Air pollution in the Republic of Korea (Korea) has been an issue for many years. In particular, the particulate matter (PM) component of air pollution is becoming increasingly important due to several factors, including geographical characteristics, the chemical evolution in Seoul, and overcrowding in urban areas. In 2016, the United States National Aeronautics and Space Administration, the Ministry of Environment in Korea, and the National Institute of Environmental Research studied PM in Korea and released the Korea–United States Air Quality Study, reporting that the PM emitted domestically may exceed the recommendations outlined in the World Health Organization (WHO) air quality guidelines [1
]. In previous studies, PM such as PM2.5
(fine inhalable particles with diameters that are generally smaller than 2.5 µm) and PM10
(inhalable particles with diameters that are generally smaller than 10 µm) have been associated with increased mortality and morbidity from multiple health conditions, including cardiovascular disease, lung cancer, acute respiratory infections, asthma, and diabetes [2
]. Moreover, other studies have reported that maternal exposure to PM during pregnancy may increase the risk of preterm birth (gestational age < 37 weeks) [10
]; low birth weight (birth weight < 2500 g) in term infants [13
]; and congenital malformation [16
] through processes related to inflammation, oxidative stress, endocrine disruption, and impaired oxygen transport across the placenta [17
]. In addition, there have been several studies reporting on the long-term effects of prenatal air pollution exposure on neurodevelopment and respiratory outcomes [18
Preterm birth has short-term effects on respiratory, central nervous system, and cardiovascular functions in the form of patent ductus arteriosus, respiratory distress syndrome, and intravascular hemorrhage [20
]. There are also, long-term consequences for physical health, neurodevelopment, pulmonary function, and adult health (cerebral palsy, asthma, growth impairment, and hypertension) [21
]. Preterm birth is the second-most common cause of mortality in children under five years of age, and low-birth weight infants (LBWIs) have a 20-fold higher mortality rate than infants with a birth weight > 2500 g [22
Given the increasing attention on the relationship between exposure to PM and adverse birth outcomes, this study aimed to assess the association of birth outcomes, such as preterm births and low birth weight in term infants, with PM in ambient air pollutants in Korea. We used a cutoff value for PM concentration of 70 μg/m3
to indicate high levels of exposure. This reference value of 70 µg/m3
was based on the interim target-1 in the WHO air quality guidelines [1
] and is associated with a 15% higher long-term mortality risk relative to the guideline level of 20 µg/m3
. We hypothesized that air pollution would be negatively associated with birth outcomes, particularly preterm births and low birth weight.
Using a nationwide registry-based study, we analyzed the associations of between preterm births and birth weight of term infants and exposure concentrations of PM10 during pregnancy.
Our results indicate that exposure to PM10
> 70 µg/m3
during pregnancy may be associated with preterm births. There is an ongoing debate regarding the health effects of exposure to PM10
during pregnancy, but our results are consistent with most other studies [11
]. Our results regarding the association of higher PM10
concentrations and increased preterm birthrate closely agree with several previous studies on the effects of PM10
exposure on preterm birth in a restricted area in Korea [31
]. However, unlike previous research, this study analyzed not only the distribution of PM but also the specific standard of 70 μg/m3
, which is associated with a higher long-term mortality risk according to the WHO [1
]. When the results are visualized geographically, it is clear that preterm births occurred more frequently in west coast areas, and some rural cities in the capital area where there are many more factories than in Seoul. Moreover, exposure to PM10
> 70 µg/m3
during pregnancy was significantly associated with very preterm births. In addition, mean exposure to PM10
> 70 µg/m3
resulted in significantly elevated adjusted odds ratios regardless of whether the location was a metropolitan or nonmetropolitan area.
Finally, we found a tendency for increases in PM10
to be associated with increased risk of adverse birth outcomes such as low birth weight in term infants. Our data showed a statistically significant (P
= 0.001) 1 g decrease in birth weight among term infants per 10 µg/m3
increase in PM10
exposure during pregnancy. This tendency is consistent with previous research, but the changes in birth weight described previously have been so modest that they may have little clinical importance [33
]. The proportion of low birth weight in term infants was higher in areas where the mean PM10
concentration was >70 µg/m3
, but the association was not statistically significant, as measured by the aOR.
Although NHIS data showed a decline in PM10
concentration over a five-year period, ambient air pollutants—specifically particulate matter—remain a burden on the economy and human society in Korea and worldwide. The Global Burden of Disease, Injuries, and Risk Factors Study of 2016 identified air pollution, especially ambient air pollution, as the sixth leading risk factor for global disease [35
]. At the same time, according to 2016 data from the Korean National Statistical Office (KNSO), the frequency of preterm births at a gestational age of <37 weeks increased by 1.5 times from the level seen 10 years previously, to 7.2%. The prevalence of LBWIs (under 2500 g at birth) increased 0.2% from the 2015 level, and steadily increased to about twice the level seen in 1996. Meanwhile, preterm birth is the second-most common cause of mortality in children under 5 years of age, and LBWIs have a 20-fold higher mortality rate than infants of normal birth weight (>2500 g). Therefore, policies are needed to ameliorate modifiable factors for adverse birth outcomes, such as air pollution, to reduce the preterm birthrate and the rate of LBWIs. A campaign is needed to educate the public, especially pregnant women and their families, on methods to reduce or avoid PM.
Our study had several strengths, including adjustment for a number of covariates, including maternal age, parity, infant sex, and parental employment, which have been associated with adverse birth outcomes in previous reports [28
]. In addition, our analysis was based on the specific criteria of 70 µg/m3
, which is relevant to higher risks. However, it is important to note some limitations. First, when we discuss the association between PM and adverse birth effects, such as preterm birth and low birth weight, we should consider other factors that may affect birth results. These include the family history of preterm births, maternal smoking history, low maternal body mass index, prior preterm birth, medical and pregnancy history, occupational exposure, and other factors. These factors could not be analyzed due to the unavailability of the necessary data. Second, until recently, the concentrations of PM10
, which is also thought to be related to adverse birth outcomes, had not been measured in Korea [13
]. Because there are various types of air pollutants, including SO2
, and NO2
, comprehensive analyses of the effects of pollutants beyond PM10
on birth outcomes are warranted. Lastly, this study did not analyze results by trimester because detailed data regarding birth history were not available from the KNSO. There was also no data regarding the date of birth or the day of gestational age; therefore, there was no possibility of dividing pregnancies into trimesters.
In April 2017, The Lancet published The Lancet Planetary Health to assess the effects of environment change on human health, but also to investigate other factors such as political, economic, and social systems that govern those effects [38
]. This reflects the increasing global emphasis on the importance of the environment on human health.