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So why has this dramatic change in the frequency of these allergic conditions occurred?
To answer this we need to consider what predisposes humans to develop allergies in the first place. One essential pre-condition is to be born with an inherited tendency for allergy. This is coded in our genes and inherited from our parents. This allergic tendency is called atopy and someone with the predisposition is known as an atopic person. However, it is certainly not guaranteed that an atopic person will always go on to become an allergic person. So has the increase in allergy been due to a change in our inherited predisposition or from a change in our environment? The answer is fairly obvious if we consider that genes do not change easily or quickly. The abrupt rise in allergy in the last fifty years would have required an equally abrupt change in our genes. This is quite impossible, and so the only valid explanation is that the increase in allergy is caused by environmental (including life style) changes. So it is not that more people have become allergy-prone, more allergy-prone people have become allergic because factors in the environment have added together to cause it. Until the end of the 19th century, agriculture and other outdoor occupations were the norm, houses offered very basic comfort and levels of hygiene were very poor. Allergy was rare and mostly affected people from the privileged classes. With the advent of the industrial revolution in the 19th century, life changed dramatically and irreversibly. People abandoned agriculture in favour of industry which was expanding and offering more employment. The population became more affluent and levels of personal comfort and hygiene improved. Housing characteristics also changed. Houses became better sealed with insulated windows and constant heating to maintain a temperature above 18-20 degrees Celsius. People mainly worked indoors and mechanisation reduced the level of physical activity. Hobbies also changed, as people abandoned physical pursuits in favour of sedentary activities. There was also a major shift in the composition of our diet. Huge increases in the consumption of sugar, salt and commercially modified fats occurred as people replaced home prepared natural foods with commercially produced convenience foods. Major changes in transportation also occurred as cars trains and planes replaced the less efficient, but also less pollutant, animal transportation. All these changes started at the end of the 19th century, but became more prominent in the second half of the 20th century, coinciding precisely with the period when the rise in prevalence of the allergic diseases occurred. Scientists are now convinced that amongst these life style changes there are allergy-inducing influences whilst, at the same time, many protective influences have been lost.