Air Pollution and Birth Weight in Connecticut and Massachusetts
Environ Health Perspect. doi:10.1289/ehp.10631 available via http://dx.doi.org [Online 15 February 2008]
Referencing: Ambient Air Pollution and Low Birth Weight in Connecticut and Massachusetts
In their paper, Bell et al. (2007) examined the effects of ambient air pollutants on birth weight in children born in Connecticut and Massachusetts between 1999 and 2002. The study is the largest among the studies conducted in the United States and has provided evidence that, even with pollutant levels that met the air quality standards, significant reductions in birth weight occurred.
The study findings are in line with other articles; however, there are several concerns that need further attention. Infants with preterm birth (born at 32–37 weeks of gestation) were included in the analysis, and this may have affected the overall and the third trimester–specific results. Although these children comprised only 6.7% of the sample, their birth weights were greatly affected (i.e., mean birth weight was 585–1,050 g less than those born at 39–40 weeks of gestation).
In the discussion, Bell et al. (2007) pointed out that the effect of air pollutants on birth weight could be mediated by the effect of these pollutants on preterm birth and/or on fetal growth. It is unclear whether the effects observed in this study were mediated by one or both competing mechanisms. Bell et al.'s Table 6 compared studies that tested pollutant effects mediated only by fetal growth (Basu et al. 2004; Parker et al. 2005; Ritz and Yu 1999; Salam et al. 2005), only by preterm birth (Rogers and Dunlop 2006; Rogers et al. 2000), or by both mechanisms separately (Wilhelm and Ritz 2003, 2005; Woodruff et al. 2003). Bell et al. (2007) could have further benefited readers if they had evaluated pollutant effects on intrauterine growth restriction (IUGR), a metric of pathological fetal growth. Although IUGR is conventionally defined based on the bottom 10th percentile of the birth weight distribution, a better approach would be to define it by < 15th percentile of predicted birth weight based on gestational age, infant sex, and maternal race.
Pollutant data were not available from all counties. As such, different counties contributed to different exposure effects. This makes the comparison between single and two-pollutant models difficult. In Figure 2 (Bell et al. 2007), for example, the effect of particulate matter < 2.5 µm in aerodynamic diameter (PM2.5; measured in 13 counties) seems to have been significantly changed when adjusted for carbon monoxide (measured in 7 counties). It would have been more meaningful if sensitivity analyses were conducted comparing one and two-pollutant models restricting to those counties that had data on all exposures.
The detrimental effect of PM2.5 on birth weight was significantly larger in black infants than in white infants. Hispanic white infants were at increased risk of low birth weight in a similar population (Maisonet et al. 2001). Thus, combining non-Hispanic and Hispanic whites into one group, it is not possible to determine whether Hispanic infants were disproportionately affected by ambient pollutants. There is significant heterogeneity in the percentages of African-American and Hispanic white population by county (U.S. Census Bureau 2007). For example, within the state of Massachusetts, 25% of residents were African American and about 18% were Hispanic in Suffolk, whereas about 2% each were African American and Hispanic in Barnstable. In addition, in counties with higher proportions of African-American and Hispanic populations, the percentages living in poverty were also much higher. Although Bell et al. (2007) acknowledged within-county heterogeneity, questions remain about whether significant heterogeneity in effects existed across counties.
In summary, the results would have been more appealing if Bell et al. (2007) could provide data to show whether pollutant effects were mediated by preterm birth and/or affected fetal growth, and whether these effects were greater in Hispanics, in mothers who smoked, and in counties with a higher proportion of people living in poverty.
The author declares he has no competing financial interests.
Muhammad Towhid Salam
Department of Preventive Medicine University of Southern California
Keck School of Medicine
Los Angeles, California
E-mail: msalam@usc.edu
References
Basu R, Woodruff TJ, Parker JD, Saulnier L, Schoendorf KC. 2004. Comparing exposure metrics in the relationship between PM2.5 and birth weight in California. J Expo Anal Environ Epidemiol 14:391–396.
