This study investigates to what extent between school differences in asthma prevalence exist and whether between school differences are attributable to an unequal distribution of individual risk factors and asthma correlates in certain schools. We find that the differential distribution of students at higher risk of asthma does not completely explain between school differences. Instead, we find evidence for a contextual effect of schools on asthma prevalence. To our knowledge this is the first study to explore the contextual influence of schools on adolescent asthma prevalence in a large nationally representative U.S. school based population.
Our findings at the individual level largely mirror those of other studies. Non-Mexican Hispanics and Blacks, those born in the U.S. and those with either a mother or father with asthma were noted to have higher prevalence of asthma. Interestingly, however, in our analysis neither marker of individual SES—household income nor maternal education level—was predictive of asthma. These findings speak to the inherent complexity in the social patterning of asthma prevalence.
Our findings add to the growing body of literature demonstrating school-level differences in health related behaviors and outcomes 
. School level differences in academic achievement independent of student demographics have long been noted 
. More recently studies have demonstrated between school differences in health related behaviors such as tobacco and alcohol use 
, and weapon carrying 
as well as our own work on physical activity 
, all of which may be partially attributable to school level norms, policies, or opportunities. Though the literature linking school environments to health related outcomes is growing, this is one of very few studies to demonstrate a school contextual effect on a health outcome
Due to the limitations in our data regarding structural, cultural, or other differences in schools, we were unable to explore possible mechanisms through which schools are influencing students' asthma status. However, we hypothesize that schools may influence students' asthma outcomes through several different pathways. First, schools may differ in their allergen exposures for students. Numerous studies have demonstrated high levels of allergens in schools including mold, cat, dog, mouse, dust mite, and cockroach allergen as well as high exposure to VOCs 
. Several European studies have linked increased exposure to specific allergens in schools to worse asthma related outcomes in both students and teachers 
. Because students spend such a significant portion of their waking hours in school and the link between allergen exposure and asthma related outcomes is so well documented, it is logical to think that schools with higher level of allergen exposure would have on average students with higher asthma prevalence.
Schools may also influence the asthma prevalence and morbidity in students through their physical structures. Studies by the Government Accounting Office have shown that the physical structures of schools vary and specifically vary by the demographics of the student body 
. Schools with high percentages of low-income and/or racial/ethnic minority students are more likely to report that the school building has an unsatisfactory environmental condition such as poor ventilation. Students attending such schools may potentially have on average worse asthma related outcomes.
Finally, the stress related school environment may influence asthma related outcomes in students. Stress and more specifically exposure to violence have been increasingly recognized as risk factors for worsening asthma related outcomes 
. School environments may expose students to stress through school-based violence, racial/ethnic or socioeconomic tension such as racism, and/or through high academic or social demands. Because stress is likely distributed unevenly between schools, we hypothesize that it may contribute to between school differences in asthma.
Because the schools were not nested within communities (i.e. students from more than one community might attend the same school and students from a single community might attend more than one school), we were unable to tease apart the effects of schools versus communities. It is noteworthy however, that when we controlled for the socioeconomic and racial/ethnic makeup of the neighborhood of residence of the students, that the between school variance was altered very little. Though we recognize that schools, especially public schools, are largely influenced by the neighborhoods in which they exist, there may be substantial differences in exposures between a students' home and school which make the question of neighborhood versus school influence on asthma worthy of pursuit.
There are several limitations to this paper that must be acknowledged. First, these data were collected more than 10 years ago and may not be reflective of the current situation among schools. However, the prevalence of asthma has continued to rise over the last decade and the social patterning of asthma has become amplified 
. At the same time, the physical structures as well as social norms in schools remain non-uniform. Thus our findings may underestimate the current situation. Given the longitudinal nature of the data, it is imperative to perform analyses on baseline data in order to understand factors that may influence the health of adolescents as they transition into young adulthood and beyond. This study lays the groundwork for our future work using additional waves of Add Health. Another limitation to these analyses is our reliance on parental reported asthma status of the child. We do not have further data on severity of illness, recent exacerbations, or hospitalizations that would help us obtain a more nuanced understanding of asthma related outcomes beyond current prevalence. However, this is a problem commonly confronted in asthma studies. Additionally, the parent is asked if the child has the condition now
. Thus, we have no information about the chronicity of the condition nor how it relates to the time attending the current school.
In conclusion, we find significant variation in asthma prevalence between schools that cannot be explained by the racial, socioeconomic or clustering of other asthma risk factors in the student body. This implies that the school has a contextual influence on asthma outcomes independent of the makeup of the student body. This has significant public health implications in that schools should be an area of focus in trying to improve all asthma outcomes and to eliminate the disparities currently seen.