The Australasian Society of Clinical Immunology and Allergy

By Professor Ronald S. Walls

Department of Immunology and Allergy, Concord Hospital, Concord, NSW 2139

1. Exposure to allergen drives allergic diseases in two ways:

(i) It is responsible for its development.
Atopy, the tendency to produce specific IgE in response to common environmental allergens, is a very strong risk factor for asthma. Up to 90% of asthmatic children are atopic (1).
The presence of asthma correlates with the degree of exposure to allergens during the early stages of life and asthma and allergic rhinitis persist with allergen exposure.

(ii) Symptoms are precipitated by allergen exposure.

The implication is that proper management of allergic subjects requires
that potential allergens be identified and removed from the environment.

2. The aim of allergen avoidance is the prevention of allergic disease and the reduction of symptoms in those in whom it has already developed.

Prevention can be defined as:
Primary prevention. In infancy the aim is to prevent sensitization when the immune system is immature. The evidence is good that this will prevent or delay the development of allergic disease. Such measures are most likely to be of benefit in high-risk families (i.e. with one or both parents being atopic).
In adults the risk of sensitisation can be reduced by advising atopic individuals to avoid careers which expose them to high levels of allergen, for example handling animals, to select houses which are well-ventilated, dry, with hard floor coverings, and to choose garden plants which are not highly sensitizing (2).
Secondary prevention. This is late intervention in patients who already have allergic disease, the aim being to minimize the precipitation of symptoms and to improve the underlying inflammatory pathology of the disease.

3. Evidence that allergen avoidance is effective.

Many studies show that strict avoidance is effective in reducing the severity of asthma and allergic disease and the need for medications. Asthmatics kept in environments free of dust mite for long periods of time improve their symptoms, but significantly, bronchial hyperresponsiveness also improves. The big question is whether these measures can be transferred to real life situations and applied in a practical and cost-effective manner.
A meta-analysis of such studies reported in 1998 (3) purported to show that allergen avoidance was ineffective. However, this study was flawed in that it included studies where allergen reduction was not achieved, and other effective studies were excluded. In five of six studies where allergen exposure was documented for six months or more, there was significant improvement in the active group (4).

4. Avoidance of indoor allergens (see Table).

The most important indoor allergens are house dust mite, cat, cockroach and mould.
The principles of allergen reduction are to remove as much allergen as possible, and in the case of house dust mite, enclose residual allergen in mattresses and pillows, using impervious covers so that subjects cannot come into contact with it.

House dust mite thrives in an even, moderate temperature, and is entirely dependent on humidity for survival. The major allergens are present in the numerous faecal pellets which at 5microns in diameter, remain suspended in the atmosphere and are of ideal size to be inhaled.

Cats are a potent source of allergen. Particles are even finer and remain airborne and on furnishings and clothes long after the cat has been disposed of. They can also be readily transmitted from one environment to another on clothing and so may be present in schools even though there are no cats in the vicinity. No breeds of cats or dogs are free of the potential to cause allergies. Cats, dogs, rabbits and other furry pets should be banned from the home, which is easier said than done. A compromise is to insist that they do not enter bedrooms, and ideally are kept out of doors. Washing a cat twice a week has been suggested but it is not very effective in reducing allergen loads.

Cockroaches are an important allergen in low socio-economic groups living in inner city areas of the United States, but their importance as allergens in Australia is not yet clear. Their dried remains are present in house dust.

Moulds are both indoor and outdoor allergens. One must ensure that houses are well-ventilated, and that there are not water leaks through walls, ceilings or floors. Bathrooms, shower recesses, closets, gutters, roofs and eaves should be checked. Install extractor fans, remove obvious mould from window-sills and bathrooms and remove pot plants.

Carpets are reservoirs for dust mite and their food, and maintain an optimal environment for dust mite growth. They also accumulate other allergens such as cat, cockroach and mould allergens (the latter particularly when damp). Until vacuum cleaners became available in the mid-1930's carpets used to be removed for cleaning or storage. Vacuum cleaners do not remove all allergen and particles and their value is uncertain. Another problem with them is the dispersion of allergen through the exhaust. Machines should be fitted with HEPA exhaust filters. Water filters are not recommended because they may produce an allergen aerosol. The most cost-effective measure is to remove the carpet.

Acaricides based on tannic acid or benzyl benzoate are commercially available. They reduce allergen loads only temporarily and need to be repeated every six months. They do not replace the need for physical cleaning.

Mattresses are a good home for dust mite. They should be vacuumed regularly, but can also be enclosed in an impervious envelope. Plastic bags were used originally. They still work well and are cheap. Modern fabrics are available which allow air passage but exclude mites. See appendix.

