Meta-analysis of the Association between Short-Term Exposure to Ambient Ozone and Respiratory Hospital Admissions

Abstract

Ozone is associated with health impacts including respiratory outcomes; however, results differ across studies. Meta-analysis is an increasingly important approach to synthesizing evidence across studies. We conducted meta-analysis of short-term ozone exposure and respiratory hospitalizations to evaluate variation across studies and explore some of the challenges in meta-analysis. We identified 136 estimates from 96 studies and investigated how estimates differed by age, ozone metric, season, lag, region, disease category, and hospitalization type. Overall results indicate associations between ozone and various types of respiratory hospitalizations; however, study characteristics affected risk estimates. Estimates were similar, but higher, for the elderly compared to all ages and for previous day exposure compared to same day exposure. Comparison across studies was hindered by variation in definitions of disease categories, as some (e.g., asthma) were identified through ≥3 different sets of ICD codes. Although not all analyses exhibited evidence of publication bias, adjustment for publication bias generally lowered overall estimates. Emergency hospitalizations for total respiratory disease increased 4.47% (95% interval 2.48, 6.50%) per 10ppb 24-hr ozone among the elderly without adjustment for publication bias and 2.97% (1.05, 4.94%) with adjustment. Comparison of multi-city study results and meta-analysis based on single-city studies further suggested publication bias.

Figure 1. Disease categories and diagnosis codes in selected studies

Note: The diagnosis codes in bold are ICD-9 unless specified. Numbers in parentheses indicate how many studies reported estimates for each specified disease category. The white-background boxes show the studies using ICD-9 codes to define disease categories. The gray-background boxes show the studies using other diagnosis codes (e.g. ICD-10 codes, APR-DRG codes). One study can be included in multiple boxes estimates are provided for multiple disease categories.

Figure 2. Comparison of overall effect estimates for total or general respiratory disease by hospital visit type, disease, and age group

Figure 3. Funnel plot of estimates of association between ozone and total or general respiratory disease emergency hospital admissions for the elderly with and without adjustment for publication bias

Note: Effect estimates show the percentage increase in risk (95% confidence interval) for a 10ppb increase in daily ozone.