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allergy education hong kong

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Nasal allergy, asthma and eczema are common problems. Since these problems are caused by contact with environmental allergens, allergen avoidance is an important aspect in the overall treatment regimen. Common allergens include house dust mite, animal dander, mould spores and pollens. In Hong Kong, house dust mite is the most common indoor allergen.

House dust mites

House dust mites need to absorb moisture in the atmosphere to remain alive. Therefore, their growth rate is directly affected by the relative humidity in the environment. Relative humidity of 55-75%, together with a temperature of 25 degrees is the ideal environment for their growth. Under these circumstances, their life cycle is around 15 days.

The droppings from house dust mites contain high levels of allergens. Since these allergens have enzymatic activities, they can easily penetrate protective layers of the skin and mucous membrane, making them very potent. House dust mites are arachnids and not insects, and are relatives of spiders and fleas. Common household insecticides are ineffective against them.

How to reduce house dust mite allergens?

In order to reduce house dust mite allergens in the environment, we need to reduce the growth of the mites as well as remove the allergens themselves. As mite growth depends on humidity, reducing the relative humidity to below 50% for 5 days can kill the mites.

Allergen-proof cover is a very effective way of reducing allergens. House dust mites mainly live in mattresses, pillows, comforters, carpets and upholstery. As we spend a lot of time in bed, bedding should be covered with an allergen-proof material. The best option is microfiber, as this material is so densely woven that the minute allergen particles cannot pass through, while allowing moisture to pass through to prevent humidity build-up. Do not use materials that cannot be washed, that are not breathable or those that contain chemicals.

In general, allergen-proof covers should be washed every two to four weeks in warm water, and regular bedsheets should be washed weekly at 55 degrees of above. Mattresses should be regularly cleaned with a vacuum cleaner fitted with HEPA filter, and professionally cleaned once or twice a year. Items that cannot be washed can be heated to above 55 degrees in a dryer for at least 10 minutes. Items that cannot be heated can be placed in a household freezer for at least 24 hours to kill the mites, and then washed in cold water. Dry cleaning might not be effective in removing dust mite allergens.

Many people believe that synthetic pillows are less allergenic than feather, but the reverse is true. Studies have shown that feather pillows harbor fewer dust mites because the casing tends to be of higher quality and is more tightly woven. Dust mite allergen particles are between 5 to 10 micrometres in diameter, and the mites themselves are around 20 micrometres. Therefore, microfiber materials with a pore size of 6 micrometres or less should effectively form a barrier against allergen penetration. It is important to have good breathability in order to reduce moisture build-up, especially in humid environments such as Hong Kong. Therefore, avoid using non-woven or too tightly woven materials.

Carpets – the ideal breeding ground for dust mites

We recommend that allergy sufferers avoid using carpets at home. Carpet is the ideal breeding ground for dust mites, and problem is particularly severe in humid environments. If floor covering is needed, choose short pile carpets and rugs and can be cleaned more easily. Although there are some products on the market that claim to eliminate dust mites, there are actually no effective way to kill these arachnids or neutralize their allergens. Therefore, regular deep cleaning is necessary.

Although there are many different types of vacuum cleaners now available, only machines with HEPA filters should be used. Ordinary vacuum bags have spore sizes greater than 10 micrometres, which allow the dust mite particles to pass through into the atmosphere. HEPA filters can remove over 99% of particles down to 1 micrometres in diameter, and can effectively trap dust mite allergens. Water filters are more effective than ordinary vacuum bags, but microbubbles formed on the water surface still allow some dust particles to escape. Articles that cannot be effectively cleaned, such as cushions and woolen clothing, should not be used.

Animal dander

The second most common type of indoor allergen is animal dander, including cats, dogs, rodents, horse and feather. Cat allergen appears to be the most potent. These allergens are produced by glands in the skin and are shed on dead skin and fur. Even in households that do not have pets, animal allergens are often detectable as they are carried on clothing worn by pet owners. Significant levels of animal allergens have been detected in schools, libraries and other public places.

Cat allergens are smaller than dust mite allergens, having a diameter of around 1 micrometres. These allergens therefore can stay airborne for a long time. Therefore, allergy sufferers will react very soon upon arriving in an area where cats are present, even if they do not have direct contact with the animals. Studies from Sweden have shown that the level of cat allergens in a classroom is directly related to the number of students living with cats, and this can lead to severe symptoms in cat-allergic students. Some schools therefore segregate students according to pet ownership.

Studies have clearly shown that patients with animal allergy suffer more severe symptoms if they live with these animals. Removing the animals from the living environment often leads to improvement. However, it this is not possible, we would recommend doing the following:

  • Install HEPA air filters
  • Wash the animal frequently, ideally two to three times a week
  • Make sure vacuum cleaner has been fitted with HEPA filter
  • Minimize use of soft furnishings
  • Regular deep cleaning of soft furnishings

It is best to quarantine the animals to one area to minimize dispersion of the allergens. Do you allow them into bedrooms. Patients should change into clean clothes before going into bedrooms.

Even after the cat has left home, the allergens will remain in the environment for many months. Soft furnishings such as carpets and cushions might never be completely cleared of animal dander. Therefore, consider discarding old furniture and carpets.


15% - 20% of allergic rhinitis or asthma sufferers are allergic to cockroach. It has clearly been shown in clinical studies that cockroach allergens can trigger allergy symptoms. Cockroach allergens are found in the feces, saliva, secretions and molted skin. These become part of household dust and can trigger allergy symptoms when inhaled.

  • Place cockroach traps in common hiding places such as behind stoves and refrigerators, and near drains. Seal cracks in walls and floor to eliminate hiding places.
  • In heavily infested homes, call in professional pest control to exterminate cockroaches.
  • Store away food and clean kitchen and bathrooms of oil stains and soap residues.

The information on this website aims to provide educational purpose only. Anyone reading it should consult physician before considering treatment and should not rely on the information above.