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Preterm Birth After the Utah Valley Steel Mill Closure: A Natural Experiment

Parker, Jennifer D.a; Mendola, Paulinea; Woodruff, Tracey J.b

doi: 10.1097/EDE.0b013e3181883d5d
Air Pollution: Brief Report
Free

Background: Prior studies have linked the Utah Valley Steel Mill closure that took place between August 1986 and September 1987 to improvements in several health outcomes. So-called natural experiments ease concerns over confounding and exposure misclassification, concerns that are common in studies of air pollution and pregnancy outcome.

Methods: We compare birth outcomes for Utah mothers within and outside the Utah Valley, before, during, and after the mill closure.

Results: Mothers who were pregnant around the time of the closure of the mill were less likely to deliver prematurely than mothers who were pregnant before or after; effects were strongest for exposure during the second trimester. Preterm birth within the Utah Valley did not change during the time of mill closure. No patterns for birth weight were observed.

Conclusions: These results support other studies that have found effects on preterm birth of air pollution exposure early in pregnancy.

From the aOffice of Analysis and Epidemiology, National Center for Health Statistics, Centers for Disease Control and Prevention, Hyattsville, MD; and bProgram on Reproductive Health and Environment and Department of Obstetrics, Gynecology, Reproductive Sciences, University of California, San Francisco, CA.

Submitted 4 February 2008; accepted 11 April 2008.

Disclaimer: The findings and conclusions in this paper are those of the authors and do not necessarily represent the views of the National Center for Health Statistics, Centers for Disease Control and Prevention.

Correspondence: Jennifer D. Parker, Office of Analysis and Epidemiology, National Center for Health Statistics, Centers for Disease Control and Prevention, 3311 Toledo Road, Room 6107, Hyattsville, MD 20782. E-mail: jdparker@cdc.gov.

An open-hearth steel mill in Utah Valley was closed from August 1986 through September 1987. Pope1 has demonstrated that during this closure, air pollution was reduced, especially particulate pollution (PM10), with improvements in respiratory disease and related hospital admissions,2,3 mortality,4 and school absenteeism.5 Toxicologic studies suggest that reductions in morbidity and mortality were not completely explained by the decline in particulate matter alone, but were likely because of reduced exposures to metals on the particles and oxidative stress.6

Research syntheses from the early 2000s suggest that birth outcomes may also be related to air quality.7–10 Some, although not all, recent studies have that suggested poor air quality may increase the risks of preterm birth11–18 and restricted fetal growth.18–22 Study results do not consistently identify particular pollutants or exposure windows. Environmental health risks can be difficult to observe because other factors, such as social class, are often correlated with both air quality and birth outcomes. Time-series designs mitigate some concerns of confounding bias for short-term exposures but are less suited for investigating the effects of more chronic exposure. Furthermore, exposures can be correlated during the course of pregnancy, making it difficult to identify risk with exposure at a particular stage of pregnancy.

Perinatal outcomes in relation to the Utah Valley steel mill closure have not previously been studied. This report describes an exploration of birth outcomes among women delivering infants in the Utah Valley before, during, and after the steel mill closure. This “natural experiment,” with its large differences in air quality over a defined time period for a given population, reduces concerns over confounding and correlated exposures.

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METHODS

We compiled birth certificate data for Utah deliveries during 1984–1990 from the National Center for Health Statistics (http://www.cdc.gov/nchs/births.htm). We divided births into 2 groups; those who resided in Utah County (Utah Valley, which included the cities of Orem and Provo), at the time of birth and those who resided in all other Utah counties (not Utah Valley). There were about 500 births a month in the Utah Valley and about 1800 births a month in the rest of Utah.

The study population was limited to singleton infants of non-Hispanic white, married mothers with both birth weight and gestational age reported on the birth certificate. In the Utah Valley, 88% of mothers fit these criteria; outside of the Utah Valley, 79% of Utah mothers fit these criteria. The proportion excluded generally did not vary by year, although there were slightly more unmarried mothers in later compared with earlier years in both areas. Higher percentages of mothers in the Utah Valley were married and non-Hispanic whites.

