Relationship between weather factors and heat stroke in Ningbo city

Objective To explore the main effects of weather factors on heat stroke. Methods Data from case report on heat stroke was collected in Ningbo city during 2011 to 2014. Temperature threshold, lag effects and interaction of weather factors on heat stroke had been analyzed, using the piecewise regression model, distributed lag non-linear model, response surface model and other methods. Results Results showed that temperature and humidity were more correlated with heat stroke than other weather-related factors. Through different models, daily average temperature always presented a better role in predicting the heat stroke, rather than maximum or minimum temperature. Positive association between daily average temperature and heat stroke was obvious, especially at lag 0-1 days, with its threshold as 29.1 (95% CI: 28.7-29.5) ℃ . The cumulative RR of heat stroke at 90th percentile of daily average temperature versus 10th percentile was 14.05 (95% CI: 7.23-27.31) in lag 0-1 days. The effects of daily relative humidity on heat stroke appeared nonlinear, with low humidity showing a negative effect on heat stroke and could lag for 1-4 days. However, the effect of high humidity was not significant, with the cumulative RR of low humidity and high humidity as 2.35 (95%CI: 1.27-4.33) and 0.86 (95%CI: 0.40-1.85) in lag of 0-4 days, respectively. We also noticed that there was an interactive effect of both temperature and humidity on heat stroke. Under high temperature and low humidity, the risk of heat stroke showed the highest. Conclusions Temperature and humidity showed obvious relationship with heat stroke in Ningbo city, with the threshold temperature as 29.1 ℃. Under high temperature and low humidity, the risk of heat stroke became the highest. Key words: Heat stroke; Weather factor; Temperature; Humidity; Threshold; Interaction