Indoor allergy (perennial allergic rhinitis)

Page last reviewed: 13/07/2011

An indoor allergy is when something in your home or work environment triggers an allergic reaction that causes the lining of your nose to become inflamed and swollen. This can cause cold-like symptoms such as:

  • sneezing,
  • a runny nose, and
  • nasal congestion.

The symptoms of an indoor allergy can range from mild to severe. While an indoor allergy is not a life-threatening condition, it can have a considerable impact on quality of life. It can also affect sleep, daily activities and performance at work or school.

An indoor allergy is also known as perennial allergic rhinitis. The word perennial is used because the symptoms can occur all year round, unlike the seasonal types of allergic rhinitis, such as hay fever, which is caused by an allergic reaction to certain types of pollen.


Anything that can cause an allergic reaction is known as an allergen. The most common triggers of an indoor allergy are:

  • dust mites (tiny creatures found in almost all homes),
  • fungus spores,
  • pets, in particular the dead skin that is shed by pets, known as animal dander, and
  • substances related to particular occupations or workplaces, such as wood dust and chemicals.

How common are indoor allergies?

Indoor allergies are very common. It is estimated that 10-20% of the population has an indoor allergy. Most indoor allergies first develop during childhood or early adulthood, with 80% of cases developing before a person is 20 years old. Men and women are equally affected by indoor allergies.


It is recommended that most people with a mild to moderate indoor allergy use medication to control their symptoms and, when possible, limit their exposure to the offending allergen.

In some cases, the symptoms of an indoor allergy can be reduced by using a treatment called immunotherapy. This involves an affected person being exposed to small amounts of allergen over time so that their body builds up a tolerance to the allergen.

The main disadvantage of immunotherapy is that it is time consuming, taking three to five years to complete. It is also not suitable for everyone.

Page last reviewed: 13/07/2011

The symptoms of an indoor allergy usually have two phases:

  • the early phase, which occurs minutes after exposure to the offending allergen, and
  • the late phase, which occurs four to eight hours after exposure.

The early phase

The early phase symptoms of indoor allergy include:

  • sneezing,
  • runny nose,
  • blocked nose (nasal congestion),
  • itchiness and irritation of your eyes, nose and throat,
  • facial pain, and
  • headache.

The late phase

The late phase symptoms of indoor allergy are similar to those of the early stage, except that less sneezing and itching occurs. The symptoms of nasal congestion also get worse and you may find yourself producing a lot of mucus.

In a small number of people, the nasal congestion associated with indoor allergy is so severe that they lose some of their sense of taste and smell. The late phase can persist for hours or days.

Over time, repeated allergic reactions can lead to more generalised symptoms such as:

  • fatigue,
  • feeling very sleepy, and
  • a general sense of feeling unwell.

Page last reviewed: 13/07/2011

Indoor allergies are caused when the immune system, the body's natural defence system against infection, reacts to allergens as if they were harmful.

The immune system produces antibodies to fight off the allergen. Antibodies are special proteins in the blood which are created by the immune system to fight off harmful viruses and infections.

When small particles of an allergen come into contact with the inside of your nose and throat, an antibody called Immunoglobulin E (IgE) is released. IgE causes other blood cells to release more chemicals (including histamine) which, together, cause the symptoms of an allergic reaction.

Histamine causes most of the symptoms that occur during an allergic reaction, such as:

  • swelling of the mucus membrane, the inside layer of your nose, which blocks the airway and causes congestion, and
  • the production of excess mucus, which occurs as a result of the swelling and causes a runny nose and sneezing.

It is still unclear why people develop over-sensitivity to allergens. However, there seems to be a genetic link (it may run in families). Exposure to cigarette smoke during childhood is also a risk factor.

Indoor allergens

An indoor allergy is triggered when you breathe in small particles of an allergen. Some common causes of indoor allergies are described below.

House dust mites

House dust mites are tiny insects that feed on the dead flakes of human skin. They can be found in:

  • mattresses,
  • carpets,
  • soft furniture,
  • pillows, and
  • beds.

Indoor allergies are not caused by the dust mites themselves, but by a chemical that is found in their faeces (stools). Dust mites are present all year round. It is estimated that one square metre of carpet could contain around 100,000 mites. Their numbers usually peak during spring and autumn, so your symptoms may be more troublesome during these times.


