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  1. In recent years, the prevalence of asthma and respiratory allergies has increased. The World Health Organisation (WHO) estimates 230 million people now suffer from asthma,[1][2][3] its largest risk factor being the inhalation of substances or particles that cause allergic reactions or irritation of the airways. Common irritants include dust mites, moulds, pet dander and pollen.[4] Contact dermatitis (skin allergy) has also increased, with nickel, latex and chemicals being its main causative agents.[5][6] In Western societies, food allergies have also been on the rise, the most common allergens being milk, eggs, nuts, soy, wheat, fish and shellfish. In fact, it is now estimated that each year at least 30,000 people in the USA require emergency treatment for food allergies, with 150-200 dying as a result.[7] Climate change, environmental pollutants and occupational exposure to specific chemicals also contribute to the development of allergies.[8]  A widely accepted theory termed the ‘hygiene hypothesis’ suggests that the proliferation of allergies is a result of an increasingly ‘hygienic’ world that limits the contact of infant immune systems with microorganisms and bacteria. This could, in theory, prevent them from inducing appropriate responses later in life.[9][10] The theory essentially suggests that our immune systems are no longer challenged in the way they used to be, and thus overreact to allergens that they come into contact with. For example, studies have shown that children who live on farms have a lower incidence of allergy, atopic and non-atopic asthma.[11][12]  The scientific basis of the hygiene hypothesis is related to the balance of two types of white blood cells in our body called T helper 1 and T helper 2 (TH1 and TH2) cells. The balance is changed by bacterial and viral infections, which increase the number of TH1 cells (designed to fight infection) and decrease ‘pro’ allergy TH2 cells (generating allergic response). Whilst this is a widely accepted theory, this hypothesis is now under debate,[13] and it has been contradicted by some studies.[10] One of these studies showed that cleaning the house with bleach did not increase sensitisation to pollen allergens, and in fact, it was shown to protect from the risks of asthma and indoor allergens.[14] To further refute the cleanliness-asthma link hypothesis, other studies found that asthma incidence is decreasing in some Western countries, despite their population is not getting less clean, while in some Latin American countries there are high rates of both infections (due to presumed less cleanliness) and asthma.[12] The increase in food allergies is thought to originate in changes in diet and increased use of antibiotics, which can both alter the bacterial flora within the gut. This suggestion is supported by the fact that studies have shown a link between the bacteria found in the gut and immune responses including allergy development.[15] [16][17][18] Studies have also shown that the exposure to bisphenol A (BPA) and phthalates – substances commonly found in plastics – during pregnancy and childhood can also cause food allergies. One study found that the concentration of BPA in the urine of pregnant women was directly correlated with wheezing in their children at five years of age, and asthma at three, five and seven years of age.[19] Similarly, higher concentrations of phthalates in pregnant women’s urine have been linked to their children’s allergy risk up to the age of two years of age, even though there was no association with phthalate levels in the children’s urine.[20] Less predominant risk factors for allergies include genetic ones. For example, children with one allergic parent are more likely to develop an allergy, and there are several genes and polymorphisms associated with asthma or atopy phenotype.[21] Despite that, it is thought that allergy to a specific factor is not hereditary, and the exact mode of inheritance is not yet understood.[22]  Although doubts have been raised over the hygiene hypothesis, avoiding over-cleaning and the non-essential use of antibiotics may still help to reduce the risks of allergies. Being aware of environmental factors is also a way to reduce the risk of allergy development, especially in children. Avoiding products that contain pesticides, phthalates, bisphenols or other toxic substances whenever possible may assist in decreasing the likelihood of developing allergies. Despite the unclear mechanism of allergies inheritance, they may indeed be hereditary, which means that by saving your own health you also save the health of your children.      [1] WHO. (2016) Asthma Retrieved October, 2016, from http://www.who.int/mediacentre/factsheets/fs307/en/    [2] Masoli, M. et al. (2004) The global burden of asthma: executive summary of the GINA Dissemination Committee report. Allergy, 59(5), 469-478. [3] Bateman ED. et al. (2007) Asthma and allergy - a global perspective. Allergy, 62(3), 213-215. [4] Silvestri M. et al. (1996) Sensitization to airborne allergens in children with respiratory symptoms. Annals Allergy Asthma & Immunology, 76(3), 239-244. [5] Kimata, H. (2004) Latex allergy in infants younger than 1 year. Clinical and Experimental Allergy, 34(12), 1910-1915. [6] Admani S. et al. (2014) Allergic contact dermatitis in children: review of the past decade. Current Allergy and Asthma Reports, 14(4), 421. [7] FDA. (2016) Food Allergies: Decreasing the Risk. Retrieved 24 October, 2016, from http://www.fda.gov/ForConsumers/ConsumerUpdates/ucm089307.htm [8] Bush, R., & Peden, D. (2009). Advances in environmental and occupational disorders in 2008. Journal Of Allergy And Clinical Immunology, 123(3), 575-578. http://dx.doi.org/10.1016/j.jaci.2009.01.062 [9] Okada H. et al. (2010) The ‘hygiene hypothesis’ for autoimmune and allergic diseases: an update. The Journal of Translational Immunology, 160(1), 1-9. [10] Brooks, C. et al. (2013) The hygiene hypothesis in allergy and asthma: an update. Current Opinion in Allergy and Clinical Immunology, 13(1), 70-77. [11] Reidler J. et al. (2000) Austrian children living on a farm have less hay fever, asthma and allergic sensitization. Clinical and Experimental Allergy, 30(2), 194-200. [12] von Mutius E. et al. (2010) Farm Living: Effects on Childhood Asthma and Allergy. Nature, 10, 861-868. [13] Yazdanbakhsh, M. et al. (2002) Allergy, parasites, and the hygiene hypothesis. Science, 296(5567), 490-494. [14] Nickmilder, M., Carbonnelle, S., & Bernard, A. (2007). House cleaning with chlorine bleach and the risks of allergic and respiratory diseases in children. Pediatric Allergy And Immunology, 18(1), 27-35. http://dx.doi.org/10.1111/j.1399-3038.2006.00487.x [15] Frei, R. et al. (2012) Microbiota and dietary interactions – an update to the hygiene hypothesis? Allergy, 67(4), 451-461. [16] Maslowski, KM. et al. (2011) Diet, gut microbiota and immune responses. Nature, 12, 5-9. [17] Stefka, AT. et al. (2014) Commensal bacteria protect against food allergen sensitization. Proceedings of the National Academy of Sciences of the USA, 111(13145-13150). [18] Berni Canani, R. et al. (2015) The role of the commensal microbiota in the regulation of tolerance to dietary allergens. Current Opinion in Allergy and Clinical Immunology,15(3), 243-249. [19] Donohue, KM. et al. (2013) Prenatal and postnatal bisphenol A exposure and asthma development among inner-city children. Journal of Allergy and Clinical Immunology,131(3), 736-742. [20] Stelmach, I. et al. (2015) The effect of prenatal exposure to phthalates on food allergy and early eczema in inner-city children. Allergy and Asthma Proceedings, 36(4), 72-78.   [21] Vercelli, D. (2008). Discovering susceptibility genes for asthma and allergy. Nature Reviews Immunology, 8(3), 169-182. http://dx.doi.org/10.1038/nri2257 [22] Los, H., Postmus, P. E., & Boomsma, D. I. (2001). Asthma Genetics and Intermediate Phenotypes: A Review From Twin Studies. Twin Research, 4(2), 81–93. http://doi.org/10.1375/twin.4.2.81