The impact of baseline incidence rates on burden of disease assessment of air pollution and onset childhood asthma: analysis of data from the contiguous United States

Purpose: Burden of disease (BoD) assessments typically rely on national-level incidence rates for the health outcomes of interest. The impact of using a constant national-level incidence rate, versus a more granular spatially varying rate, remains unknown and understudied in the literature. There has been an increasing number of publications estimating the BoD of childhood asthma attributable to air pollution, as emerging evidence demonstrates that traffic-related air pollution (TRAP) leads to onset of the disease. In this study, we estimated the burden of incident childhood asthma cases which may be attributable to nitrogen dioxide (NO₂), a criteria pollutant and a good marker of TRAP, in the contiguous United States. We used both a national-level and newly generated state-specific asthma incidence rates and compared results from the two approaches.

Methods: We estimated incident childhood asthma cases which may be attributable to NO₂ using standard BoD assessment methods. We combined child (<18 years) counts with 2010 NO₂ exposures at the census block level, concentration-response function, and state-specific asthma incidence rates. NO₂ concentrations were obtained from a previously validated land-use regression model. We sourced the concentration-response function from a meta-analysis on TRAP and risk of childhood asthma. We estimated incidence rates using raw data collected in the 2006-2010 Behavioral Risk Factor Surveillance System and Asthma Call-back Surveys. We stratified the estimated BoD by urban versus rural status and by median household income, explored trends in BoD across 48 states and the District of Columbia, and compared our results with a published BoD analysis which used a constant national-level incidence rate across all states.

Results: The overall mean (min-max) NO₂ concentration(s) was 13.2 (1.5-58.3) ug/m³ and was highest in urbanized areas. The estimated national aggregate asthma incidence rate was 11.6 per 1000 at-risk children and ranged from 4.3 (Montana) to 17.7 (District of Columbia) per 1000 at-risk children. The 17 states that did not have data to estimate an incidence rate were assigned the national aggregate asthma incidence rate. Using the state-specific incidence rates, we estimated a total of 134,166 (95% confidence interval: 75,177-193,327) childhood asthma incident cases attributable to NO₂, accounting for 17.6% of all childhood asthma incident cases. Using the national-level incidence rate, we estimated a total of 141,931 (95% confidence interval: 119,222-163,505) incident cases attributable to NO₂, accounting for 17.9% of all childhood asthma incident cases. Using the state-specific incidence rates therefore reduced the attributable number of cases by 7,765 (5.5% relative reduction), compared with estimates using the national-level incidence rate. Across states, the change in the attributable number of cases ranged from -64.1% (Montana) to +33.8% (Texas). California had the largest absolute decrease (-6,190) in attributable cases, whereas Texas had the largest increase (+3,615). Stratifying by socioeconomic status and urban versus rural status produced new trends compared with the previously published BoD analysis showing high heterogeneity across the states.

Conclusions: We estimated new state-specific asthma incidence rates for the contiguous United States. Using state-specific incidence rates versus a constant national incidence rate resulted in a small change in the NO₂ attributable BoD at the national level, but had a more prominent impact at the state level.