Preterm birth was defined as delivery before 37 weeks. Gestational age and birth weight were compared for consistency, and less than 0.5% of birth records were excluded in both areas based on Zhang and Bowes cut-points.23

Given the uncertainty in the literature regarding the critical time windows for exposure, and the fact that not all pregnancies completely overlapped with the mill closure, we defined exposure in several ways. First, we identified the first month of pregnancy by using the date of last menstrual period (LMP) on the birth record. Because this information was not available to us for records after 1988, first month of pregnancy for those pregnancies was approximated based on the gestational age and month of birth, with a specific day of birth randomly assigned within the month. We excluded the few birth records before 1989 for which the month of LMP differed by more than 1 month from the “first month” extrapolated from reported gestational age (<0.1%). Using the first month of pregnancy as a benchmark, we defined the following time windows for exposure to mill closure:

  • Exposure in first trimester: first month of pregnancy August 1986 through May 1987.
  • Exposure in second trimester: first month of pregnancy May 1986 through March 1987.
  • Exposure in third trimester: first month of pregnancy February through December 1986.
  • Exposure in first and second trimester: first month of pregnancy August 1986 through February 1987.
  • Exposure in second and third trimester: first month of pregnancy April 1986 through December 1986.
  • Exposure in whole pregnancy: first month of pregnancy August 1986 through December 1986.

Note that we define “exposure” to mean exposure to the reduced air pollution during mill closure. Given these 6 possible (and overlapping) exposure windows, we then compared the percent preterm births in Utah Valley-exposed pregnancies with the percent preterm births among Utah Valley pregnancies during the same months of the 2 preceding years and the 2 following years (“unexposed”). For example, Utah Valley mothers whose first month of pregnancy was from August 1986 to May 1987 were defined as “exposed to mill closure in first trimester.” The comparison group comprised Utah Valley mothers whose first month of pregnancy was August to May of other years (1984–1985, 1985–1986, 1987–1988, 1988–1989, but not 1986–1987). A comparison of preterm rates in the exposed and unexposed pregnancies provides a relative risk for each of the 6 exposure categories of pregnancies.

In the same way, we compared births to women from the rest of Utah during exactly the same time periods. This generated relative risks for the rest of Utah for the same time periods of exposure, even though there was no exposure to women in the rest of Utah. These women provided a control for time trends that might confound our exposure measure.

We used a locally weighted regression smoother (lowess24,25) to depict visually the time trends in birth outcomes before, during, and after the steel mill closure. A relatively narrow bandwidth of 10% was used to preserve seasonal variation.

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RESULTS

Despite the potential economic hardship in the Utah Valley due to the steel mill closure, the number of births during the period of closure was similar to the numbers before and after the closure. We observed from graphic inspection (Fig. 1) that preterm birth was lower for pregnancies that began during the mill closure than for those that began in other intervals. However, mean birth weight and percent low birth weight among term infants were similar throughout the time period (not shown).

FIGURE 1.

FIGURE 1.

Table 1 shows the percentage of births delivered preterm for each of the 6 exposure time periods and for the 2 geographic areas. The percent preterm delivery was generally lower for pregnancies in the Utah Valley exposed to the mill closure. The strongest reduction was with exposure to the mill closing in the second trimester of pregnancy. In contrast, the percent preterm delivery was unchanged during these periods for births outside of the Utah Valley. Similar tabulations of mean birth weight or low birth weight for births in the Utah Valley or outside the Utah Valley did not suggest effects of mill closure on fetal growth (not shown).

TABLE 1

TABLE 1

Extending the preterm birth analysis, we looked at the first trimester time period in a different way. As it took time for the air quality to improve after the mill closure, we limited the first trimester time period to October through March rather than August through March; results were similar to the main findings (not shown).

Although we are unable to rule out a differential self-selection of women becoming pregnant during the mill closure, we found no evidence to support this. Maternal education, parity, and age (factors available on the birth certificate) did not explain the findings (not shown).

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DISCUSSION

This is the first report of reproductive outcomes associated with the Utah Valley mill closure, a natural experiment that has demonstrated the impact of airborne pollutants on respiratory health, morbidity, and mortality. We found a decreased risk of preterm delivery for pregnancies during the mill closure, particularly for pregnancies with second trimesters in the lower exposure period. Although this analysis was limited by the small number of births and imprecise exposure assessment, these findings are consistent with a growing literature on preterm delivery risk associated with exposure to air pollution. Results for birth weight did not indicate effects on fetal growth.

Observing changes in health associated with large and time-limited changes in environmental exposure provides more convincing evidence of risk because much of the concern about residual confounding is allayed. Because the birthrate remained relatively stable in Utah Valley during the mill closure, we do not believe there was a major change in the underlying population due to the mill closure.

