Association between PM2.5 concentrations and the daily number of outpatient visits for urticaria in Beijing city

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Objective To investigate the association between concentrations of environmental air pollutants and the daily number of outpatient visits for urticaria in Beijing. Methods Time series analysis in combination with distributed lag non-linear model (DLNM) and Spearman's rank correlation test was used to analyze the association between PM2.5 concentrations and the daily number of outpatient visits for urticaria. All effect estimates were expressed as a risk ratio (RR) for the daily number of outpatient visits with an interquartile range (IQR) increase of 82.8 μg/m3 in PM2.5. Results From January 1st 2013 to June 30th 2014, the daily average PM2.5 concentration, temperature, relative humidity and number of visits for urticaria were 89.3 μg/m3, 12.7 ℃, 51.9% and 27.3 respectively. Spearman's rank correlation test showed that the daily average PM2.5 concentration and number of visits for urticaria were correlated with daily mean temperature (rs =-0.21, 0.36, respectively, P 0.05). There was no significant correlation between daily number of outpatient visits for urticaria and daily PM2.5 concentration lagged by 0-14 days (RR = 1, 95% CI: 0.89, 1.13). After potential confounding factors were adjusted, an 82.8 μg/m3 increase in daily PM2.5 concentration at lag 0-20 days was significantly associated with a 22% (95% CI: 3%, 46%) increase in daily number of outpatient visits for urticaria, suggesting that the lagged effect of PM2.5 was obviously increased. Conclusion The association between PM2.5 concentrations and the number of visits for urticaria is easily affected by meteorological factors and biological confounding factors, and relative humidity and mean temperature are both positively correlated with the number of visits for urticaria. Key words: Urticaria; Air pollutants; Environmental pollution; Humidity; Statistics; PM2.5