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Identifying Allergens: Which Present a Risk?


Some researchers are suggesting that in the majority of cases, continued exposure to relevant indoor allergens could be the cause of chronic allergic asthma, and that the criteria for causality have now been met (1). For this reason, it is extremely important for people with asthma to understand which allergens are factors in their disease and to take steps to reduce their exposure to those allergens.

Indoor Allergens

The major indoor allergens of relevance in Great Britain come from house dust mites; cats; dogs; and, less commonly, other furry pets, molds, and cockroaches.

House Dust Mites

Mites are thought to feed either on dead skin or on the fungi that grow on dead skin. They are commonly found wherever dust can accumulate. Typically, this means they can be found in large numbers in mattresses, carpets, cuddly toys, soft furnishings, duvets, and pillows (2).

It is commonly said (although the source of the saying is unknown) that up to 1/10th of the weight of an old pillow could be made up of dead skin, molds, mites, and mite feces.

Dust mites are sensitive to humidity. All their water needs must be met from the air around them, and the water must be taken in across their cuticles (2). House dust mites thrive in conditions of high humidity; at 25 [degrees] C and 75 percent relative humidity, their populations develop most quickly (3). When the level of water vapor in the air is below approximately 7 grams per kilogram (g/kg), populations decrease (4).

Recent research has found that "asthmatic patients attending a hospital asthma clinic were two to three times more likely to live in a dwelling with evidence of dampness than an age- and sex-matched random sample of the general population living in the same area," suggesting that dampness may allow house dust mites or molds to flourish (5).

The main mite allergen, Dermatophagoides pteronyssinus 1, is a digestive enzyme produced by the mite and excreted in dry, brittle fecal pellets (6). The pellets fracture and can easily become airborne. In mite-sensitized individuals, these allergens can cause allergic rhinitis (stuffy or runny nose, particularly in the morning or inside the home); atopic dermatitis; and asthma.

Cat and Dog Allergens

These allergens are mainly found on the animal's fur. They are produced by the sebaceous glands in the skin and, to a lesser extent, in the saliva glands. Cat allergens are small and light and can remain airborne for long periods, which is why cat-sensitive individuals may experience symptoms almost immediately on entering homes with cats (7).

Pet allergens will remain on carpets and soft furnishings for many months, if not years, and the allergens can be detected in areas where pets are rarely found (e.g., hospital outpatient departments), having been carried there on pet owners' clothes (8).


Sensitization to cockroaches is believed to be less common than other allergies in the United Kingdom. Nevertheless, cockroach allergen levels above the proposed threshold for risk of sensitization were found in 65 percent of classrooms (9).


Mold spores are ubiquitous in outdoor and indoor air (10). In one study, 95 percent of patients in whom a skin-prick test was positive for molds also were sensitized to other common indoor allergens (11). Mono-sensitization was found to be significantly less prevalent for molds than for other common allergens. Evidence suggests that sensitization to molds mainly occurs in people who have a high potential for being sensitized to common allergens; however, in this Danish study, sensitization to molds was not associated with living in a damp dwelling (11).

Allergen Avoidance

A recent study in Bristol, England, demonstrated a 50 percent reduction in the use of inhaled steroids in a group of children whose exposure to indoor allergens had been decreased for 24 weeks, primarily through the use of barrier covers (12). That study is one of many that have demonstrated the potential benefits of reducing exposure to indoor allergens. Also, in one of the few studies on primary prevention, a substantial reduction in the frequency of allergic manifestations has been observed when early allergen avoidance is combined with dietary manipulation (13).

Pharmaceutical companies are unlikely to focus on the environmental approach to asthma management. Yet the National Asthma Campaign, in its publication "Purchasing and Providing Asthma Care," points out that 92 percent of general practitioners and practice nurses used patient education materials for asthma and 91 percent obtained those materials from pharmaceutical companies (14).

These educational materials contain scant advice on how to reduce exposure to indoor allergens or improve air quality in the home. This might be the reason for apparently low levels of compliance with allergen avoidance advice. All available research suggests that only 25 percent of children with severe asthma are sleeping in bedrooms that have been altered to reduce their exposure to allergens (15). Their asthma is being controlled solely by medication.

Environmental controls in the home are based on reducing exposure to airborne allergens and generally improving indoor air quality. Pregnant women and parents of very young children - particularly if there is any family history of predominantly allergic diseases such as asthma, eczema, or rhinitis - may also wish to consider carrying out allergen avoidance measures as a precautionary step.

