Maternal exposure to ambient air pollution has been associated with a number of adverse pregnancy outcomes including low birth weight, defined as a full-term infant weighing less than 2,500 g (about 5 lb, 8 oz) at birth. But its been difficult to translate these findings into health-protective policies because of inconsistencies across studiessomething the International Collaboration on Air Pollution and Pregnancy Outcomes (ICAPPO) is working to remedy. This worldwide, multicentre project used a common analysis protocol to derive combined effect estimates of maternal exposure to coarse and fine particulate matter (PM10 and PM2.5, respectively) and to assess differences in how the centres conducted their individual studies. ICAPPO researchers at 14 centres in 9 countries generated effect estimates for ambient PM10 and PM2.5 in relation to more than 3 million live, singleton, term births. For this analysis, the authors used a meta-analysis approach to synthesize contributing factors such as study setting, availability of air pollutant data, birth data, and maternal socioeconomic status. They estimated combined effects across centres and used meta-regression analysis to determine how each centres particular characteristics and exposure assessment methods influenced differences in effect estimates among centres. All these factors can contribute to variation in results across studies. Higher maternal exposure to PM10 and PM2.5 was associated with lower birth weight across the study population. The researchers discovered a positive association between low birth weight and increased PM10 and PM2.5 exposure during the entire pregnancy, as well as a negative association between increased PM10 exposure and term birth weight when all averages were compared. Differences in average PM2.5 exposure levels and PM2.5/PM10 ratios influenced effect estimate outcomescentres with higher PM2.5 levels and higher PM2.5/PM10 ratios reported stronger associations with birth weight. These findings could indicate a geographical variation in the association between air pollution and low birth weight. Meta-regression analyses also showed that centres reported stronger associations if they conducted a temporal exposure assessment only (i.e., exposure was determined according to the time period that each woman was pregnant) compared with those that accounted for both spatial and temporal elements of exposure (i.e., exposure was determined by time of pregnancy and where the woman lived). Although the study does not include direct measurements of personal exposure levels, it does provide a comprehensive estimate of the global effects of maternal exposure to particulate matter on birth weight. This could prove useful for shaping meaningful public health policies regarding air pollution.
Environmental Health Perspectives, 1 March 2013 ;http://ehp03.niehs.nih.gov ;