of Allergies) British Medical Journal v316, n7137 (April 4, 1998):1075 COPYRIGHT 1998 British Medical Association (UK) The practice of avoiding exposure to allergens (allergen avoidance) for allergic
diseases such as asthma is not a new idea. In the 16th century the Archbishop of St Andrews had a miraculous remission of his intractable asthma by getting rid of his feather bedding, and in 1927 Storm van
Leeuwen created a "climate chamber" in Holland in an attempt to recreate the beneficial environment of high altitude sanatoriums. This article focuses on how to avoid indoor allergens in homes in
temperate climates and the potential benefits for sensitized patients with a topic disorders.
Nowadays most people spend more than 90% of their lives indoors. Over the past 30 years, the home environment
has changed enormously with the introduction of soft furnishings, fitted carpets, and central heating. Indoor ventilation has . decreased--the rate at which indoor air is exchanged for fresh air is now 10
times lower than it was 30 years ago, with a considerable increase both in humidity and in concentrations of indoor pollutants and airborne allergens. As exposure to allergens is an important cause of
symptoms in sensitised patients, reducing exposure should improve disease control. In spite of this, few patients in Britain with asthma, eczema, or perennial rhinitis, or any combination of these, are skin
tested.
Characteristics of indoor allergens
The predominant indoor allergens in Britain are from mites, cats, and dogs, and they have dramatically different aerodynamic characteristics. Mite allergens
are present on large particles in beds, soft furnishings, and carpets, which become airborne only after vigorous disturbance and settle quickly. In contrast, about 25% of cat and dog allergens are associated
with small particles [is less than] 5 [micro] m in diameter, which after disturbance remain airborne for prolonged periods. This in part explains the difference in clinical presentation between asthmatic
people who are sensitive to mites and those sensitive to their pets. Patients allergic to dust mite may be unaware of the relation between asthma and exposure to mites, as this is a predominantly low grade
chronic exposure occurring overnight in bed. Patients allergic to cats or dogs may develop symptoms within minutes of entering a home with these animals or simply by stroking an animal, as a result of
inhaling large amounts of easily respirable cat and dog allergen.
For symptoms to occur, atopic patients need to be exposed to allergens to which they are sensitised. Allergen avoidance should be
recommended only to those symptomatic individuals who are sensitised on the basis of skin tests or specific serum IgE concentration.
Mites
Lessons from high altitude
At high altitude, low levels of
humidity mean that mites cannot survive, and so mite allergen concentrations are low. Asthma control in mite sensitive patients moving to high altitude improves, although full improvement may take 6-12
months to achieve. The real challenge facing physicians in Britain is to create a low allergen environment in patients' homes that is sufficiently flexible to suit individual needs and at the same time not
prohibitively expensive. Many avoidance measures have been tested, but only a few have been subjected to randomised controlled trials.
Controlling concentrations of mite allergens
Bedrooms
The
single most effective measure is to cover the mattress, pillows, and duvet with covers that are impermeable to mite allergens. These covers used to be made of plastic and were uncomfortable to sleep on.
Now fabrics permeable to water vapour (either microporous or polyurethane coated) but also both impermeable to mite allergens and comfortable to sleep on are available. Allergen concentrations decrease by up
to 100-fold after such covers are introduced.
All exposed bedding should be washed at 55 [degrees] C. This kills mites and removes allergen; although the "cold" cycle (30 [degrees] C] of laundry
washing dramatically reduces allergen concentrations, most mites survive it. The covers should be wiped down at each change of bedding.
Buying a new mattress produces only a temporary benefit as
reinfestation may occur within a few months from other reservoirs, such as carpets. Ideally, bedroom carpets should be replaced with sealed wooden or vinyl flooring, and the curtains should be hot washed
regularly or replaced with wipeable blinds. In this way exposure to mite allergens in the bedroom at night can be virtually abolished. Substantial clinical benefit of effective mite avoidance has been shown
in mite sensitised asthmatic patients and in patients with eczema or perennial rhinitis.
Rest of the house
Intensive vacuum cleaning with high filtration cleaners reduces the size of the allergen
reservoir, but no benefit has been established in a clinical trial. Older vacuum cleaners with inadequate exhaust filtration should be avoided as they provide one of the few ways to get large amounts of mite
allergen airborne. Sensitised asthmatic patients who have to use a vacuum cleaner should use one with a built-in high efficiency particulate air filter and also use double thickness bags.
