Associations of urban greenness with asthma and respiratory symptoms in Mexican American children

Asthma is one of the most common childhood chronic conditions with approximately 19 million adults and seven million children affected in the United States alone. 1 The etiology of asthma and other allergic conditions is not fully understood, but environmental and lifestyle factors have been implicated in the inception of these conditions. In the United States, the prevalence of asthma and asthma related symptoms is disproportionately high in urban poor neighborhoods.2-4 Although this might be due partly to sociodemographic factors, studies have suggested that urbanization and its detrimental impact on the natural environment and lifestyle might increase the risk of respiratory conditions.3-6 In fact, children raised or born on a farm or in rural areas have a reduced risk of respiratory conditions.7-10 Recently, there has been an increased emphasis on the effect of residential surrounding greenness on the risk of respiratory conditions. However, studies linking urban greenness and vegetation with respiratory health are limited and have yielded inconsistent findings.11 While some studies linked greenness with increased risk of asthma and adverse respiratory conditions,12-15 other investigations failed to demonstrate a clear association between greenness and childhood respiratory health.16,17 A few studies, however, have demonstrated protective effects of urban greenness on the risk of allergies and respiratory conditions.18-20 It has been speculated that increased urban greenness may protect against respiratory conditions through its positive impact on environmental biodiversity21,22 and subsequently the human microbiome.22,23 Recent research has shown variations in outdoor urban microbiome due to differences in surrounding vegetation23 and there is an increasing body of literature linking the microbiome with allergic conditions.24 Increased access to urban residential greenness has also been linked with increased physical activity, lower rates of overweight/obesity, reduced psychological stress, and better air quality;20,25 factors that have been linked with respiratory health.26-31 The limited evidence for 24 favorable effects of urban greenness on respiratory health suggests that the protective effect of greenness may be modified by individual and neighborhood characteristics, such as individual and neighborhood low socioeconomic status (SES).19,20 In this study, we examined the associations of residential surrounding greenness with respiratory conditions among an urban sample of Mexican American children and tested whether these associations were explained or modified by individual- and neighborhood-level factors known to contribute to the development of asthma.