Ricki Lewis, PhD
October 15, 2013
Full Story: http://www.medscape.com/viewarticle/812604
Exposure in utero to air pollution and particulates from traffic accounts for “a substantial proportion” (18%) of low birthweight (LBW) cases, according to a study of newborns in European citiespublished online October 15 in the Lancet Respiratory Medicine.
Particulates (especially those smaller than 2.5 μm in diameter) in air pollution and traffic exhaust are an established risk factor for low birthweight (LBW), which itself is associated with impaired lung function in childhood. Air pollution is an ever-changing mixture of particulates, metals, polycyclic aromatic hydrocarbons, and other volatilized organic compounds, any or all of which can affect fetal growth and development in a variety of ways, including impairing placental function and causing oxidative stress or estrogen disruption.
Assessing Exposure
Marie Pedersen, PhD, from the Centre for Research in Environmental Epidemiology in Barcelona, Spain, and colleagues evaluated data from 14 population-based mother–child cohort studies conducted in 12 European countries. The study spanned 1994 to 2011 and included a total of 74,178 women.
The study considered birthweight, gestational age, and sex. The primary outcome was LBW (weight <2500 g) in a full-term (more than 37 weeks’ gestation) singleton pregnancy. By studying newborns, the investigators were able to eliminate confounding environmental factors introduced in studies that examine effects of exposure to particulates in children. In addition, by restricting their analysis to term pregnancies, they reduced the risk for bias from growth-impaired fetuses.
The researchers estimated concentrations of particulate matter in the women’s homes with an aerodynamic diameter of less than 2.5 μm (the “fine” fraction), less than 10 μm, and between 2.5 μm and 10 μm (the “course” fraction) present during pregnancy, as well as absorbance and concentrations of nitrogen dioxide and nitrogen oxides. The investigators also examined associations with traffic density on the nearest road and total traffic load.
Risk for LBW was significantly associated with exposure to particulates smaller than 10 μm in diameter, nitrogen dioxide, and high traffic density. Specifically, a 5 μg/m3 increase in concentration of fine particulates during pregnancy was associated with an 18% increased risk for LBW at term (adjusted odds ratio [OR], 1.18; 95% confidence interval [CI], 1.06 – 1.33).
The researchers saw an increased risk even when a woman’s overall exposure was below the present European Union annual limit of 25 μg/m3 . For example, participants exposed to concentrations of less than 20 μg/m had a 41% increased risk for every 5 μg/m3 increase in exposure (OR1.41; 95% CI, 1.20 – 1.65).
Increased exposure to particulates 10 μm or less in diameter was associated with a 16% increased risk of LBW(OR for 10 μg/m3 increase, 1.16; 95% CI, 1.00 – 1.35), and nitrogen dioxide exposure was associated with a 9% increased risk (OR for 10 μg/m3 increase, 1.09; 95% CI, 1.00 – 1.19).
Increased risk for LBW was also seen for increases in traffic density near the home (OR for increase of 5000 vehicles per day, 1.06; 95% CI, 1.01 – 1.11). The researchers estimate that reducing exposure to the fine fraction to a maternal concentration of 10 μg/m3 could decrease cases of LBW 22% (95% CI, 8% – 33%).
The study also assessed head circumference, which is associated with intelligence, and reduced circumference with impaired neurodevelopment. Previous studies did not consider head circumference. The researchers recorded reductions in head circumference with each 5 μg/m3 increase in maternal exposure to fine particulates and found reductions only among women exposed to concentrations below 20 μg/m3, not below 15 μg/m3.
A Controllable Risk Factor
The researchers conclude that the additional risk that exposure to air pollution introduces is similar to that of maternal smoking. They also found that the association between inhaling small particulates and LBW was stronger for women who had sons, for women with low or medium education, and for pregnancies dated from the last menstrual periods compared with those dated using ultrasound.
The new study improves on meta-analyses by accounting for heterogeneity in past studies, eliminating confounders, and assessing a large population over a large geographical area. Limitations include the possibility that pollution and traffic changed in geographic areas during the study, restriction to exposure to the home environment, and lack of data on particulate size and traffic for some locations.
The researchers conclude that a reduction in air pollution in European cities could prevent some cases of LBW. In an accompanying comment, Jonathan Grigg, MD, from the Blizard Institute, Queen Mary University of London, United Kingdom, agrees. “Dissemination of Pedersen and colleagues’ results to the wider public could therefore further increase the pressure on policy makers to reduce exposure of urban populations to particulate matter.”
However, Dr. Grigg cautions against giving advice to pregnant women until further studies can determine individual exposures to particulate matter in cities.
Patrick O’Brien, MD, spokesperson for the Royal College of Obstetricians and Gynaecologists, put the study into perspective. “Exposure to some level of air pollution is unavoidable in day-to-day life, and the risk still remains fairly low. Other factors, such as smoking, high blood pressure or excessive alcohol consumption, may contribute more to the risk of having a low birth weight baby,” Dr. O’Brien said in a related news release from the college.
The authors have disclosed no relevant financial relationships. Dr. Grigg is cochair of the Royal College of Physicians’ Working Party on air quality and life effects.
Lancet Respir Med. Published online October 15, 2013. Article abstract