Killing mites
with chemicals (acaricides) is feasible in the laboratory, but convincing evidence of clinical benefit is lacking. Mites can be killed in carpets either with steam or by freezing with liquid nitrogen, but
both techniques are unlikely to produce sufficient benefits to warrant the huge effort required (in addition, regular treatment is necessary). Air filters and ionisers are of no clinical benefit as mite
allergen does not stay airborne.
A final alternative might be to reduce humidity to suppress the growth of mites. This has been attempted by the use of mechanical ventilation and with portable
dehumidifiers. Neither of these techniques reduces humidity levels in typical British houses sufficiently to suppress mite growth, and allergen concentrations have not been shown to be reduced.
Pet allergens
Pet allergens are present in huge concentrations in houses with cats and dogs, but they are also transferred on clothing, so concentrations are detectable in homes without pets and in public
buildings and transport
For an asthmatic patient who owns a pet and is sensitized to the animal, the best way to reduce exposure is to get rid of the pet, but this is rarely feasible as owners are usually
very attached to their pets. Even after permanent removal of a cat or dog from the home, it may take many months before the reservoir allergen concentration returns to normal. So if patients do get rid of
their pets they should not expect their symptoms to improve immediately--it may take 6-12 months for full benefit
If the pet remains in the home, advice can be given about methods that are known to reduce
airborne pet allergens. The pet should be kept out of the bedroom and preferably outdoors or in a well ventilated area--for example, a kitchen. In homes with a pet, the concentration of airborne pet
allergens will be considerable higher when the pet is actually in a room than when it is elsewhere in the house.
Ideally, carpets should be removed, as the concentration of pet allergens can be as much as
100 times higher in carpets than in polished floors. If carpets remain then regular cleaning with a high filtration cleaner is advised. Washing the animal thoroughly and as often as possible, combined with
the use of a high efficiency particulate air cleaner, is the best way to reduce allergens, but whether these practices are clinically effective is as yet unproved.
Allergen avoidance in primary prevention of atopic diseases
Sensitisation to allergens seems to be related to exposure to allergens in early life. The key question is whether early allergen avoidance can
prevent allergic disease developing in the first place.
The innovative Isle of Wight study has examined the effect of avoiding mite allergens and certain foods from birth onwards on the development of
atopy and asthma. This study used acaricides, and hence the reduction in the concentration of mite allergens was relatively modest Even these minor reductions, however, were associated with a reduction in
eczema and episodic wheezing.
Several prospective cohorts of babies at high risk of atopy or asthma have been randomised to more effective protocols for avoiding exposure to mite allergens and are
currently being studied. Population based studies are also being planned.
The future
Exposure to indoor allergens may have contributed to the observed increase in asthma prevalence. The important issue
is whether asthma can be prevented by allergen avoidance in early life. Primary allergen avoidance is premature outside the context of a clinical trial but may be an important aspect of preventive therapy by
the end of this century. Major changes in the domestic environment are likely over the next decade, with the removal of dust mite habitats--that is, a return to hard flooring--a reduction in indoor humidity,
and the encasing of all bed and bedding with mite proof covers.
Concentrations of indoor allergens
* The highest concentration of mite allergens is found in beds; patients spend 6-8 hours every night
in close contact with their mattress, pillow, and bedding, so the reduction of exposure in the bedroom is critical
* Most exposure to pet allergens probably occurs in living areas other than the bedroom,
and this must be taken into account when planning avoidance strategies
* In public areas--for example, cinemas and public transport--mite allergen levels are low, but exposure to airborne pet allergens
can be substantial even in hospital outpatient areas
* Exposure to cockroach allergens may be important in schools and some high rise blocks of flats
First steps in treating atopic diseases
*
Diagnose disease by taking a history and by either performing skin testing or measuring specific serum IgE concentrations
* Minimise the impact of identified environmental risk factors such as mites, cats, and dogs
* Regard allergen avoidance as an integral part of the overall management of sensitized asthmatic patients
The ABC of allergies is edited by Stephen Durham, honorary consultant physician in respiratory medicine at the Royal Brompton Hospital, London. It will be published as a book later m the year.
