Smith RB, Fecht D, Gulliver J, et al. Impact of London's road traffic air and noise pollution on birth weight: retrospective population based cohort study. BMJ. 2017;359:j5299.
This was a retrospective population-based cohort study that investigated the relationship between birth weight and exposure to both air and noise pollution from road traffic.
The study included all live births in the greater London area that occurred between 2006 and 2010. The M25 orbital motorway was used as the boundary. Using official birth registries, the researchers identified 671,509 singleton births. Of these, 7,493 births were eliminated because maternal residences were close to the study area boundary. Births were excluded for a number of other reasons including births at less than 24 weeks, implausible birth weights, and missing gestational ages. In the end, 540,365 births were included in the analysis.
Modern software allowed geocoding maternal home residences to fractions of a meter. Average monthly concentrations of nitrogen dioxide (NO2), nitrogen oxides (NOx), ozone (O3), particulate matter (PM) with diameter <10 µm (PM10) from traffic exhaust, and PM with diameter of <2.5 µm (PM2.5) from exhaust were estimated for 20 m x 20 m grids across the city. Road traffic noise levels were modeled to 0.1 dB resolution for all geocoded maternal addresses.
Low birth weight was defined as birth weight <2,500 g (5.5 lbs) at a gestational age of at least 37 weeks. Small for gestational age (SGA) was defined as birth weight for gestational age less than the 10th centile by sex and ethnicity.
Air pollution from road traffic in London adversely affects fetal growth. The authors estimate that 3% of term low birth weight deliveries in London are directly attributable to high exposure during pregnancy to PM2.5 (>13.8 μg/m3).
Low birth weight is an important predictor of an infant’s future health.
These data did not suggest that traffic noise affected birth weight. Each interquartile increase in air pollution exposure was associated with a 2% to 6% increase in odds of a low birth weight delivery. Statistical trends were seen for decreasing birth weights with increasing road traffic noise but were weakened when adjusted for the traffic-related air pollutants. Exposure to PM2.5 both from traffic exhaust and other sources was consistently associated with increased risk of term low birth weight.
Let’s start out by clarifying why low birth weight is such a concern.
In the near term, underweight babies are more likely to have adverse outcomes such as early delivery, fetal distress, Cesarean section, low Apgar scores, hypoglycemia, hospital admission, and death.1 Women born with low birth weight are more likely to experience preeclampsia when they themselves become pregnant decades later, so that this trait is passed between generations.2 Very low birth weight is associated with brain structure abnormalities and cognitive function impairments.3
Low birth weight is an important predictor of an infant’s future health. Patterns set in motion at birth by low birth weight can significantly affect body composition and development as the child matures, being possibly predictive of weight problems and various health problems later in life.4,5 Low birth weight is associated with high low-density lipoprotein (LDL) and total cholesterol levels in men, and hypertension and diabetes mellitus in women aged 40 to 69.6
Before examining the results of this study by Smith et al, we need to pause for a moment and consider this study in the context of several other recent papers. A number of studies published in the same time period have attempted to answer whether air pollution increases risk of low birth weight. This paper from Smith et al is just one of 4 similar studies published in December 2017, according to the PubMed database.
Also already published in December is a study by Kingsley et al that examined effects of ambient air pollution on preterm births in Rhode Island in a hospital-based study. The authors note that the air is relatively clean in Rhode Island with low levels of airborne particulate matter compared to locations studied in earlier reports. Nevertheless, Kingsley’s group reported in the Journal of Epidemiology and Community Health that increased PM2.5 exposure was associated with a 12- to 16-gram reduction in birth weight (N=61,640). Kingsley’s group estimated that for every 2.5 µg/m3 increase in PM2.5 exposure during pregnancy, risk of preterm birth increased 4%.7
Liu et al examined maternal exposures to particulate matter in Shanghai China and, in a paper published in December 2017, reported more dramatic effects than the researchers in Rhode Island. The effects were probably greater because the pollution in Shanghai is significantly worse: The annual average concentration of PM2.5 in Shanghai was 56.19 μg/m3 in 2013. The authors estimate that 33% of preterm births and 23% of low birth weight births in Shanghai are directly attributable to PM2.5 exposure.8
In yet another December publication, Ng et al reported their detailed analysis of data on births in California (N=1,050,330). Interquartile increases in exposure to total PM2.5 were associated with a 7.7% increased risk of low birth weight. This study detailed the type of particulates and reported varying risks depending on what the particulates were made of; incidence of low birth weight was increased 7.7% by ammonium sulfate particulates, 5.6% by soil particulates, and 3.1% by ammonium nitrate. Regional differences, such as inland vs coastal and northern vs southern, also came into play.9
As mentioned, these studies were published in the first week of December. A less than diligent search of the medical literature going back through the summer of 2017 finds a series of similar reports from locations across the world including Jinan, China,10 Scotland,11 Connecticut,12 and Cape Cod, Massachusetts.13 The results from this storm of research are fairly consistent, enough that we should be paying careful attention: Air pollution exposure during pregnancy is very likely to increase a woman’s risk of having an infant with low birth weight.
In mice there seem to be discrete periods during pregnancy when experimental exposure to fine particulates are more detrimental to pregnancy and are more likely to increase risk of low birth weight.14 This appears true for the present study (Smith et al) as well. Exposure to primary traffic-related air pollutants in the second and third trimester had greater impact on low birth weight than first trimester exposure. Conversely, exposure to PM2.5 during the earlier trimesters was more closely associated with SGA than exposure during the third trimester. It looks as if there is no good time for exposure.
Walter Crinnion, in a review on air pollution published in a 2015 issue of this journal, concluded:
Much more focus needs to be placed on recognizing the important role that common air pollutants hold in health, with commensurate actions being taken to reduce the levels of common air pollutants in the home—the one environment most people are in control of. It is quite possible that one of the most effective preventive medicine modalities would be the installation of a high-quality air purifier in the home.15
His conclusion now appears to be especially true for women during pregnancy.