Some people become ill after contact with substances which have no effect on most other people. This is called an allergic reaction. Or you can say that the person has 'an allergy' to the substance.

You can have an allergic reaction if you breathe in, eat or have physical contact with the substance you are allergic to. An allergic reaction can show itself in different ways, such as:

• sneezing
• an itchy or runny nose
• red eyes
• breathing problems and coughing
• common skin problems such as hives or eczema

The substances which cause these reactions are known as allergens. Common examples of allergens that affect the lungs include:

• pollen from grass and trees
• house dust mites
• animal fur

Some people - between a quarter and a third of us - have a tendency to develop allergies. This runs in families. However, not all the people with this tendency will actually get allergy problems. 

Common allergies

Some allergies can cause problems all through the year; others, called seasonal allergies, only affect us at certain times.

The most common cause of year-round allergy is the house dust mite. These tiny mites, found in every home and work place, feed off the skin which we shed naturally. They are especially common in bedding, such as pillows and mattresses.

Another common cause of year-round allergy is skin and fur from cats, dogs, horses and other animals. These are not just found in places where animals live. Pet allergens are often found in schools - they are brought in from home by the pupils and teachers.

Some people have allergies because of substances that are found mainly at their work, such as certain kinds of dust.

Allergens that cause problems only at certain times of the year include: pollen from trees (springtime), grass (high summer) or weeds (late summer). Some plants may produce mould spores which can cause allergy in late summer and autumn.

How do allergies affect us?

By far the most common allergy in the UK is to grass pollen: it's called summer hayfever, or 'seasonal allergic rhinitis'. Hayfever usually causes:

• itching
• sneezing
• a runny or blocked nose
• sore eyes
• breathing problems

Levels of pollen in the air vary depending on the weather. They can rise a lot on dry, sunny days.
Perennial rhinitis - the so-called 'permanent cold' - refers to the same symptoms which may occur all year round. This is usually in response to dust mites and animal fur.

Most people with rhinitis find that they feel much better if they regularly use treatments like nose sprays and anti-histamine tablets. Your GP or pharmacist can tell you what will suit you best. Allergies can also trigger asthma, particularly in the young and middle-aged. Many people have both asthma and rhinitis. It is important to recognise and treat both conditions, including how to avoid provoking allergens where possible.

Asthma caused by substances breathed in at work can be serious and long-lasting. Some people find they still have it even after they have left work or the substance is removed. It is important to find out what is causing the problem, and to avoid it if possible. 

Allergens that can cause lung problems are more common in some jobs than others.
Farmers who work with mouldy hay may be prone to 'farmer's lung'. This is a condition
which can be mistaken for an infection because the symptoms are similar. People who handle birds (including those who keep them as a hobby) can get a form of allergy which affects the air sacs in the lungs.

Remember that most people who do these jobs are not likely to have problems. Up to a third of people have a family tendency to get allergies, and some of them may react to the allergens. However, although conditions such as 'pigeon- fancier's lung' are rare, they may cause lung damage that will keep getting worse until you find out what causes the problem, and then avoid it.