Allergy Triggers

In general, triggers are the things that make asthma or other allergies worse. Avoiding triggers is an important way of controlling asthma.

The most common indoor environmental allergens triggering asthma and other allergic conditions like eczema and rhinitis are listed below.

House Dust Mites

House dust mite allergens are important allergens in asthma and rhinitis. A large proportion of asthmatics are sensitive to house dust mites and their droppings. These allergens are believed to play a role in the development of asthma as well as triggering asthma attacks. Recent research has shown that children exposed to high levels of house dust mite allergen during the first year of life are statistically more likely to develop asthma in childhood or during their teens.

House dust mites are distantly related to spiders. They are almost invisible to the naked eye, being only 0.3mm long. Mites are most commonly found in dark, warm and humid areas of the home, such as bedding, carpets, upholstery and curtains. Their favourite habitat is the bed where, during the night, they thrive in the warmth and humidity from the body and the ready food supply of dead skin cells.

A bed may harbour thousands of mites. Dust mites produce allergens in the form of protease enzymes, which can attack the protective lining of the nose, throat and lungs. Moving around in bed makes the allergens airborne and easy to inhale. This stimulates the body’s immune system, triggering the symptoms of asthma and rhinitis.

Cat dander (hair, skin etc)

It is estimated that around 50% of children with allergic asthma are sensitised to cats. Symptoms due to actual sensitisation to cat allergens may not appear immediately, but take months to develop after initial close contact with the animal. Most people will know if they are allergic to cats, as asthma symptoms can be intense and develop rapidly when they are exposed to cat allergens.

Cat allergens come from the skin, saliva and urine of the animal and are deposited around the home. Lack of ventilation in the home will make exposure worse. Individual cats may produce differing levels of allergens so it is possible to be affected by particular cats and not others. The allergens are very persistent, lingering in houses where no cat has lived for years. They are also very fine and lightweight, so they are easily made airborne and inhaled. Cat allergens may be carried on clothing and so can be found in houses without cats, in schools and other public buildings.


The main pollens that cause allergic reactions like hayfever, rhinitis and asthma are usually grasses or tree pollens but also some weed species such as docks, plantains and ragweed. Each plant species has its own pollen producing-period throughout the year. In Britain in the spring (February to April) tree pollens such as birch and hazel are high, and during the summer (May to September) pollens from grasses and weed species are high. Fifty per cent of asthmatics are allergic to pollens of one sort or another.

Dog dander

Generally, the problems associated with dogs are similar to those due to cat allergens, but fewer people tend to be allergic to dogs.


Moulds send out millions of small spores that are spread in the air and these can trigger allergy symptoms. Indoors, moulds and mildew thrive in areas with increased humidity, such as damp cellars, kitchens, bathrooms and conservatories. They can be seen as black or coloured patches around window frames, refrigerator door seals, on damp walls and on shower curtains or in shower cubicles. They can also be found on stale bread and in the soil of houseplants. They most commonly occur in the home during the winter months when moisture and condensation is highest.

Indoor exposure to mould spores may also result from dry or wet rot. These are often present in old damp buildings such as churches or old halls. Flood-damaged houses may also suffer from serious mould problems while drying out.

You may also be exposed to spores outside during the autumn when moulds and other fungi are producing fruiting bodies such as mushrooms and toadstools.


In some areas in the USA, Japan, Taiwan and parts of France, cockroach skin particles and droppings have been shown to cause asthma and other allergies. In inner city housing in the USA cockroach allergens have been shown to be the major cause of asthma. Generally, allergic reactions to cockroach allergens are found in warmer climates, as the insects do not survive well in colder climates.