Asthma has been defined as a common and chronic inflammatory condition of the airways in which a narrowing of the medium-sized air passages of the lungs causes wheezing, shortness of breath, and coughing (Elsom, 1996; Williams, 2005). The cause of asthma is not completely known, and a combination of host and environmental factors appear to be involved with the development and trigger factors of asthma (Thurston and Ito, 1999; Tunnicliffe and Ayres, 2001). Environmental elements such as tobacco smoke, pet dander, american house dust mites (Dermatophagoides farinae), and american cockroaches (Periplaneta americana) contribute to asthma irritation and asthma severity in the indoor environment (Edelman, 1997; Lara et al., 2001). Outdoor air pollution is also known to trigger asthma symptoms in both children and adults (Walling, 2002; Woodruff et al., 2003).
Childhood asthma is the most common chronic disease in children, with increases in the number of cases occurring in industrialized countries (Bellenir, 2006; O’Connell, 2004; Walling, 2002). Asthma affects ≈20 million people in the United States, and children make up more than one-third of those affected (Sommers et al., 2007). In 2000, the American Academy of Pediatrics reported that asthma rates for those less than 18 years old had increased by more than 70% from 1982 to 1994 (Neidell, 2004). It was also reported that the percentage of children with asthma doubled from 3.6% in 1980 to 7.5% in 1995, and a survey administered in 2001 indicated that 6.3 million or 8.7% of children had asthma (Akinbami, 2006; Woodruff et al., 2003). The total cost for asthma in 1994 was ≈$5.8 billion (Smith et al., 1997). O’Connell (2004) estimated this annual cost increased to $14 billion in 2004.
Numerous time-series studies of hospital admissions and emergency visits have suggested a connection between particulate air pollution and asthma attacks (Dockery and Pope, 1996). Pollutants including sulfur dioxide, ozone, acid aerosols, fine particulates, and nitrogen dioxide are all believed to sensitize people to asthma (Elsom, 1996). Studies of children with asthma at summer camps found that air pollution, particularly ozone, was significantly and consistently correlated with acute asthma exacerbations, chest symptoms, and lung function decrements (Thurston and Ito, 1999). Inflammation of allergic asthmatics has been shown to increase with air pollutants such as ozone and that repeated exposure to ozone would likely have a detrimental effect on lung function of asthmatics (Devlin et al., 1996). Finally, research has suggested that long-term exposure to urban air pollution is related to prevalence of asthma and allergies, primarily those pollutants emitted by traffic in urban areas (Pénard-Morand et al., 2010).
Trees remove airborne dust and chemical matter, or particulate matter from the air, where it is stored on leaves, twigs, and trunks (Beckett et al., 1998). Particulary, leaf magnetization studies have found high values from leaves on the side of a tree closer to the roadside and lower values at the distal side, confirming the ability of trees to reduce aerosol concentrations related to vehicular pollutants in the atmosphere (Matzka and Maher, 1999). Tree cover or “green belts” around factories and other industrial locations reduce air pollution by serving as a sink for pollutants and reducing the flow of dust (Rao et al., 2004). Rao et al. (2004) found that planting around the industrial area reduced air pollutant emissions by as much as 63%, including a reduction of sulfur dioxide by 39%, nitrogen oxides by 40%, 37% of particulate matter, and a 93% reduction in carbon monoxide levels. In 1984, it was estimated in urban areas that planting 1 million new trees would remove 200 tons of particulate pollution each day after the trees reached 10 years of age (Petit et al., 1998). Research has also suggested that using grasses and shrubs on rooftops could help reduce air pollution in urban areas by increasing the amount of vegetation in areas with limited space (Currie and Bass, 2008). Furthermore, research has suggested that an increase in tree density is related to a decrease in the prevalence of asthma in urban areas such as New York City (Lovasi et al., 2008).
Texas has a wide variety of climate and geography conditions across the state, which leads to a diversity in native vegetation classified as forests, woodlands, shrublands, grasslands, herbaceous vegetation, swamps, and marshes (Diamond et al., 1987). The main objective for this study was to determine if childhood asthma rates were related to tree cover and vegetation in the MSAs of Texas.
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