It has been well documented that air pollution is associated with adult and infant mortality and respiratory symptoms in children. 1 More recently, there have been reports that air pollution is associated with birth outcomes, such as low birth weight, 2–5 prematurity, 4,6 and intrauterine growth retardation, 7 although two smaller studies produced negative findings. 8,9
These recent studies have been largely conducted in populations exposed to relatively low levels of pollution. The chance to demonstrate an effect, if it exists, is better when the range of exposures is wide. We have analyzed data from the British 1946 birth cohort; because the levels of air pollution were much higher in Britain in 1946 than today, this study provides an opportunity to address the question whether birth weight was associated with air pollution.
The participants were drawn from the MRC National Survey of Health and Development (NSHD), a social class stratified birth cohort study initially consisting of 5,362 people selected from all live births that occurred in England, Scotland, and Wales during 1 week in March 1946. 10 Birth weight was taken from hospital or midwife records; gestational age, however, was not recorded. Information about sociodemographic factors has been obtained by interview conducted by a health visitor (community nurse). 11
As exposure to outdoor air pollution, we used an air pollution index developed by the original investigators of the 1946 cohort. This index was based on domestic coal consumption per square mile; each of the 2,689 areas of residence (communities) of each study participant was classified into one of four groups. 12 The air pollution index (based on 1940s data) agreed well with measurements made in a subset of the areas in 1962; the mean annual concentrations of smoke in the four pollution groups were 67, 138, 217, and 281 μg/m3, respectively; the mean concentrations of sulfur dioxide were 90, 130, 191, and 257 μg/m3, respectively. 12 We used linear regression to estimate the differences in mean birth weight (in grams) between air pollution groups, after controlling for paternal social class, maternal education, and geographical region. In addition, we adjusted for birth order, time since the birth of the previous child, and the quality of housing (running hot water, not shared kitchen, own bathroom).
There was a strong association between the air pollution index and birth weight (Table 1). Babies born in areas with the highest air pollution were on average 87 grams lighter than those born in the areas with the cleanest air. Controlling for gender, father’s social class, mother’s education, and region did not change these estimates, nor did further adjustment for birth order, birth interval, and housing quality.
The 1946 birth cohort has been regarded as a representative sample of the UK population legitimately and singly born in the immediate post-war era. 11 As the air pollution index was based on domestic coal consumption, it was probably, at least to some extent, related to urbanization. Nevertheless, socioeconomic characteristics, the most likely potential confounders, were not related to air pollution. Because food rationing was in place, a reasonably uniform distribution of maternal nutrition can be assumed, although some differences in nutrient intake probably existed. Data on maternal smoking were not available, but it is unlikely that maternal smoking in 1946 would be related to air pollution independently from social class (and there was no clear social gradient in female smoking at that time 13).
As with most studies in which exposure is based on place of residence, we cannot rule out confounding by unmeasured factors. It is also possible that coal burning (the basis for the exposure classification) influenced the quality of indoor air, which in turn could have affected birth weight. We do not have data to assess this possibility. It is assuring, however, that the adjustment for a range of the most likely potential confounders did not reduce the effect estimates.
The way of classifying air pollution in 1946 does not allow us to express its effects in absolute units (eg per 100 μg/m3). Nevertheless, the effects appear stronger than those found in the more recent studies. 2–5 This result is most likely due to the fact that exposures in 1946 Britain, 10 years before the clean air legislation, were several times higher than those seen currently, even in less developed countries. Because of the lack of data on gestational age, we were unable to examine separately the association of air pollution and intrauterine growth retardation and prematurity.
The major problem with the hypothesis linking air pollution with birth weight are the biological mechanism for such a relation. 4 A number of potential mechanisms have been put forward, including infection, rheological factors (eg blood viscosity or clotting), and direct effect of specific pollutants on DNA, 14–18 but there is little direct evidence on these mechanisms. Perhaps the most convincing is the finding that infants born in high pollution areas had high levels of DNA adducts, lower birth weight, and smaller head circumference. 19 Polycyclic aromatic hydrocarbons, important products of coal burning, are related to placental DNA adducts 20 and may account for the effect of particulate pollution on fetal growth. 21 Carbon monoxide, another coal burning product, was also found to be associated with low birth weight. 5 So far, however, the major support for the hypothesis that air pollution is related to birth outcomes is the consistency of the findings in different populations, including this historical British birth cohort.
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