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There has been a notable increase in the commonness of allergies in the past decades, and there are multiple hypotheses explaining this phenomenon. This is in part because we know what they are, in contrast to earlier humans who would think that the symptoms pointed towards a non-important illness.
Increasing use of chemicals
One theory is the exponential use and abuse of chemicals in affluent nations since the second world war. Vast numbers of chemicals are introduced into our indoor and outdoor environments with little or no testing[citation needed] regarding their toxicity to living beings. Many believe[attribution needed] that air quality is getting worse rather than better, particularly if one considers indoor air quality as well as outdoor. (Indoor air quality has become significantly worse since building codes changed in the 1970s to make buildings more air-tight to conserve energy. This affects buildings built since that time.) Adverse reactions to toxins vary considerably from one person to another, and can involve extremes in symptoms including the neurological and endocrine systems as well as the more commonly recognized allergy symptoms listed above.
In 2004, a joint Swedish-Danish research team found a very strong link between allergies in children and the phthalates DEHP and BBzP, commonly used in PVC. Allergies are also viewed by some medical practitioners as a negative consequence of the use and abuse of antibiotics and vaccinations. This mainstream Western approach to treatment and prevention of infectious disease has been used in the more affluent world for a longer period of time than in the rest of the world, hence the much greater commonality of allergies there. It is hypothesized that use of antibiotics and vaccination affect the immune system, and that allergies are a dysfunctional immune response.
The "hygiene hypothesis"
Main article: Hygiene hypothesis
The hygiene hypothesis maintains that children in more affluent countries are leading a cleaner and cleaner life in modern times (less exposure to dirt, extra use of disinfectants, etc), their immune systems have less exposure to parasites and other pathogens than children in other countries or in decades past. Their immune systems may, therefore, have many "loaded guns", cells which might have targeted, say, the intestinal worms that no longer cause trouble in affluent neighbourhoods. Having no reasonable target, these cells inadvertently become activated by environmental antigens that might only cause minor reactions in others. It is the symptoms of this exaggerated response that is seen as the allergic reaction.
Many common allergies such as asthma have seen huge increases in the years since WW2, and many studies[citation needed] appear to show a correlation between this and the increasingly affluent and clean lifestyles in the West. This is supported by studies[citation needed] in less developed countries that do not enjoy western levels of cleanliness, and similarly do not show western levels of incidences of asthma and other allergies. During this same period, air quality, at one time considered the "obvious" cause of asthma, has shown a considerable improvement. This has led some researchers[attribution needed] to conclude that it is our "too clean" upbringing that is to blame for the lack of immune system stimulation in early childhood.
Evidence for the hygeine hypothesis appears to be mounting more so then for the chemical hypothesis. Such data is still open to interpretation. Day care for children offers protective effects against asthma, as do early episodes of viral infection. Children raised on a farm also have a decreased atopy. Exposure to endotoxin and other components of bacteria may reduce atopic diseases.
Environmental endotoxin showed a strong protection against hay fever and allergy. Endotoxin exposure reduces peripheral blood leukocytes release of inflammatory cytokines after lipopolysaccharide, including cytokines such as tumor necrosis factor alpha, interferon-gamma, interleukin-10, and interleukin-12 . Toll-like receptors are thought to be involved. This is the basis for the new DNA vaccine being developed by Peter Creticos and others at the Johns Hopkins Division of Allergy.
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