Can meteorological conditions modify the associations between NO₂ and SO₂ exposure and congenital limb defects (CLDs) during the first trimester of pregnancy? Increases in NO₂ and SO₂ exposure were consistently associated with higher risks of CLDs during the first trimester of pregnancy; both low- and high-temperature exposure and high air humidity act synergistically with the two air pollutants on CLDs. Animal studies have indicated air pollutants are associated with CLDs, but corresponding epidemiological studies are limited with equivocal conclusions. Meteorological conditions are closely connected to the generation, diffusion, distribution and even chemical toxicity of air pollutants. This case–control study included 972 cases of CLDs and 9720 controls in Changsha, China during 2015–2018. Cases from the hospital based monitoring system for birth defects (including polydactyly, syndactyly, limb shortening, and clubfoot) and healthy controls from the electronic medical records system were studied. Complete data on daily average NO₂ and SO₂ concentrations and meteorological variables were obtained from local monitoring stations to estimate monthly individual exposures during the first trimester of pregnancy, using the nearest monitoring station approach for NO₂ and SO₂ concentrations, and the city-wide average approach for temperature and relative humidity, respectively. The 25th and 75th percentiles of daily mean temperature, as well as the 50th percentile of daily mean relative humidity during the study period were used to classify high- and low-temperature exposure, and high humidity exposure based on existing evidence and local climate characteristics. Multivariate logistic regression models were used to estimate the independent effects per 10 μg/m³ increase in NO₂ and SO₂ on CLDs, and the attribute proportions of interaction (API) were used to quantify the additive joint effects of air pollutants with meteorological conditions after including a cross product interaction term in the regression models. NO₂ and SO₂ exposures during the first trimester of pregnancy were consistently and positively associated with overall CLDs and subtypes, with adjusted odd ratios (aORs) ranging from 1.13 to 1.27 for NO₂, and from 1.37 to 2.49 for SO₂. The effect estimates were generally observed to be the strongest in the first month and then attenuated in the second and third months of pregnancy. Synergistic effects of both low and high temperature in combination with NO₂ (with APIs ranging from 0.07 to 0.38) and SO₂ (with APIs ranging from 0.18 to 0.51) appeared in the first trimester of pregnancy. Several significant modifying effects by high humidity were also observed, especially for SO₂ (with APIs ranging from 0.13 to 0.38). Neither NO₂ nor SO₂ showed an interactive effect with season of conception. The methods used to estimate individual exposure levels of air pollutants and meteorological factors may lead to the misclassification bias because of the lack of information on maternal activity patterns and residential mobility during pregnancy. Moreover, we were unable to consider several potentially confounding factors, including socioeconomic status, maternal nutrient levels, alcohol use and smoking during early pregnancy due to unavailable data, although previous studies have suggested limited change to the results after when including these factors in the analysis. The findings are helpful for understanding the combined effects of air pollution and meteorological conditions on birth defects. Environmental policies and practices should be formulated and implemented to decrease air pollutant emissions and improve meteorological conditions to reduce their harmful effects on pregnancy. Additionally, pregnant women should be suggested to reduce outdoor time when the air quality is poor, especially when ambient temperature is higher or lower than what is comfortable, or when it is excessively humid. The study is funded by Major Scientific and Technological Projects for Collaborative Prevention and Control of Birth Defects in Hunan Province (2019SK1012), Major Research and Development Projects in Hunan Province (2018SK2060) and Scientific and Technological Department Projects in Hunan Province (2017SK50802). There are no competing interests. N/A.

A growing body of studies have investigated the association between air pollution exposure during early pregnancy and the risk of orofacial clefts, but these studies put more emphasis on particulate matter and reported inconsistent results, while research on the independent effects of gaseous air pollutants on orofacial clefts has been quite inadequate, especially in China. A case–control study was conducted in Changsha, China from 2015 to 2018. A total of 446 cases and 4460 controls were included in the study. Daily concentrations of CO, NO₂, SO₂, O₃, PM_2.5 and PM₁₀ during the first trimester of pregnancy were assigned to each subject using the nearest monitoring station method. Multivariate logistic regression models were applied to evaluate the associations of monthly average exposure to gaseous air pollutants with orofacial clefts and its subtypes before and after adjusting for particulate matter. Variance inflation factors (VIFs) were used to determine if the effects of gaseous air pollutants could be independent of particulate matter. Increase in CO, NO₂ and SO₂ significantly increased the risk of cleft lip with or without cleft palate (CL/P) in all months during the first trimester of pregnancy, with aORs ranging from 1.39 to 1.48, from 1.35 to 1.61 and from 1.22 to 1.35, respectively. The risk of cleft palate only (CPO) increased with increasing NO₂ exposure levels in the first trimester of pregnancy, with aORs ranging from 1.60 to 1.66. These effects sustained and even exacerbated after adjusting for particulate matter. No significant effect of O₃ was observed. Our study suggested that maternal exposure to CO, NO₂, and SO₂ during the first trimester of pregnancy might contribute to the development of orofacial clefts, and the associations were potentially independent of particulate matter.