1. Study population
A retrospective case-control study was conducted in the Department of Pediatrics, Xiangya hospital, Central South University from May 2017 to April 2019. We recruited 699 children under age of 14 who were diagnosed with pneumonia as the case group, and 811 healthy children who registered for physical examination as the control group.
2. Demographic data
We obtained the demographic data from the electronic medical records system, including child's gender, age, birth season, gestational weeks, parity, mode of delivery, and parental atopy. Parental atopy is the history of maternal or paternal allergic diseases.
3. Exposure assessment
3.1. Exposure time windows
Exposure timing-windows included preconceptional and prenatal time periods in this study. The preconceptional exposure mainly included two time windows: one year before conception and three months before conception. The prenatal time period was defined from the first day of the mother’s last menstrual period to the delivery date. The prenatal exposure was further divided into three trimesters: the first trimester (the 1st to 12th weeks of pregnancy), the second trimester (the 13th to 27th weeks of pregnancy), and third trimesters (the 28th weeks of pregnancy to birthday of child).
3.2. The data of diurnal temperature variation (DTV)
We obtained the data of daily temperature (including the mean, maximum, and minimum temperatures) of 10 regions in Changsha during 2003-2019 from China Meteorological Administration. The diurnal temperature variation was calculated as the difference between the daily maximum and minimum temperature. The 10 regions of Changsha included Kaifu district, Yuhua district, Furong district, Liuyang county, Mabling, Ningxiang county, Tianxin district, Wangcheng district, Yuelu district, Changsha county. Child's exposure was estimated by DTV at the region where the residence was located. Then individual exposure to outdoor DTV was counted by average of the daily DTV during preconceptional and prenatal period.
3.3 Exposure to outdoor air pollution
Exposure to air pollution was considered as confounding variables for pneumonia in this study. We obtained daily 24h average concentrations of three main air pollutants, including nitrogen dioxide (NO2), sulfur dioxide (SO2), and particulate matter ≤10 µm in diameter (PM10), from 7 municipal air quality monitoring stations in Changsha from 2003 to 2019. Individual air pollution exposure during different time windows was estimated using inverse distance weight (IDW) method .
4. Statistical analysis
Statistical analyses were performed by SPSS software (version 23.0, SPSS Inc., Chicago, USA). The relationship between outdoor DTV exposure during preconceptional and prenatal period and childhood pneumonia was assessed by using multiple logistic regression model, and with adjusting potential variables in Table 1 and three air pollutants (NO2, SO2, and PM10). The correlation in regression analysis was calculated by odds ratio (OR) of 95% confidence interval (95% CI). In our study, OR (95% CI) was estimated by per 1 ℃ increase in exposure level for outdoor DTV. p≤0.05 were considered significant.