Household pets can often trigger an indoor allergy. Dogs and cats are the most common cause, although some people are affected by rabbits, guinea pigs, hamsters and birds.

Animal fur and feathers are not responsible for allergic reactions, which are caused by flakes of dead animal skin and the animals' urine and saliva.

Some indoor allergies can be triggered by an infestation of pests, such as mice or cockroaches.

Fungal moulds and spores

Fungi are plant-like organisms. Unlike plants, they cannot produce food using sunlight (photosynthesis). In order to feed, fungi break down natural materials, such as wood. They do this by growing into the material. These growths are known fungal moulds.

Fungal moulds can grow anywhere inside your house where there is natural material, such as wood, paper, food, plants and some types of insulation.

Fungi particularly thrive in warm, damp areas, such as:

  • the bathroom,
  • the kitchen,
  • around pipes and boilers, and
  • in basements.

The moulds themselves do not cause the allergic reaction, but they can release tiny spores into the air which can be ingested (swallowed), triggering an indoor allergy.

Occupational allergies

A number of allergens that are found in certain workplaces can trigger an indoor allergy. These include:

  • chemicals, which can affect people who work in manufacturing plants and factories, or other areas such as hairdressing,
  • wood dust, which can affect carpenters or joiners,
  • latex, which can affect healthcare workers,
  • animals, which can affect veterinarian and agricultural workers, as well as people who work in laboratories, and
  • flour dust, which can affect bakers and people who work in catering.

Page last reviewed: 13/07/2011

Your GP should be able to diagnose an indoor allergy by asking you about your symptoms and your personal and family medical history.

They will also ask you about any allergens that seem to cause a reaction, and whether the reaction occurs at a particular place or time. If your symptoms only develop when you are at home or at work, it is very likely that they are the result of an indoor allergy.

Physical examination

Your GP may examine the inside of your nose to check for the presence of nasal polyps. Nasal polyps are fleshy swellings that grow on the lining of the nose or sinuses (small air-filled cavities inside your cheekbones and forehead). Polyps are caused by the inflammation associated with an indoor allergy.

Your GP may also recommend that you take antihistamines (a widely used anti-allergy medication) to see whether your symptoms improve. If your symptoms do improve, it is very likely that they are the result of an allergic reaction.

To identify which allergen is causing your symptoms, your GP may recommend allergy testing.

Allergy testing

The two most widely used tests to identify allergens are:

  • the skin prick test, and
  • a blood test.

Skin prick test

The skin prick test is usually the first test to be carried out when trying to determine which allergen is causing your symptoms. The allergens will be placed on your arm and introduced into your skin by pricking it with a short pin. If there is a positive reaction, your skin will become itchy, red and swollen.

Blood tests

Blood tests for indoor allergies involve checking your blood for specific immunoglobulin E (IgE) antibodies.

Page last reviewed: 13/07/2011


The three medications that are most widely used to treat an indoor allergy are:


Antihistamines are usually recommended as the initial treatment for a mild to moderate indoor allergy.

Antihistamines block the action of the chemical histamine, which the body releases when it thinks it is under attack from an allergen. Antihistamines can be taken as a tablet (oral),a liquid oral medicine, as nasal sprays or nasal drops (topical intranasal).

Oral antihistamines are available over-the-counter (OTC) without prescription from your pharmacist. Antihistamine nasal sprays are only available with a prescription.

In the past, antihistamines caused drowsiness, but new types were developed to avoid this side effect. However, around 10% of people are still affected by drowsiness.

If, during the night, your symptoms are particularly troublesome, you may prefer to take an older type of antihistamine because the side effect of drowsiness may help you to sleep better.

If you are taking antihistamines, see how you react to the medication before driving or operating heavy machinery. Antihistamines can cause drowsiness if you take them and drink alcohol.

Oral antihistamines are not suitable for children under two years of age. Antihistamine nasal sprays are not suitable for children under five years of age.

You can take antihistamines if you are pregnant. However, to minimise any risk to your unborn child, it is recommended that you use a nasal spray rather than tablets. Older antihistamines, such as chlorphenamine, loratidine and cetirizine, are preferred to newer medicines because there is more information available about their safety.