Advice currently available on allergen avoidance is provided in the sidebar below. Interestingly, much of the advice about basic, low-technology measures (related to dust, ventilation, and airing of bedding) was routinely given to all mothers of newborn children earlier this century (16,17). This wisdom appears to have been lost in many cultures since then, only to reappear in the guise of specialist advice on the control of allergic disease (2).


Certain ethnic groups and people on low incomes are known to suffer disproportionate morbidity from asthma (18). This circumstance probably results partly from poor housing conditions. Also, the existing health care system centers around providing people with drugs to manage asthma. Other than social security loans, families on low incomes are denied access to the physical measures required to control allergic asthma, such as barrier covers, better vacuum cleaners, and a dry internal environment.

In Denmark, local authorities offer grants to enable low-income families to pay for environmental controls. The authorities anticipate that, on balance, the grants should save money Grant sizes range from enough to pay for barrier covers (a full set could cost [pounds]100 or more in the United Kingdom) to, in rare cases, the cost of a whole-house mechanical ventilation system with heat recovery (19).

To counteract misconceptions about the disease and help people take preventive measures, better general health promotion work in schools and the community at large is needed on asthma, diet, and indoor pollution. Environmental health officers (EHOs), when being interviewed about "pollution" and asthma, should mention the role of indoor pollution and remind people about the high prevalence of asthma in areas with low levels of outdoor pollution (e.g., the Isle of Skye). Articles in council tenant magazines could ensure that every tenant receives accurate information on asthma and on how to tackle problems with dampness and mold.

At Swindon, EHOs have attended seminars and training courses in hospitals and allergy centers to identify how their work can have a positive effect on asthma. EHOs have urged the housing department to consider the potential adverse impacts of improvement programs on indoor air quality and have suggested the provision of trickle vents in all new UPVC windows, as well as the use of mechanical ventilation with heat recovery for certain properties. The next stage might be to consider including a percentage of low-allergen homes in new developments.

Also, making the planning, transportation, and building control departments aware of the issues may influence improved and new road and housing schemes.

Health Promotion

Expectant parents are extremely unlikely to receive information on how to create a low-allergen bedroom, or on the implications of a family history of the disease. The Health Education Authority's Birth to Five book, given free to all first-time mothers, does not include information on environmental conditions other than temperature. Many antismoking leaflets do not even state that if the mother smokes the child is at increased risk for asthma.

The EHOs at Swindon hope to develop existing links with the health trusts, health promotion, and maternity department. Better liaisons among health visitors, midwives, and environmental health departments would enable advice to be given to parents during routine home and clinic visits. Pages on asthma and allergy in the "red book" (child's health record book) also would be a step forward.

Local asthma task groups should include housing professionals; however, in 1996, it was a struggle just ensuring that local general practitioners followed national guidelines on which drugs to prescribe - let alone getting them to think of the environmental aspects of the disease. Research evidence suggests that many long-term asthmatics return from the hospital to homes full of the very factors that aggravate asthma.

The home environment always should be considered as a way of controlling symptoms in asthmatics, and environmental tip sheets for patient education should be developed.

The environmental health department in Swindon is running a large project researching parents' awareness of the role the indoor environment plays in their children's asthma. The project will report the percentage of parents who have modified the home environment in response to a child's asthma. All children in each infant school intake will be considered during their first routine health assessments.

Funding also has been secured to begin an intervention project. This project will administer skin tests and provide barrier covers for children with asthma who are from low-income families and who do not presently have any allergen avoidance equipment. A local group of the National Asthma Campaign has provided funds to train a local asthma nurse in skin testing. The hope is that local doctors will consider the home environment in more detail.

Although at least 90 percent of schoolage children with asthma are allergic to at least one common indoor allergen, allergy testing is not routinely offered to all school-age children with asthma. In some areas of the country (Swindon being one of them), even children who have been in and out of the hospital with asthma and who are on high-dose steroids are not tested to determine which allergens could be relevant to their disease.

Evidence suggests that local authorities could have an important role to play in reducing the prevalence and severity of asthma through their links with local health trusts and their work on environmental health, social housing, planning, building control, and, in some cases, traffic management. Joint health promotion strategies, focused on smoking, exercise, diet, and breast-feeding, are essential to encouraging long-term health benefits.

(Adapted with permission from Environmental Health, a publication of the Chartered Institute of Environmental Health in London, January 1998.)


Advice for Avoiding Allergens

Key issues that an allergen avoidance leaflet might address are suggested below.

