This study aimed to investigate the association between PM2.5 exposure during pregnancy and metabolic dysfunction through personalized measurement of pollutant concentrations. The sources of particulate matter are diverse, and include industrial emissions, vehicles, and wildfires, with 25% coming from burning fuel. However, most activities were carried out indoors because of decreased mobility owing to pregnancy or the increased popularity of working from home; thus, the indoor sources of particulate matter should be considered when analysing the effect of particulate matter in pregnancy (2, 9).
This study found a positive association between PM2.5 exposure during pregnancy and metabolic dysfunction. Exposure to high PM2.5 altered baseline blood pressure, lipid metabolism, and glucose homeostasis. Pregnant women exposed to high PM2.5 concentrations showed elevated blood pressures in their third trimester, increased TGs, and had higher odds of developing GDM. Moreover, this study revealed that the greater the exposure to PM2.5, the higher the TG/HDL-C ratio was, which represents cardiovascular risk more effectively than does the lipid profile itself.
The results of this study corresponded well with those of earlier studies. A higher level of PM2.5 was associated with metabolic syndrome, hypertriglyceridemia, and high fasting blood glucose (8) (1, 28). Recent studies also showed the association between exposure to PM2.5 and GDM (5, 26). Researchers demonstrated that long-term exposure is a risk factor for dyslipidaemia and that a high TG/HDL ratio was associated with GDM (23, 29)
In contrast, the results of some studies were inconsistent with our findings. Exposure to PM2.5 during the pre-conception period and the 1st trimester did not increase the risk of GDM. Moreover, there was no association between PM2.5 and glucose intolerance, but HbA1c was affected by air pollution (4, 30–32). We suggest that the lack of prospective studies may be the reason for these discrepancies.
Although the exact biological mechanism of the effect of particulate matter on metabolic dysfunction has not yet been established, a hypothesis of oxidative stress is emerging. There are various sources of oxidative stress in pregnancy, including exposure to metals such as cadmium, mercury, methylmercury, lead, and chromium; tobacco; airborne particulate matter; and plastics (33). Previous studies found that inhalation of particulate matter affects oxidative stress generation and causes systemic inflammation, vascular dysfunction, atherosclerosis, and cardiovascular disease in non-pregnant populations (33, 34). Oxidative stress induces inflammation and causes metabolic syndrome beyond insulin resistance (35–37). Furthermore, studies have shown that reactive oxygen species and oxygen-centred free radicals induced by particulate matter are involved in the impairment of the insulin signalling pathway (35, 36, 38–42). In addition, oxidative stress induced by particulate matter exposure was associated with various obstetric complications, such as recurrent pregnancy loss, prematurity, intrauterine growth restriction (IUGR), diabetes, and preeclampsia (43–46). However, there were no significant differences between inflammatory markers in this study (Supplemental Table 3). Some studies attempted to prevent inflammation using antioxidants that contain flavonoids, arginine, vitamin C, vitamin E, carotenoids, resveratrol, and selenium. These studies have shown that antioxidants have a positive effect in preventing inflammation and metabolic syndrome (47–49).
The relationship between exposure to particulate matter and the risk of developing metabolic syndrome was consistently observed in the obese population (6, 50, 51). Moreover, obese patients also tended to have higher levels of inflammatory markers, including C-reactive protein (CRP). This pro-inflammation tendency is thought to increase the risk of various obstetric complications, such as GDM, preeclampsia, large for gestational age, and IUGR (52, 53). In this study, BMI > 25 kg/m2 before pregnancy was associated with a remarkably high CRP level; however, the effect of particulate matter in obese patients was not significant (Supplemental Table 4).
The importance of the in-utero environment of the foetus is emerging in aspects of foetal programming (54–56). The researchers suggested that external stimuli experienced by the foetus in the uterus can affect future health. In a retrospective study, particulate matter appeared to cause future foetal neurodevelopmental delay, which can be reasoned from an inflammatory perspective; however, this aspect is not fully understood (57, 58). This study showed us that exposure to particulate matter during pregnancy could influence maternal metabolic dysfunction. According to the foetal programming theory, particulate matter exposure could influence the intrauterine environment, which has an important role in future foetal health, beyond maternal well-being.
Our study had several strengths. First, this was the first prospective study examining the effects of particulate matter on pregnant women in South Korea. Second, this study focused on the individual effects of particulate matter by measuring personalized particulate matter concentrations. However, this study also had several limitations. First, it was limited by its small sample size. However, there is potential to increase the number of participants because this study is ongoing. Second, the study was performed during the 2019 coronavirus pandemic; hence, the outdoor concentration of PM2.5 might be poorly represented owing to reduced outdoor activities. Third, the PM2.5 data in the pre-conception period was absent; thus, this study could not represent the effect of PM2.5 before pregnancy. Last, the effect of climate factors, such as temperature, humidity, and seasonal variations, which could be a risk factor for adverse pregnancy outcomes, were not considered in this paper (59–61).