If your symptoms are frequent or persistent and you have a nasal blockage or nasal polyps, your GP may recommend that you use a nasal spray which contains corticosteroids (topical intranasal corticosteroids), either as a single treatment or in combination with antihistamines.

Corticosteroids block many of the functions of the immune system and also help to reduce inflammation. They take longer to work than antihistamines, but their effects last longer.

Side effects from inhaled corticosteroids are rare, but can include dryness and irritation of the nose, and nose bleeds.

If you require rapid relief from your symptoms, you may be given a short-term course of oral antihistamines to take in combination with your corticosteroid nasal spray.

If you experience severe symptoms and are worried that they will have an impact on an important event, such as a driving test or exam, let your GP know. They may recommend taking a seven-day course of oral corticosteroids to control your symptoms.

The use of oral corticosteroids on a long-term basis to control an indoor allergy is not usually recommended. Long-term use can cause a range of side effects, such as weight gain and thinning of the skin.

Nasal decongestants

Nasal decongestants are usually available over-the-counter (OTC) in the form of a nasal spray. Only use a decongestant spray for short-term relief of your symptoms. Using decongestant spray for longer than five to seven days may make our symptoms worse, particularly when you stop them, and can cause long term nasal congestion

If you are taking a type of antidepressant known as a monoamine oxidase inhibitor (MAOI), do not use nasal decongestants. The combination of the two types of medication can cause serious side effects.



Immunotherapymay be considered if you have severe and difficult to control symptoms. It involves gradually increasing your exposure to an allergen to make you less sensitive to it. This process is known as desensitisation. It makes you less likely to react adversely to the allergen when you are exposed to it in the future.

Immunotherapy has proved particularly effective in treating people with an allergy to:

  • cats,
  • dust mite,
  • fungal spores, and
  • cockroaches.

As immunotherapy involves a three- to five-year course of treatment, it is usually only recommended if your symptoms cannot be controlled with medication.

Immunotherapy may not be suitable for you if you have:

  • severe asthma,
  • a weakened immune system, either as a result of a condition, such as HIV, or due to a side effect of a medical treatment, such as chemotherapy, or
  • a serious health condition, such as heart disease or cancer.

There are two main ways that immunotherapy can be administered:

  • subcutaneous immunotherapy, where you are given regular injections of the allergen into your skin, and
  • sublingual immunotherapy, where you are given a small capsule containing a sample of the allergen, which you dissolve under your tongue.

Evidence suggests that subcutaneous immunotherapy may be more effective at relieving the symptoms of an allergy, although sublingual immunotherapy may be your preferred option if you do not like having regular injections.

It will usually take between six and twelve months before you notice an improvement in your symptoms.


Immunotherapy carries a small but significant risk of triggering a serious allergic reaction, known as anaphylaxis. Symptoms of anaphylaxis include:

  • an itchy sensation in your throat,
  • rapid swelling of your throat, mouth, lips and face,
  • an itchy, red skin rash that quickly spreads across your body,
  • increasing breathing difficulties due to swelling and tightening of your neck,
  • a sudden intense feeling of apprehension and fear (which has been described as a 'sense of impending doom'),
  • a sharp and sudden drop in your blood pressure, which can make you feel light-headed and confused, and
  • unconsciousness.

Anaphylaxis is a medical emergency that needs immediate treatment with a medication called adrenaline. Dial 999 and ask for an ambulance if you or someone you know experiences symptoms of anaphylaxis.

Due to the risk of anaphylaxis, immunotherapy should only be administered by a healthcare professional with the training and necessary equipment to treat anaphylaxis.

Page last reviewed: 13/07/2011


Sinusitis is a common complication of an indoor allergy that involves inflammation or infection of the sinuses (small air-filled cavities that are located inside the cheekbones and forehead). This can occur if your sinuses become filled with mucus (fluid), which is usually drained away. If the mucus cannot drain away, for example due to a blockage, it may become infected with bacteria.

Sinusitis is a common problem for people with an indoor allergy because allergens can irritate the sinuses, in the same way that they irritate the lining of the nose. There is also a risk of secondary infection occurring because an excess amount of mucus or nasal polyps can stop the sinuses from draining properly.

