Allergy is on the increase

Allergies are increasing rapidly; in the UK, 1 in 4 people suffers from an allergy at some time in their life.  Studies that have examined individual conditions have confirmed that allergic conditions are increasing at roughly 5 per cent a year.  For example, a study conducted in Leicester for the National Asthma Campaign showed that the rate of asthma and wheezing in under five's years has almost doubled since 1990¹⁰⁰.

There is similar evidence that allergic diseases are increasing in many countries. Allergy seems to be a problem of 'western' civilisations.  For example, Population based studies have shown that there are large geographical differences in the prevalence of allergic disease, with countries such as Britain, Australia, and New Zealand having figures around ten times higher than countries in central and eastern Europe and Asia.  Environmental differences are much more likely than genetic differences to account for these geographical variations. 

The increasing prevalence of allergic disorders has been especially noticeable in the last twenty years (click to view).  It is most noticeable in young people and the available evidence suggest that it is linked to a Western lifestyle.  This evidence points to  changes to maternal and infant diets, exposure to antibiotics in infancy, exposure to indoor air pollutants (especially cigarette smoke) and exposure to other airborne allergens as the most likely culprits.

The Hygiene Hypothesis

The chance of having eczema or hay fever or positive allergy skin tests is higher for someone with few or no brothers or sisters, and lower for someone with more brothers or sisters.  Professor Strachan, who reported this in 1989, thought an explanation for this might be that children with more brothers or sisters have more infections in childhood, and that the infections might protect against allergy.  It appears that when the immune system gears itself up to make antibodies against germs, it promotes a kind of immune system boost that discourages allergy.  This theory has attracted a lot of attention, and has been called the hygiene hypothesis.  As an off-shoot of this idea, scientists are now looking at the possibility of boosting the kind of antibody response we make to germs in order to protect children against allergies in later life.

Why has asthma become so much commoner?

Although the incidence of asthma has been increasing steadily since about 1960, the reasons for this increase are still not clear.  Whilst we know that the more people are exposed to an allergen, the more likely they are to become allergic to it, the rate of increase has occurred at the same rate in geographical locations where quite different allergens predominate as main triggers of asthma.  So it seems unlikely that an increase in the allergens we are exposed to is to blame.

It has also been suggested that the increase may be connected with with the fact that children are now much less likely to be exposed to parasites and major childhood infections (the 'Hygiene Hypothesis').  However looking at the evidence for this, these improvements occurred at least 100 years ago, and it was not for fifty or more years after this that the rise in asthma took place.

Other theories have suggested a variety of other possible causes.  The wide-scale abandonment of breast feeding, the increased us of antibiotics in infancy, changes in dietary habits, obesity, the reduction of outdoor play in children have all been blamed.  Changes in lifestyle are certainly likely to contribute to the increase and it seems certain that the cause is not down to just one factor.

Two interesting findings have emerged from several studies.  Neither is fully understood, although there may be a common explanation.  Living on a farm reduces rather than increases the risk of asthma, and people who live, or have lived, in a house with a cat are less likely rather than more likely to be allergic to cats.  Clearly much more work needs to be done before we will know why asthma has become so much commoner.

Has pollution caused the increase in allergies?

The idea that pollution has caused allergies to increase is not new.  Recent research suggests that there may be some truth in this idea. For example hay fever is commoner in cities than it is in country areas even though the pollen counts may be higher in the country.  However this is likely to be only one of a number of reasons, and more research is needed.

Although the evidence linking pollution to the increase in allergies remains unclear, there is no doubt that certain allergies are more troublesome when allergen exposure is accompanied by exposure to certain kinds of pollution.  The most troublesome found so far is diesel exhaust (diesel particulates) mainly from lorries and buses.  Research indicates that some pollen allergen can be transferred on physical contact from pollen grains to particles contained in diesel exhaust.

Why do only some people get allergies?

Asthma, eczema, hay fever and allergic rhinitis are all allergic conditions that are more likely to develop in someone who already has one of these conditions.  

As a group these illnesses run in families and we now know that there are a number of different genes which, when inherited, cause a person to have a tendency to get any one or all of the conditions; the tendency is called atopy,   People who have the tendency are called atopic.  

However if you have the genes for allergy it does not necessarily mean that you will suffer from allergy.  What is more, inheriting an allergy from someone, does not mean that you will develop the allergy to the same substance(s) that they did. Interestingly, it does not seem that the proportion of the population inheriting the genes for allergy has increased, it appears that those people with these same genes did not have so many troublesome allergies in the past.  So either there have been changes in the environment, or else something that we are doing must be different to have cause this change to occur; so there is great interest in finding out what environmental factors may cause an infant to develop allergy.  We now know that persistence of the pre-disposition to allergy appears to be favoured by changes that have occurred in infant feeding, the lack of exposure to infection (the first-born infant is at greater risk) and the use of antibiotics within the first year of life.  Exposure in the first few weeks of life to a strong allergen such as grass or silver birch pollen or cats, may increase the risk of developing an allergy.

What about the environment?

There has now been quite a bit of research to establish why people with allergy genes actually develop an allergy.  What we are exposed to in the first three to six months of life appears particularly important, in other words, there is a 'vulnerable period' in early childhood. This has given rise to the idea of allergy prevention by avoiding early contact with things which cause allergy.  To give a simple example, children born in the spring or early summer, who are therefore exposed to pollens during their first three months, are more likely to develop hay fever when they are older.

However, at present much of this information is confusing, particularly as far as it affects diet and early feeding, so that as yet no clear guidelines have emerged to guide allergic parents how to have less allergic offspring.