Air pollution reduces birth weight

Relation

Smith RB, Fecht D, Gulliver J, et al. Effects of air and noise pollution from London road traffic on birth weight: retrospective population-based cohort study.BMJ. 2017;359:j5299.

Draft

This was a retrospective population-based cohort study that examined the association between birth weight and exposure to air and noise pollution from road traffic.

Participant

The study included all live births in the London area between 2006 and 2010. The M25 ring motorway was used as the border. Using official birth records, the researchers identified 671,509 singleton births. Of these, 7,493 births were eliminated because the mothers' places of residence were close to the study area boundary. Births were excluded for a variety of other reasons, including births under 24 weeks, implausible birth weights, and missing gestational age. In the end, 540,365 births were included in the analysis.

pollution

Modern software made it possible to geocode the locations of mothers down to fractions of a meter. Average monthly concentrations of nitrogen dioxide (NO₂), nitrogen oxides (NOx), ozone (O₃), fine dust (PM) with a diameter <10 µm (PM₁₀) from traffic exhaust gases and PM with a diameter of <2.5 µm (PM_2.5) from exhaust gases were estimated for 20 m x 20 m grids across the city. Road traffic noise levels were modeled for all geocoded maternal addresses at a resolution of 0.1 dB.

Target parameters

Low birth weight was defined as a 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 a gestational age less than 10^thPercentiles by gender and ethnicity.

Key insights

Air pollution from traffic in London affects fetal growth. The authors estimate that 3% of low birth weight births in London are directly attributable to high PM exposure during pregnancy_2.5(>13.8 μg/m^₃).

Low birth weight is an important indicator of an infant's future health.

These data do not suggest that traffic noise affects birth weight. Each interquartile increase in air pollution exposure was associated with a 2% to 6% increase in the odds of a low birth weight birth. Statistical trends showed decreasing birth weights as road traffic noise increased, but were attenuated when adjusted for traffic-related air pollutants. Exposure to particulate matter_2.5Both traffic emissions and other sources were consistently associated with an increased risk of low birth weight.

Practice implications

Let's start by clarifying why low birth weight is such a problem.

In the short term, underweight babies are more likely to experience adverse outcomes such as preterm birth, fetal distress, cesarean section, low Apgar scores, hypoglycemia, hospitalization, and death.¹Women born with low birth weight are more likely to develop preeclampsia when they become pregnant decades later, so this trait is passed down from generation to generation.²Very low birth weight is associated with abnormalities in brain structure and impairments in cognitive function.³

Low birth weight is an important indicator of an infant's future health. Patterns set in motion at birth by low birth weight can significantly influence body composition and development as the child matures, potentially predicting weight problems and various health problems later in life.^4.5Low 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 years.⁶

Before reviewing the results of this study by Smith et al. examine, we need to pause for a moment and consider this study in the context of several other recent works. A number of studies published during the same period have attempted to answer whether air pollution increases the risk of low birth weight. This paper by Smith et al. is just one of 4 similar studies published in December 2017, according to the PubMed database.

Also published in December was a study by Kingsley et al., who examined the effects of air pollution on premature births in Rhode Island in a hospital-based study. The authors note that Rhode Island's air is relatively clean compared to locations studied in previous reports, with low levels of particulate matter in the air. Nevertheless, Kingsley's group reported in theJournal of Epidemiology and Community Healththe increased PM_2.5Exposure was associated with a 12 to 16 gram reduction in birth weight (N = 61,640). Kingsley's group estimated this at 2.5 µg/m each³Increase in PM_2.5Exposure during pregnancy, risk of premature birth increased by 4%.⁷

Liu et al. studied maternal exposure to particulate matter in Shanghai, China, and reported more dramatic effects than the researchers in Rhode Island in a paper published in December 2017. The impact was likely greater because pollution in Shanghai is significantly worse: the annual average concentration of particulate matter_2.5in Shanghai was 56.19 μg/m³in 2013. The authors estimate that 33% of preterm births and 23% of low birth weight births in Shanghai are directly attributable to PM_2.5Exposure.⁸

In another December publication, Ng et al. about their detailed analysis of data on births in California (N=1,050,330). Interquartile increases in total PM exposure_2.5were associated with a 7.7% increased risk of low birth weight. This study details the type of particles and reports different risks depending on what the particles are made of; The incidence of low birth weight was increased by 7.7% by ammonium sulfate particles, by 5.6% by soil particles, and by 3.1% by ammonium nitrate. Regional differences such as inland vs. coast and north vs. south also played a role.⁹

As mentioned, these studies were published in the first week of December. A less-than-careful search of the medical literature dating back to the summer of 2017 finds a number of similar reports from locations around the world, including Jinan, China,¹⁰Scotland,¹¹Connecticut,¹²and Cape Cod, Massachusetts.¹³The results from this storm of research are fairly consistent, so we should pay close attention: Exposure to air pollution during pregnancy very likely increases a woman's risk of having a low birth weight child.

In mice, there appear to be discrete periods during pregnancy in which experimental exposure to particulate matter has a greater impact on pregnancy and is more likely to increase the risk of low birth weight.¹⁴This also seems to apply to the present study (Smith et al). Second and third trimester exposure to primary traffic-related air pollutants had a greater impact on low birth weight than first trimester exposure. Conversely, PM exposure_2.5during the earlier trimester was more closely associated with SGA than exposure during the third trimester. It looks like there is no good time for exposure.

Walter Crinnion, in a report on air pollution published in this journal in 2015, concluded:

Much more attention needs to be paid to recognizing the important role that common air pollutants play in health, and appropriate measures must be taken to reduce the levels of common air pollutants in the home - the only environment most people have control over. It is entirely possible that one of the most effective preventive medical modalities would be to install a quality air purifier in the home.^fifteen

His conclusion now seems to apply particularly to women during pregnancy.