Risk factors for developing asthma

Public Health Significance of Urban Pests Allergic asthma 11 10 Several determinants have been identified as risk factors for allergy. There is a genetic component, which has been observed in studies of monozygotic and dizygotic twins Marsh, Meyers Bias, 1981; Ownby, 1990; Duffy, Mitchell Martin, 1998; Borish, 1999. In general, studies seem to show that a child with one atopic parent is twice as likely as a child with non-atopic parents to develop atopy, and a child with two atopic parents is four times as likely WHO, 2003. Compared with rural life, urban life has been shown to be associated with an increase in atopy Braback et al., 1994; Ronmark et al., 1999. However, the variation in risk between urban and rural communities are difficult to attri- bute, since lifestyle differences can include many determinants, such as exposure to ani- mals, increased exposure to bacteria, housing type and physical activity, all of which have been evaluated independently as risk factors for allergy Crater Platts-Mills, 1998. While many studies have identified exposure to environmental tobacco smoke ETS as a risk for developing asthma, the relationship between exposure to ETS and allergic sen- sitization does not seem as clear Rylander et al. 1993; Chang et al., 2000; Gold, 2000; Ronmark et al., 1998. The first few years of life are thought to be an important period in the development of sensitization. Several studies have even suggested that prenatal exposure to allergens may influence the immune response Warner et al., 2000; Miller et al., 2001b. The develop- ment of sensitization, as judged by skin test or serum IgE level, has been shown to occur during the early years of life through the teenage years; however, exposure to new aller- gens as an adult can lead to the development of new sensitizations Dowse et al., 1985; Sporik et al., 1990. Exposure to some substances, referred to as adjuvants, can influence the development of sensitization to environmental allergens. Two examples of environmental exposure that are currently being studied for their potentially relevant immunomodulatory effects are bacterial cell wall components and products of diesel combustion. In the 20 th century, increased cleanliness at the community level has generally resulted in lower exposure to bacteria early in life. That this increased cleanliness is responsible for a more allergic population is part of the proposed hygiene hypothesis. Measuring endo- toxin, a lipopolysaccharide from Gram-negative bacteria, has been used to assess exposure to bacteria. Experiments that use animal models have shown that exposure to high levels of endotoxin, in conjunction with an allergen, leads to a non-IgE response, while low doses of endotoxin and exposure to allergens result in an IgE response Eisenbarth et al., 2002. Cross-sectional studies have shown that rural children with high levels of endo- toxin in their bedroom had a lower prevalence of allergic sensitization than those with low exposure von Mutius et al., 2000; Braun-Fahrlander et al., 2002. Also, recent birth cohort studies from two American cities, Boston Phipatanakul et al., 2004 and New York Perzanowski et al., 2006, have found exposure to endotoxin in the home to have a modest inverse relationship to the allergic skin disease eczema in the first two years of life. In urban environments, exposure to diesel exhaust particulates is common. Components of diesel particulates can shift an allergen-specific immune response to an enhanced IgE response. This has been demonstrated in people, where nasal challenges with diesel par- ticulates increased the IgE antibodies to ragweed allergen Diaz-Sanchez et al., 1997. Due to difficulties in measuring diesel in epidemiological studies, few studies have dealt with its associations with asthma. Higher levels of particulate matter and proximity to traffic, which have been used as proxies for exposures that would include diesel exhaust particulates, have been associated with asthma and other respiratory symptoms in seve- ral studies Riedl Diaz-Sanchez, 2005.

1.1.1.2. Risk factors for developing asthma

As already mentioned, a primary risk factor for developing asthma is allergic sensitiza- tion Platts-Mills et al., 1997. Allergy to dust mites, cockroaches, mice, cats, dogs and the fungus Alternaria have all been shown to be significantly associated with asthma Call et al., 1992; Peat et al., 1994; Sporik et al., 1995; Custovic et al., 1996a; Halonen et al., 1997; Rosenstreich et al., 1997; Perzanowski et al., 1998; Ronmark et al., 1998; Phipatanakul et al., 2000a; Matsui et al., 2004a. In fact, a strong association between asthma and allergy has been reported in virtually all studies in westernized communities Platts-Mills et al., 1997. When evaluating asthma at an early age before the age of 6 years, sensitization to an inhaled allergen is a strong risk factor for asthma symptoms that persist into later childhood Martinez et al., 1995. As with allergic sensitization, having a family history of asthma also contributes to the risk of becoming asthmatic Bracken et al., 2002. Exposure to ETS in the home is also associated with asthma Gold, 2000. Moreover, having two or more siblings in day care and frequent respiratory infections early in life have been shown to be protective against asthma, and these findings are sighted as evi- dence for the hygiene hypothesis von Mutius et al., 1999; Ball et al., 2000. Furthermore, exposure to ozone has been associated with exacerbating asthma d’Amato et al., 2005. Psychosocial stress has also been associated with asthma Wright, Rodriguez Cohen, 1998. Finally, there also may be dietary and physical-activity components to the deve- lopment of asthma Crater Platts-Mills, 1998; Sprietsma, 1999; Shore Fredberg, 2005. The current hypothesis for the genesis of asthma in children involves a complex interac- tion, in the first years of life, between the two arms of the immune system: the innate and adaptive pathways. Viruses and other immunostimulatory substances may modulate the immune system’s response to non-pathogenic allergens, such as those from dust mites, cockroaches and mice. This occurs at a critical point in lung growth and development in children, and effects of this immune response could have permanent effects on lung func- tion Holt, Upham Sly, 2005.

1.1.1.3. Exposure to allergens is a risk for exacerbating asthma