Most asthma begins in childhood, and a number of environmental and lifestyle factors are thought to contribute to the onset of asthma. These factors include outdoor pollutants, including ozone and diesel exhaust, and indoor pollutants such as tobacco smoke and NO2
. A number of recent studies have focused on the hygiene hypothesis, and the latest iteration of this theory proposes that exposure to farm animals, pets, and nonpasteurized milk or fermented beverages may promote healthy development of the immune system to reduce rates of allergic diseases and asthma.1
On the other hand, lack of exposure could lead to a Th2-bias in immune responses, allergic sensitization, and asthma. Lifestyle and nutrition may also be important for lung and immune development, and several studies suggest that nutrients (e.g. omega-3 fatty acids, vitamin D) and consumption of fruits and vegetables protect against asthma, while obesity and lack of exercise could have the opposite effect.2-4
Virus-induced wheezing episodes in the first 2-3 years of life are a strong risk factor for asthma, particularly in children with other atopic features such as allergic sensitization or atopic dermatitis.5
Whether these episodes contribute to asthma causation or instead reveal an underlying predisposition to asthma has not yet been established. Finally, other factors that have been linked to the development of wheezing and asthma include genetics, ethnicity, month of birth, and exposure to high levels of stress.6-8
It is expected that diseases with a strong environmental component would not be evenly distributed throughout the population, and this is the case for asthma. The ISAAC study has documented over 30-fold differences in the local prevalence of childhood wheezing and asthma on a global scale,9
and although there is less information about the distribution of asthma within the US population, the same appears to be true.10
Asthma in the US appears to more frequent in ethnic minorities and in children growing up in poor urban neighborhoods, and is least common in rural areas in combination with farm animal exposure.11
Notably, asthma morbidity follows a similar pattern.
These findings raise questions about the reasons for increased asthma in densely populated urban areas. What are the specific factors or combinations of factors that lead to asthma in American inner cities? Considering the list of asthma risk factors, the urban environment has a number of features that could have adverse effects on children's respiratory health, especially during the first few years of life when the lung and immune system are rapidly developing. It is possible that asthma risk is high because of exposure to adverse conditions, such as pollutants, cockroach or mouse allergens, stress, or the development of obesity. Conversely, it is also possible that children who grow up in an urban environment lack exposures or experiences that are necessary for healthy lung and immune development. Potential examples include reduced exercise, less availability of nutrients such as vitamin D, or low exposure to beneficial microbes that promote normal immune development. Although the urban environment is not usually considered hygienic, it may in fact be deficient in dirt (soil)!
To begin to identify environmental and lifestyle factors related to asthma causation in economically disadvantaged neighborhoods in large US cities, the Inner City Asthma Consortium initiated the Urban Environment and Childhood Asthma (URECA) study in 2004.12
The study hypothesis, design, and early findings from this observational birth cohort study are described in the following sections.