A phrase from the Beatles’ tune Blue Jay Way goes “There’s a fog upon L.A.” The phrase could be appropriately morphed into “There’s a smog upon L.A.” It is well known that the grungy, yellowish-gray gunk is unhealthful. Now, a new study has noted another health impact from smog. An international team of researchers from 14 sites in nine nations, including Seoul, South Korea; Atlanta, Georgia; and Vancouver, British Columbia found that a pregnant woman's exposure to outdoor air pollution may increase the risk of her infant being born at a lower birth weight. The study was published on February 6 in the journal Environmental Health Perspectives.
The researchers compiled the average levels of particulate air pollution to which women were exposed during the course of their pregnancy. Sources of particulate air pollution include traffic exhaust, power plants, and even dust. The infants examined the birth weights of infants that were carried to term. The study group comprised approximately 3 million pregnancies and births; thus, the study was the largest to date that assessed the relationship between maternal air pollution exposure and low birth weight.
The investigators found that for every 10 microgram increase of pollution particles per cubic meter of air, birth weight decreased by 8.9 grams (approximately one-third of an ounce); furthermore, the infants were 3% more likely to be a low birth weight. A low birth weight infant is defined as one that weighs less than 5 pounds 8 ounces at birth. The condition carries a risk of infant mortality as well as heart, respiratory (breathing), and behavior problems later in life.
Pollution levels found by the study sites ranged from approximately 10 to 70 micrograms per cubic meter of air. The authors noted that the levels that could be found at many places around the globe. Previous studies have been conducted that assessed the relationship between maternal exposure to air pollution and a variety of adverse pregnancy outcomes, including low birth weight, preterm birth, stillbirth, and congenital abnormalities; however, their findings varied. Some found a strong association between outdoor air pollution and fetal growth, while others did not.
The authors of the new study cautioned that the lower birth weights were primarily due to air pollution levels because some other factors may have been present that contributed to low birth weight such as the mother's socioeconomic status and whether or not she smoked. Both these factors have been linked to low birth weight in previous studies.
Infants were considered full term and included in the study if they delivered between 37 to 42 weeks of gestation. Thus, the researchers note that some infants were up to six weeks older than others at delivery, which could partially explain the results. A fetus can gain up to eight ounces a week during the last weeks of pregnancy. However, the fact that the investigators found a small but consistent but consistent shift in birth weight across a large number of pregnancies is a significant finding.
It is unknown why maternal exposure to air pollution could impact fetal growth; however, one theory is that air pollution impacts the connection between the placenta and the uterus. This could reduce the transfer of nutrients to the fetus. Another is that air pollution may stress the mother’s body, which could affect fetal growth. Air pollution has been linked to asthma in children and adults. It also is linked to heart disease, diabetes, and stroke in adults.
Take home message:
In view of this study, it would be prudent for a pregnant woman to reduce exposure to air pollution. Moving to a less smoggy location would be beneficial; however, for many that is difficult or impossible. Spending as much time as possible and limiting exercise during smoggy days can help. A major source of air pollution is motor vehicle exhaust. During a recent closure of the San Diego Freeway, air pollution dropped dramatically in areas adjacent to the freeway.