Up to 20% of the global population suffers from allergic symptoms: An estimated 1.3 Billion people!
The number of people with an allergy increases every year, particularly among children. This increase is caused by a combination of factors. A common theory is that we pay so much attention to hygiene in our lives that our immune system is insufficiently 'stimulated', while this stimulation is necessary to 'harden' the immune system. A great many factors appear to be involved, the most important of which are discussed below.
The strong increase in hygiene in our modern society appears to play an important role in inducing allergies. At birth, our immune system is still entirely 'pristine'. As it becomes exposed to all manner of foreign and harmful agents while the body grows, it is activated and strengthened. Of course, our resistance is not stimulated nearly as much as it used to be in the past, as we are living in much cleaner surroundings. It is increasingly likely that the more 'hygienically' children are raised, the more prone they are to developing an allergy. A good example are children who grow up on farms: they generally have very few allergies at a later age.
Air pollution is often mentioned as a key cause of the rapidly increasing incidence of allergic symptoms. However, polluted air does not so much seem to cause allergic symptoms, it rather exacerbates them. Polluted air irritates the airways, increasing the likelihood of allergic reactions. Pollen floating in polluted air adheres to the soot and dust particles, causing their structure to change and enhancing their allergenic action. This increases the likelihood of more serious hay fever attacks.
Where you live and in what conditions, has great influence on your chances of developing an allergy. If you live in a warm, fairly damp house, you create an agreeable climate for house dust mites, increasing the odds of an allergic reaction. Of course, smoking is known to be detrimental to our health. If you are allergy-prone, you had better consider giving up smoking, as smoking is often the last straw. It accelerates the development of allergic reactions, partly because it makes the airways more sensitive to allergens. Also avoid second-hand smoke (passive smoking).
There is clearly a hereditary factor involved in allergy. For example, if one of your parents has an allergy, you will have a higher risk of contracting an allergy (20-40%) than if both your parents are non-allergic. If both your parents are allergic, this risk becomes even higher (40-75%). However, a hereditary predisposition alone does not mean that you will actually develop allergic symptoms, as more factors are involved.
Most allergies manifest themselves between ages 15 and 25. An exception is cow's milk allergy in small children. The earlier one develops allergic symptoms, the higher the risk of developing more allergies later in life. On the other hand, some people grow out of an allergy. In many people, allergies diminish after the age of 40.
However, allergic symptoms appear to manifest themselves in an increasing number of older people. The causes of this trend are not yet entirely clear. What we do know is that moving houses a lot may have an impact. In the old days, it was not nearly as customary to move once every few years. Every time you move to another place, the body is exposed to substances with which it is not yet familiar.
Allergic symptoms are more common in men than in women. For every 100 men aged around 20 with an allergy, there are about 80 women. The difference disappears with age. Women and men are not equally sensitive to certain allergens. For instance, house dust mite allergy is more common among men, whereas women are more prone to hypersensitivity reactions to cats and tree pollen.