Bell ML, Ebisu K, Belanger K. 2007. Ambient air pollution and low birth weight in Connecticut and Massachusetts. Environ Health Perspect 115:1118–1125.
Maisonet M, Bush TJ, Correa A, Jaakkola JJ. 2001. Relation between ambient air pollution and low birth weight in the northeastern United States. Environ Health Perspect 109(suppl 3):351–356.
Parker JD, Woodruff TJ, Basu R, Schoendorf KC. 2005. Air pollution and birth weight among term infants in California. Pediatrics 115(1):121–128
Ritz B, Yu F. 1999. The effect of ambient carbon monoxide on low birth weight among children born in southern California between 1989 and 1993. Environ Health Perspect 107:17–25
Rogers JF, Dunlop AL. 2006. Air pollution and very low birth weight infants: a target population? Pediatrics 118(1):156–164
Rogers JF, Thompson SJ, Addy CL, McKeown RE, Cowen DJ, Decoufle P. 2000. Association of very low birth weight with exposures to environmental sulfur dioxide and total suspended particulates. Am J Epidemiol 151(6):602–613
Salam MT, Millstein J, Li YF, Lurmann FW, Margolis HG, Gilliland FD. 2005. Birth outcomes and prenatal exposure to ozone, carbon monoxide, and particulate matter: results from the Children's Health Study. Environ Health Perspect 113:1638–1644.
U.S. Census Bureau. State and County QuickFacts. 2007. Washington, DC:U.S. Census Bureau. Available: http://quickfacts.census.gov/qfd/index.html [accessed 4 July 2007].
Wilhelm M, Ritz B. 2003. Residential proximity to traffic and adverse birth outcomes in Los Angeles County, California, 1994–1996. Environ Health Perspect 111:207–216.
Wilhelm M, Ritz B. 2005. Local variations in CO and particulate air pollution and adverse birth outcomes in Los Angeles County, California, USA. Environ Health Perspect 113:1212–1221.
Woodruff TJ, Parker JD, Kyle AD, Schoendorf KC. 2003. Disparities in exposure to air pollution during pregnancy. Environ Health Perspect 111:942–946.
Air Pollution and Birth Weight: Bell et al. Respond
Environ Health Perspect. doi:10.1289/ehp.10631R available via http://dx.doi.org [Online 15 February 2008]
In our recent study (Bell et al. 2007) we identified associations between air pollution and low birth weight in Connecticut and Massachusetts based on 358,504 births from 1999 to 2002. Salam raises several important concerns about potential limitations in the analysis and interpretation of our results. In particular, he discusses gestational period, effects by race, confounding by co-pollutants, and competing biological mechanisms among other issues.
We conducted several sensitivity analyses as suggested by Salam in his letter, and detailed results are available upon request. The original study considered confounding by co-pollutants for pairs of pollutants that were not highly correlated, and found that model results were robust to adjustment by other pollutants. Salam correctly notes that pollutant data were not available from all counties; therefore, a given observation may have data for some exposure variables and not others. We performed a new analysis comparing results based only on observations with data for the two pollutants considered for gestational exposure. For example, we calculated the association between particulate matter < 2.5 µm in aerodynamic diameter (PM2.5) and birth weight, adjusted by carbon monoxide, and the association between PM2.5 and birth weight not adjusted by CO, but including only the subset of observations with CO data available. The new analysis did not change the results or interpretation.
In our initial analysis we omitted births with gestational length < 32 weeks or > 44 weeks, and adjusted for gestational length at 2-week intervals. As noted in our article (Bell et al. 2007), births with gestational length of 32–36 weeks accounted for 6.7% of the original observations. Salam proposed analysis of gestational and third-trimester exposure restricted to observations with 37–44 weeks gestation. We performed this analysis and generated effect estimates for first- and second-trimester exposure as well. Effects estimates based on the subset analysis (37–44 weeks) were very similar to those from the original analysis.
