Abstract
Background Parks are promoted as a public health resource, and neighborhoods with limited park access often face co-occurring air pollution and socioeconomic disadvantage. The primary objective of this study was to estimate the association between census tract park proximity and emergency department (ED) visit rates for asthma and acute myocardial infarction (AMI) in Los Angeles County, California, adjusted for fine particulate matter (PM2.5), ozone, and poverty. The two outcomes were co-primary, and no directional hypothesis was prespecified. A secondary objective was to assess whether the association persisted after accounting for spatial dependence. Methods This cross-sectional ecological study included 2,305 census tracts with residents and complete data. Park proximity was measured as each tract's mean distance along the 2016 road network of the U.S. Census Bureau (Washington, DC, USA) from its Census 2010 population-weighted center to the three nearest recreational spaces in the 2024 county inventory. The straight-line distance from the tract centroid was used as a sensitivity analysis. Age-adjusted asthma and AMI ED visit rates per 10,000 residents (2015-2017), PM2.5 (2015-2017), ozone (2017-2019), and the percentage of residents below twice the federal poverty level (2015-2019) were drawn from CalEnviroScreen 4.0 (Office of Environmental Health Hazard Assessment, Sacramento, CA, USA). The data years did not fully overlap. Population-weighted regressions with standard errors clustered by ZIP (Zone Improvement Plan) code were compared with spatial error models, and associations were divided into within- and between-ZIP-code components. Results In conventional models, an interquartile range (IQR) increase in the park distance (0.38 miles) corresponded to 1.64 more asthma ED visits per 10,000 residents, with a 95% confidence interval (CI) of -0.14 to 3.41 (p = 0.070), and to 0.34 more AMI ED visits (95% CI 0.06 to 0.63; p = 0.018). Park distance explained less than 1% of residual variation. Straight-line distance from the tract centroid gave larger estimates (2.40 and 0.40 per its own IQR of 0.28 miles). Residuals were strongly spatially autocorrelated (Moran's I 0.65 and 0.60). In spatial error models, the park-distance coefficients were negative (asthma -1.01, 95% CI -1.61 to -0.41; AMI -0.12, 95% CI -0.25 to 0.00), and neither was statistically significant after a post hoc exclusion of one peripheral region (-0.46 and -0.09). The positive coefficients arose between ZIP codes rather than within them. In unsmoothed ZIP-code asthma counts, the association was positive when no spatial term was included, but it was absent when one was added. Poverty was positively associated with both outcomes in the main conventional and spatial models and with asthma in the ZIP-code count models. Conclusions In conventional models, a greater distance from parks coincided with slightly higher AMI ED visit rates across Los Angeles County, and the asthma estimate was also positive but not statistically significant with the primary street-network measure. Both associations were small, appeared between ZIP codes rather than within them, and did not persist in spatial error models. These ecological data, whose outcomes vary mainly between ZIP codes, cannot separate park-related differences from regional confounding and do not support causal or prescriptive conclusions.