Pillows should be encased in dust-mite impervious fabrics. Until recently it was believed that feather or down pillows and doonas were contra-indicated because of their potential for being allergenic and for harbouring dust mite. However, recent studies have shown lesser amounts of house dust mite in feather than in synthetic pillows. The reason may be that the fabric encasing the contents is much more substantial to hold in the feathers or down, and therefore more of a barrier to other allergens.

Synthetic bedding has the advantage of being able to be washed. Water must be over 550C to kill dust mite. An alternative is to pre-soak for at least 30 minutes in a washing machine with cold or warm water to which is added a mixture of 100ml eucalyptus oil and 25ml hand dishwashing detergent. This approach, however, may flare eczema, which often coexists. Four consecutive hours of sunlight kills house dust mite. Bedding should be hung out frequently and carpets be placed with the back facing the sun out of doors.

People who are building new homes or renovating have an excellent opportunity to adopt building practices which limit allergen build-up in their homes. Measures include using hard-coverings for floors, ensuring good ventilation throughout the house, including the under-floor area. Air-conditioning may be helpful in controlling house dust mite populations, if it is effective in reducing humidity. It should incorporate an efficient air filtration system. A Breathe-Easy^TM Home Design CD is available from the Asthma Foundation of Victoria (See appendix).

Some patients report that electrostatic air purifiers and precipitators help their symptoms, although objective trials have been unable to demonstrate efficacy.

5. Avoidance measures for other allergens.

Pollen can travel great distances on wind and it may appear a hopeless task to avoid them, but there are some simple measures which can reduce exposure to them (see Appendix).
It may not be possible to put into practice all these measures, but at least some of them will be effective in reducing allergen exposure.

6. Cost-effectiveness of allergen avoidance.

Patients are becoming more dissatisfied with the need for continuing drug therapy, and are seeking other ways of controlling disease. Some asthma studies have shown that allergen avoidance reduces the need for medications.

The advantages of allergen avoidance are:

• It is safe and without side-effects.
  
• It decreases the amount of medications required. This can lead to significant financial savings.
  
• It improves efficacy of medication.
  
• It is safe in pregnancy.
  
• It may reduce the risk of development of allergic disease in young children.

Allergen avoidance measures often require a change in behaviour patterns. This is easier to achieve if patients are told that it has been shown to be an effective adjunct in patient management.

7. References.

• Sears MR, Burrows B, Herbison GP, Hewitt CJ, Holdaway MD. Atopy in childhood. II Relationship to airway responsiveness, hayfever and asthma. Clin Exp Allergy 1993;23:949-56.
  
• Bass DJ. The Low Allergen Garden. Available from The Asthma Foundation of New South Wales. 1995.
  
• Gotzshe PC, Hammarqvist C, Bur M. House dust mite control measures in the management of asthma: meta-analysis. BMJ 1998;317:1105-10
  
• Platts-Mills TAE, Chapman MD, Wheatley LM. Control of house dust mite in managing asthma: conclusions of meta-analysis are wrong (letter). BMJ 1999;318:870-1.

8. Appendices.

Useful addresses:
ASCIA Education Resources (AER): A comprehensive range of patient information material is available. ASCIA web site: http://www.allergy.org.au/. 
This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

Breathe-Easy^TM Home Design CD is available at cost of $15.50 plus postage and packing, from Asthma Foundation of Victoria.

Multiple Choice Questions

Which of the following statements regarding house dust mite is/are true?

1. They cannot survive in a dry environment
   
2. They are destroyed by household insecticides
   
3. Measures to reduce dust mite populations are clinically effective
   
4. By killing house dust mite, their allergenicity is destroyed
   
5. Regular vacuuming removes house dust mite from carpets

Correct: 1,3

Which of the following statements regarding pollen allergens is/are correct?

1. Australian native bushland vegetation is a major source of allergen
   
2. Insect-pollinated plants are the major source of pollen allergens
   
3. Pollens are in low numbers in the atmosphere immediately after rain
   
4. Most plants release pollens in the early morning
   
5. Exposure to pollens can be reduced by keeping car windows closed

Correct: 3,4,5

Which of the following statements is/are correct?

1. Exposure to cigarette smoke in early life predisposes to development of asthma later
   
2. Exposure to dust mite in infancy is a risk factor for later development of asthma
   
3. Removal of house dust mite allergens from the environment does not affect underlying allergic inflammation
   
4. Feather pillows are a greater source of allergen than synthetic pillows
   
5. Cat allergen is no longer detectable within houses by one month after the cat has been removed.

Correct: 1,2

Which of the following statements is/are true?

1. Cat allergen can be responsible for precipitating allergies in susceptible subjects in school classrooms
   
2. House dust mite levels in school classrooms often exceed those found in lounge-rooms of homes
   
3. Prolonged avoidance of inhalant allergens can lead to reduced bronchial hyperresponsiveness
   
4. Exposure to high levels of cat allergen can lead to protection from cat allergy
   
5. Appropriate house design can reduce allergen levels in homes

Correct: 1,3,4,5