Salam notes that combining non-Hispanic and Hispanic whites, as done in our study, does not allow for distinction between Hispanics and non-Hispanics, and that race may be associated with socioeconomic status. Although we controlled for race and for socioeconomic status (through mother's education), we agree that the analysis has limitations. Research of effects by race is further complicated by distinction of non-Hispanic black versus Hispanic black and by other subdivisions of racial and ethnic categories (e.g., Mexican vs. Cuban heritage), as well as multiracial infants. Other restrictions may arise from lack of sufficient sample size to investigate various racial categories. To date, few low birth weight and air pollution studies have specifically investigated race (e.g., Basu et al. 2004; Bell et al. 2007; Maisonet et al. 2001), although others have included race as a covariate or restricted observations by race.
Although Salam's letter is directed at an individual study, it highlights some of the challenges of air pollution and pregnancy outcome studies more broadly. Many epidemiologic studies evaluate exposure from monitoring networks implemented for regulatory compliance, and not all areas have monitors. Multipollutant analysis is additionally complex because of the high correlation of some pollutants and by the variation in the chemical composition of particulate matter. Factor analysis and other source apportionment techniques (e.g., Thurston et al. 2005) that have been used to investigate the association between particles and other health outcomes may be used to estimate exposure based on sources for birth outcomes research. A recent study of 1,016 births in the Munich, Germany, metropolitan area assessed exposure to traffic-related pollution accounting for PM2.5 and nitrogen dioxide levels, land use, road characteristics, and population density (Slama et al. 2007).
A critical question is the biological mechanism through which air pollution affects fetal growth, and the potential competing mechanisms of impacts on preterm delivery and fetal growth, as mentioned by Salam. He also notes that other useful results would include analyses of mothers who smoke and of counties with a higher proportion of people living in poverty. In our study (Bell et al. 2007), we adjusted for mother's smoking and education, as indicators of socioeconomic status, in all models; however, we did not investigate the effect modification of smoking or economic conditions. Although such a study would be informative, our data set is not well suited to this analysis because of the lack of extensive data on smoking or socioeconomic status. More detailed and accurate information on these variables may be available from cohort studies.
Limitations of the current scientific literature on birth outcomes and air pollution prompted two recent international workshops, the International Workshop on Air Pollution and Human Reproduction in Munich in May 2007, and the Methodological Issues in Studies of Air Pollution and Perinatal Outcomes Workshop in Mexico City in September 2007. These workshops explored exposure assessment, confounding and effect modification, the relevant window of exposure before or during pregnancy, biological mechanisms, spatial analysis, and the public health implications of observed associations. Reports from the workshops are forthcoming.
The authors declare they have no competing financial interests.
Michelle L. Bell
Keita Ebisu
School of Forestry and Environmental Studies
Yale University
New Haven, Connecticut
E-mail: michelle.bell@yale.edu
Kathleen Belanger
Department of Epidemiology and Public Health
School of Medicine
Yale University
New Haven, Connecticut
References
Basu R, Woodruff TJ, Parker JD, Saulnier L, Schoendorf KC. 2004. Comparing exposure metrics in the relationship between PM2.5 and birth weight in California. J Expo Anal Environ Epidemiol 14:391–396.
Bell ML, Ebisu K, Belanger K. 2007. Ambient air pollution and low birth weight in Connecticut and Massachusetts. Environ Health Perspect 115:1118–1124.
Maisonet M, Bush TJ, Correa A, Jaakkola JJ. 2001. Relation between ambient air pollution and low birth weight in the northeastern United States. Environ Health Perspect 109(suppl 3):351–356.
Slama R, Morgenstern V, Cyrys J, Zutavern A, Herbarth O, Wichmann HE, et al. 2007. Traffic-related atmospheric pollutants levels during pregnancy and offspring's term birth weight: a study relying on a land-use regression exposure model. Environ Health Perspect 115: 1283–1292.
Thurston GD, Ito K, Mar T, Christensen WF, Eatough DJ, Henry RC, et al. 2005. Workgroup report: workshop on source apportionment of particulate matter health effects—intercomparison of results and implications. Environ Health Perspect 113:1768–1774.