Effective allergen avoidance reduces exposure to allergens.^{54-57 [LE II]}

In sensitised individuals with asthma this can lead to reduced asthma symptoms and reduce the requirement for medications.

Measurement of dust mite allergen

Tests that measure environmental dust mite levels are available. However, evidence of their clinical usefulness is yet to be demonstrated. A threshold level of 2ug/g Der p1 of dust for mite sensitisation and 10ug/g Der p1/g for development of acute asthmatic symptoms has been proposed.⁵⁸ Average Sydney mattress levels vary widely but are in the vicinity of 30ug/g, which is fifteen and three times the threshold level respectively for sensitisation and development of asthma.

House dust mite

House dust mites, mostly Dermatophagoides pteronyssinus(Dp) and Dermatophagoides farinae(Df):

• are the major allergens associated with asthma in Australia
• are microscopic mites that live indoors and feed off human skin scales
• thrive in temperate and humid climates such as coastal Australia
• produce allergen particles greater than 10uM – these particles require vigorous disturbance such as vacuuming to become airborne, and settle after 20-30 minutes.

Exposure of people with asthma who are mite allergic to high levels of dust mite allergen has been shown to increase:

• airway reactivity
• the risk of having current asthma symptoms
• the risk of hospital readmission in children admitted for an asthma exacerbation.⁵⁸

House dust mite avoidance

House dust mite avoidance measures should only be recommended to patients with proven sensitisation to house dust mite. They aim to:

• kill house dust mites
• remove the allergens they produce
• eliminate places where mites can live and breed.

A combination of measures with these aims is recommended.⁵⁹

The greatest exposure to house dust mites occurs in domestic homes, predominantly in the bedroom from beds and bedding. Therefore the bedroom should be the first priority.

1 The bedding

• Cover mattress, pillow and quilt with dust mite resistant covers.^{59 [LE II]} The covers must be washed every 2 months.
• Some health funds may provide a rebate for purchase of these items. If covers are not available, wash blankets and non-encased washable doonas at least every 2 months using the method described for sheets.^{60-62 [LE II]}
• Wash sheets and pillow cases weekly in hot water (>55˚C). This will kill mites and wash away the allergen they produce.⁶³ Alternatively, if washing in cold water, use a commercial product containing essential oils such as tea tree or eucalyptus oil, formulated to kill dust mites.^64,65 If washing normally, hot tumble drying of washed items for 10 minutes after they are dry will kill mites.⁶⁶ Dry cleaning is not as effective as these methods, as it will kill house dust mites but not remove the allergen they produce.
• Remove sheepskin or woollen underlays.^67,68
• Remove all soft toys from the bed and bedroom. Replace them with wooden or plastic toys which can be washed. If a soft toy is allowed it should be washed weekly using the method described for sheets. Freezing soft toys overnight will kill mites but won’t remove allergen.

2 Other measures

• Damp dust or use electrostatic cloths to clean hard surfaces (including hard floors) weekly.^69,70
• Vacuum carpets weekly. Vacuuming increases the amount of house dust mite allergen in the air for up to 20 minutes. So, if possible, ask someone else to do the vacuuming and wait for 20 minutes before re-entering the room. HEPA (high efficiency particulate air) filter vacuum cleaners may remove more allergen than other vacuum cleaners. However, they still increase the amount of mite allergen in the air.
• Consider replacing carpets with hard floors such as wood, tiles, linoleum or concrete, where practical and affordable. Carpets can contain large amounts of house dust mite and animal allergens which cannot be completely removed by vacuuming.^{71 [LE II-IV]} Although removing carpets will reduce allergen exposure, the benefit on asthma control is yet to be conclusively proven.
• Reduce humidity^{72-76 [LE II-IV]} – have a dry and well ventilated house, adequate floor and wall insulation. Avoid evaporative coolers.
• Windows – venetian blinds or flat blinds are easier to clean. Washable curtains or external shutters are other options.
• Consider house dust avoidance measures when building a new home.

It is important to note that:

• A combination of bedding covers, washing and reduction of reservoirs will reduce mite allergen levels by 90 per cent.⁵⁹
• House dust mite avoidance measures require an ongoing commitment.
• As some mite avoidance measures can be expensive, they may be inaccessible to some patients.

These measures help some but not all mite allergic people with asthma. At present we cannot predict which individuals will benefit. It is also unclear if the reduced levels can be maintained long-term.⁷⁷

A meta-analysis published in 2001 concluded that dust mite avoidance measures were not useful for asthma. However, there are significant concerns regarding the methodology, as measures that did not reduce mite levels were also included in the analysis.⁷⁸

Products that are not recommended

• Sprays (acaracides) – there is limited information on their effectiveness and safety.^64,79,80
• Domestic dehumidifiers have not been shown to benefit asthma control.^{81 [LE III-1]}
• Air cleaning devices – house dust mite allergen is only in the air for a short time after dust-raising activities such as vacuuming.

Animals

In asthma patients sensitised to pets, exposure to pet allergens can be a potent trigger of asthma. This exposure can precipitate asthma symptoms⁸² and/or worsen asthma control.⁸³

Cats and dogs are a major source of allergens in the home environment. The allergens come from the sebaceous glands in cats and the salivary glands in dogs. As all cats and dogs have sweat and salivary glands there are no breeds that do not contain allergens, although the amount of allergen released can vary between breeds.

As allergens adhere to the pet’s own skin or hair, they become airborne for prolonged periods when the pets shed their hair.

Cat allergen is ubiquitous and difficult to eliminate even after removal of the cat.⁸⁴ It is carried from place to place on clothing and can therefore be found in places where cats have never lived, such as schools and offices.⁸⁵

Pet avoidance measures

The most effective method of allergen avoidance for people with asthma who are allergic to cats or dogs is removal of the pets from the home. It can take many months before allergens reduce to low levels.

Other avoidance measures that have been shown to reduce allergens include:

• keeping pets out of bedrooms and living areas
• washing pets^{86-89 [LE III-3 - IV]}
• weekly vacuuming of carpets using HEPA (high efficiency particulate air) filters^{90 [LE III-2]}
• using HEPA air filters.^91-94

Studies have not yet evaluated the direct benefit of these measures on asthma control.

Occupational exposure to rats, mice, guinea pigs, rabbits, cattle and horses is a known trigger of acute asthma attacks for sensitised individuals. Avoidance of the animal may be necessary in cases of occupational asthma.

Some (but not all) recent studies have reported that early exposure to cats or dogs in the home may prevent the development of allergies and asthma. However this is controversial and no conclusions can be drawn from the available data (see page 12). This should not be confused with the clearly documented role of cat and dog allergens in triggering asthma symptoms in patients with asthma.