on flakes of dead skin, which we constantly shed.

Dust mite waste products, which are very tiny and light, get into the air easily and can provoke an allergic reaction when inhaled. Allergies to dust mites are very common.

Am I allergic to dust mites?

Signs that you might have an allergy to dust mites include wheezing when you are vacuuming or dusting, or when you enter a dusty room or house, or asthma symptoms during the night or first thing in the morning. If you suspect house dust mite waste is affecting your asthma, you can ask your doctor for a 'skin prick test' which can help indicate the likelihood of a substance as a trigger. Also see: Asthma and allergy

What can we do about house dust mites?

Although we can't completely get rid of dust mites, there are a number of practical things which can be done to minimise exposure to their waste products. House dust mites are sensitive to dry conditions, changes in temperature, and ultraviolet light.

Bedroom and bedding

• Get bedding covers which provide a barrier to dust mite allergens. Look for evidence that the cover has been tested against dust mites and achieves a 90% efficacy rate; has strong seams; totally encloses the bedding item; can be damp dusted, rather than having to be removed frequently to be washed; and allows moisture out. Check to see that it has a water vapour permeability index of 40% or more. Contact your local asthma society for suppliers or contact Air Flow Products (ph 0800 247-3569 or www.airflow.co.nz). If your asthma is triggered by house dust mite waste, 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.
• Sleep with windows open (if practical) to reduce the amount of moisture in the bedroom from breathing.
• Avoid keeping things under the bed. Also don't keep things on top of wardrobes - because they gather dust.
• Discard all old and dirty mattresses, bedding, pillows and furnishings
• Choose feather filling: Feather fill bedding is recommended over synthetic fill bedding. Recent New Zealand research has shown that feather bedding contains lower levels of fungal cell products, which may have a negative effect on people with asthma and respiratory problems. This is possibly because the material around synthetic fill has a looser weave. However, effective barrier covers are still needed for both types of fill. 

Vacuuming and cleaning

• Vacuum regularly. Regular and thorough weekly vacuuming doesn't eliminate dust mites, but can reduce the level of dust mite allergen, as long as the carpet is not heavily worn. If your asthma is triggered by dust mite waste - have someone else do it for you.
• Vacuuming upholstery, mattresses and pillows is also recommended.
• Use a vacuum with filter system. Use vacuum cleaners with a micro-filter, an 'S-class' filter, or a HEPA ('High Efficiency Particulate Air') filter system. Machines without these are more likely to recirculate the fine dust mite waste through their exhaust vent, back into the air. If your vacuum cleaner does not have a filter, then open the windows wide when you vacuum, so that allergens not retained in the cleaner and blown into the air are displaced by fresh air. After vacuuming, leave the house to allow the dust to settle for a half hour. Avoid using vacuum cleaners which expose you to the contents when you empty them.
• Damp dust weekly. Use a damp (not wet) cloth. Avoid using feather dusters. The barrier covers on mattresses and bedding (see above) can also be damp dusted once a week, but not removed.

Washing and drying

• Wash sheets and pillow cases in hot water. Dust mites are killed at a temperature of 60 degrees C, which is higher than most domestic machines. Although mites will not be killed by washing in a domestic machine, the mite allergen will be removed by washing at this lower temperature. 
• Regularly wash or dry clean bedding including pillows and duvets - at least once every six to eight weeks. Recent research has shown that prolonged hot tumble drying can reduce house dust mites in duvets and blankets.
• Use the sun to dry washing - direct sunlight kills the dust mite. When doing washing, hang sheets and pillowcases out in the sun to dry. Alternatively, if you use a drier, run it an extra half hour after the contents are dry.

Special measures for young children

In addition, with any young children:

• Soft toys: Put soft toys in the deep freeze for at least 72 hours every four to six weeks, and then wash (to remove the stored allergen) and thoroughly dry - hot prolonged tumble drying will help destroy house dust mites, so washable soft toys that will not shrink in the dryer are recommended
• Sheepskins: Avoid using sheepskins (and especially avoid using them as infant bedding)
• Bunk beds: Do not put young children on a bottom bunk.

On average, we spend a third of our lives in bed. Measures to minimise exposure to dust mite waste are, therefore, especially important in the bedroom.

Related topics

See also Asthma and allergy and Asthma triggers

Original material provided by The Asthma Foundation of New Zealand, 2010. Edited by everybody, December 2010.