Common symptoms of sinusitis include:

  • Pain and tenderness of the infected sinus. You may experience a throbbing pain, which is worse when you move your head, and toothache or pain in your jaw when you eat.
  • A blocked or runny nose. Your nose may produce a greenish or yellowish mucus. If your nose becomes blocked with mucus, the pain and tenderness in the affected area may become worse.
  • A high temperature (fever) of 38°C (100.4°F) or above.

The symptoms of sinusitis can be relieved by using over-the-counter (OTC) painkillers, such as paracetamol, ibuprofen or aspirin. These will help to relieve a headache, high temperature and any pain or tenderness you may have around the infected area.

Children under 16 years of age should not take aspirin. Ibuprofen is not recommended for people with asthma or those who have (or have had in the past) stomach conditions, such as ulcers. Speak to your GP if you are not sure about this.

Antibiotics may be recommended if you develop a secondary infection in your sinuses. If your sinusitis remains a long-term problem (chronic sinusitis), you may require surgery to improve the drainage of your sinuses.

Quality of life

The adverse effects of an indoor allergy on a person's quality of life are often underestimated. For example, in a survey, 30% of people with allergic rhinitis said that the condition had an adverse effect on their home, work and social life.

Children may experience problems at school if their sleep is disrupted by an indoor allergy.

Visit your GP if you have an allergy and you feel that your quality of life is suffering, or if you are worried about your child's performance at school. You or your child may need to have your treatment plan reassessed.

Page last reviewed: 13/07/2011

The best way to prevent an allergic reaction is to avoid any exposure to the offending allergen. In reality, this may not always be possible.

The following advice may help you limit your exposure to the known offending allergen as much as possible.

Dust mites

Dust mites are probably the hardest indoor allergen to avoid. Keeping your house free from them is difficult, requires long-term commitment and can be very expensive.

There is also limited evidence to suggest that eradicating dust mites from your home is an effective way of reducing or preventing symptoms of an indoor allergy.

If you want to reduce the number of dust mites in your home, you may find some of the following techniques useful.

  • Put down wood or hard vinyl floor coverings instead of carpet.
  • Instead of curtains, fit roller blinds which can be easily wiped clean.
  • Remove cushions, soft toys and other upholstered furniture. Soft toys can also be put in a bag in a freezer for 6 hours every 1-2 weeks to eradicate the dust mite.
  • Use synthetic pillows and acrylic duvets instead of woollen blankets or feather bedding.
  • Wash bedding weekly using a hot wash of 55-60°C (131-140°F).
  • Use a vacuum cleaner that is fitted with a high efficiency particulate air (HEPA) filter, which can remove more dust than an ordinary vacuum cleaner.
  • Wipe surfaces using a damp, clean cloth as dry dusting can spread the allergens further.
  • Dust inside cupboards as dust can settle on the objects inside.


Giving up your pet is the most effective way to prevent exposure to this type of allergen. This can obviously be a difficult decision for many people to make.

If your personal circumstances prevent you from permanently removing your pet from the house, you may find the following advice useful.

  • Keep pets outside as much as possible, or limit them to one room, preferably one without carpet.
  • Do not allow pets in bedrooms because their skin flakes can remain in the air for a long time.
  • Wash pets at least once a fortnight (ideally, twice a week).
  • Install a HEPA air filter.
  • If possible, remove carpets and replace them with wooden or hard vinyl floor coverings.
  • Castration (removal of some or all of the male reproductive organs so that the animal is unable to breed) will reduce the production of allergens by male cats and dogs.

Fungal moulds and spores

The following advice may help you limit your exposure to fungal moulds and spores.

  • Keep your windows closed to prevent spores from coming in from the outside.
  • Use a dehumidifier to reduce the moisture in your house.
  • Install a HEPA air filter.
  • Replacing carpets with wood or hard vinyl floor coverings should help reduce the amount and spread of spores.
  • Regularly inspect high-risk areas for fungal contamination, such as your basement and around pipes, radiators, boilers and air conditioning units. Use a fungicide spray or cream to remove any contamination.

Content provided by NHS Choices and adapted for Ireland by the Health A-Z.

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