Whole-House Controls

* Ensure adequate background ventilation or supply ventilation to all rooms to reduce indoor humidity and to reduce the concentration of any airborne allergens or other pollutants produced indoors.

* Tackle causes of dampness, mold, and high humidity.

* Provide efficient, affordable heating systems. It is imperative that bottled-gas heaters not be used. Warm-air heating systems may exacerbate asthma by increasing levels of airborne dust; however, there is limited information on the significance of this effect. Do not fit low-allergen housing with radiators that have dust traps at the rear or in the center (double radiators).

* Ensure that the openable windows and background ventilation in every habitable room can be used - even at night or when people are absent from the house - without threatening security or allowing undue heat loss. This is particularly relevant to ground-floor rooms.

* Provide safe clothes-drying areas or subsidized laundry facilities in communal flats or housing blocks to discourage people from drying clothes indoors.

* Consider the use of whole- or part-house ventilation systems with heat recovery. These systems will draw air from the front or rear of the property and pass it through a 90 percent efficient plastic heat recovery cartridge. Clean, warmed, dry air is supplied to bedrooms and living rooms and air is extracted from kitchens and bathrooms. In the winter, humidity is kept below the level needed by mite populations (4). There is a bonus of energy recovery. Pollen filters can be fitted to these systems to provide relief from hay fever symptoms during the summer. Apart from their health benefits, these systems may be helpful in reducing costs associated with treating condensation and mold (23).

Environmental Controls Within Homes, Specifically to Reduce Exposure to Existing Allergens and Indoor Pollutants (24)

* Provide durable, mite-proof barrier covers on high-density mite habitats such as mattresses, pillows, and duvets.

* Hot-wash (above 60 [degrees] C) unprotected pillows, blankets, and duvets to kill mites and flush out allergens.

* Air beds daily with the bedroom window open.

* Regular vacuum cleaning of carpets, soft furnishings, and mattresses often is recommended to people with asthma without any regard to the condition of the person's vacuum cleaner. Some cleaners simply suck up allergens and blow them into the air, thereby increasing exposure; some families on low incomes may borrow cleaners, possibly from neighbors with pets. Powerful high-filtration vacuum cleaners can remove dead mites and loose allergens. It may be possible to fit additional filters to existing vacuum cleaners. The person with asthma normally should not carry out the cleaning; however, an effective dust mask may help reduce their exposure.

* If possible, replace bedroom and lounge carpets with hard floor coverings such as wood or linoleum (7).

* Reduce clutter and soft furnishings in the bedroom and the lounge.

* Cull all cuddly toys that cannot be washed at 60 [degrees] C. Hot-wash the remainder frequently.

* Damp-dust regularly.

* Remove furry pets. If this is impossible, pets should be kept outside, or at least restricted to one room downstairs. They never should be allowed into bedrooms. Washington pets on a weekly basis with plain water will remove much of the allergen from the fur and skin (7). If pets are to remain in the house, it is essential to remove carpets and reduce soft furnishings to lower allergen levels. Once that is done, high-efficiency, particulate air-filtered (HEPA) cleaning may reduce cat allergen levels even further. Housing departments should consider the potential health risks of rehousing families with pet-sensitive individuals into properties that have been vacated by tenants with pets.

* Provide portable dehumidifiers. These may not have the capacity to influence mite numbers, but they will remove many pints of water from the air.

* Consider using high-pressure carpet and upholstery steamers and hot air heat treatments for mattresses. Medivac now offers a commercial carpet steamer capable of denaturing certain allergens and killing mites (25). Sleep safe has produced a patented heat treatment for settees and mattresses, now being used in a large-scale study in Southampton (26).

* Treat any mold that remains, once the cause of the mold has been dealt with.

* People with asthma may wish to consider the relative merits of electric ovens over gas (27).

It may take many weeks for allergen levels to decrease to adequately low levels and even longer for there to be an improvement in symptoms. Total avoidance rarely is possible, particularly as pet allergens accumulate in public places and in schools after being carried there on pet owners' clothes.


1 Custovic, A., and M.D. Chapman (1997), "Indoor Allergens as a Risk Factor for Asthma," Asthma, Philadelphia: Lippincott-Raven Publishers, pp. 1-21.

2. Fain, A., and B.J. Gerin, "Mites and Allergic Disease," Allerbio.

3. Hallas, T.E. (1991), "The Biology of Mites," Allergy, 46(Suppl. 11):6-9.

4. Korsgaard, J., and M. Iversen (1991), "Epidemiology of House Dust Mite Allergy," Allergy, 46(Suppl. 11):14-16.

5. Williamson, I.J., A.G. Fennerty, C.J. Martin, G. McGill, and R.D.H. Monie (1997), "Damp Housing and Asthma: A Case-Control Study," Thorax, 52:229-234.

6. Schou, C., and P. Lind (1991), "The Antigenicity of House Dust Mites," Allergy, 46(Suppl. 11): 1013.

7. De Blay, F., M.D. Chapman, and T.A.E. Plaits-Mills (1991), "Airborne Cat Allergen (Fel. d 1)," Am. Rev. Respir. Dis., 143:1334-1339.

8. Fletcher, A.M., A. Custovic, R.M. Green, C.A.C. Pickering, A. Smith, and A. Wookcock (1995), "Airborne Cat and Dog Allergen in Hospital," Thorax, 50(Suppl. 2).

9. Custovic, A., M.D. Chapman, R.M. Green, C.A.C. Pickering, A. Smith, S.C.O. Taggart, and A. Woodcock (1995), "Cockroach Allergen in Schools in Manchester," Thorax, 50(Suppl. 2).

10. IEH Assessment on Indoor Air Quality in the Home (1996), Leicester: Institute for Environment and Health (IEH).

11. Iverson, M., and R. Dahl (1995), "Characteristics of Mould Allergy," J. Invest. Allergol. Clin. Immunol., 5(4):205-208.

12. Carswell, E, K. Birmingham, A. Crewes, J. Oliver, and J. Weeks (1996), "The Respiratory Effects of Reduction of Mite Allergen in the Bedrooms of Asthmatic Children - A Double-Blind Controlled Trial," Clinical and Respiratory Allergy, 26:386-396.

13. Arshad, S.H., C. Gant, B.W Hide, and S. Matthews (1992), "Effect of Allergen Avoidance on Development of Allergic Disorders in Infancy," Lancet, 339:1493-1497.

14. Purchasing and Providing Asthma Care: A Summary of Good Practice (1995), London: National Asthma Campaign.

15. Strachen, D.P, and I.M. Carey (1995), "Home Environment and Severe Asthma in Adolescence: A Population based Case-Control Study," BMJ, 311:1053-1056.

16. Liddiard, M. (1928), The Mothercraft Manual, London: J. and A. Churchill, pp. 33-35.

17. Hewer, J. (1921), Our Baby: For Mothers and Nurses, London: John Wright and Sons, pp. 72-73.

18. Watson, J.P., and R.A. Lewis (1995), "The Relationship Between Asthma Admissions and Deprivation in a Health District," Thorax, 50(Suppl. 2):A59.

19. Olsen, O.F., R. Elbeck, J. Lund, and H. Mosbech (1991), "Social, Political, Economic and Health Consequences of Environmental Treatment of House Dust Mites," Allergy, 46(Suppl. 11):39-44.

20. Peters, S.E., K. Anderson, and E. Kearns (1995), "Perception of Risk of Air Pollutant Exposure in Patients with Asthma," Thorax, 50(Suppl. 2):78.

21. Personal communication.

22. Duff, A.L., et al. (1993), "Risk Factors for Acute Wheezing in Infants and Children: Viruses, Passive Smoke, and IgE Antibodies to Inhalant Allergens," Pediatrics, 92(4):535-540.

23. Personal communication from ADM Indux (now BAXI Air Management).

24. Colloff, M.J., J. Ayres, P.H. Howarth, et al. (1992), "The Control of Allergens of Dust Mites and Domestic Pets: A Position Paper," Clin. Exp. Allergy, 22(Suppl. 2):1-28.

25. Colloff, M.J., T.G. Merrett, and C. Taylor (1995), "The Use of Domestic Steam Cleaning for the Control of Dust Mites," Clin. Exp. Allergy, 25:1061-1066.

26. Personal communication.

27. Jarvis, D., P. Burney, S. Chinn, and C. Luczynska (1996), "Association of Respiratory Symptoms and Lung Function in Young Adults With Use of Domestic Gas Appliances," Lancet, 347:426431.

* Editor's note: Because this paper was originally published in Environmental Health, a publication of the Chartered Institute of Environmental Health, London, England, the references are not consistent with the Journal of Environmental Healths normal style format.
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Author:Ashmore, Ian
Publication:Journal of Environmental Health
Date:Oct 1, 1999
Previous Article:Community Environmental Health Assessment.
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