Monthly pollution exposure varied seasonally in the Utah Valley, even during the period of the steel mill closure. Additional analysis should examine more detailed pollution data from Utah Valley to identify which pollutants have the strongest impact on risk (particulates, gases, etc.) and to quantify the relationship between adverse birth outcomes and specific pollutant levels. Additional data on residential address and more precise timing of pregnancy would be useful to improve the exposure assessment for a more comprehensive examination of the risks.

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REFERENCES

1. Pope CA III. Particulate pollution and health: a review of the Utah valley experience. J Expo Anal Environ Epidemiol. 1996;6:23–34.
2. Pope CA III. Respiratory hospital admissions associated with PM10 pollution in Utah, Salt Lake, and Cache Valleys. Arch Environ Health. 1991;46:90 –97.
3. Pope CA III. Respiratory disease associated with community air pollution and a steel mill, Utah Valley. Am J Public Health. 1989;79:623–628.
4. Pope CA III, Schwartz J, Ransom MR. Daily mortality and PM10 pollution in Utah Valley. Arch Environ Health. 1992;47:211–217.
5. Ransom MR, Pope CA III. Elementary school absences and PM10 pollution in Utah Valley. Environ Res. 1992;58:204 –219.
6. Ghio AJ. Biological effects of Utah Valley ambient air particles in humans: a review. J Aerosol Med. 2004;17:157–164.
7. Glinianaia SV, Rankin J, Bell R, et al. Particulate air pollution and fetal health. A systematic review of the epidemiologic evidence. Epidemiol. 2004;15:36–45.
8. Maisonet M, Correa A, Misra D, et al. A review of the literature on the effects of ambient air pollution on fetal growth. Environ Res. 2004;95:106 –115.
9. Sram RJ, Binkova B, Dejmek J, et al. Ambient air pollution and pregnancy outcomes: a review of the literature. Environ Health Perspect. 2005;113:375–382.
10. Lacasana M, Esplugues A, Ballester F. Exposure to ambient air pollution and prenatal and early childhood health effects. Eur J Epidemiol. 2005;20:183–199.
11. Hyunh M, Woodruff TJ, Parker JD, et al. Relationships between air pollution and preterm birth in California. Paediatr Perinat Epidemiol. 2006;20:454–461.
12. Ritz B, Wilhelm M, Hoggatt KJ, et al. Ambient air pollution and preterm birth in the environment and pregnancy outcomes study at the University of California, Los Angeles. Am J Epidemiol. 2007;166:1045–1052.
13. Sagiv SK, Mendola P, Loomis D, et al. A time-series analysis of air pollution and preterm birth in Pennsylvania, 1997–2201. Environ Health Perspect. 2005;113:602–606.
14. Leem JH, Kaplan BM, Shim YK, et al. Exposures to air pollutants during pregnancy and preterm delivery. Environ Health Perspect. 2006; 114:905–910.
15. Ponce NA, Hoggatt KJ, Wilhelm M, et al. Preterm birth: the interaction of traffic-related air pollution with economic hardship in Los Angeles neighborhoods. Am J Epidemiol. 2006;163:392–393.
16. Hansen C, Neller A, Williams G, et al. Maternal exposure to low levels of ambient air pollution and preterm birth in Bristbane, Australia. BJOG. 2006;113:935–941.
17. Jalaludin B, Mannes T, Morgan G, et al. Impact of ambient air pollution on gestational age is modified by season in Sydney, Australia. Environ Health. 2007;7:16.
18. Wilhelm M, Ritz B. Local variations in CO and particulate air pollution and adverse birth outcomes in Los Angeles County, California, USA. Environ Health Perspect. 2005;113:1212–1221.
19. Parker JD, Woodruff TJ. Influences of study design and location on the relationship between particulate matter air pollution and birthweight. Paediatr Perinat Epidemiol. 2008;22:214 –227.
20. Parker JD, Woodruff TJ, Basu R, et al. Air pollution and birth weight among term infants in California. Pediatrics. 2005;115:121–128.
21. Bell ML, Ebisu K, Belanger K. Ambient air pollution and low birth weight in Connecticut and Massachusetts. Environ Health Perspect. 2007;115:1118 –1124.
22. Slama R, Morgenstern V, Cyrys J, et al. 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. 2007;115:1283–1292.
23. Zhang J, Bowes WA Jr. Birth-weight-for-gestational-age patterns by race, sex, and parity in the United States population. Obstet Gynecol. 1995;86:200 –208.
24. StataCorp 2007. Stata Statistical Software: Release 10. College Station, TX: StataCorp LP.
25. Cleveland WS. Robust locally weighted regression and smoothing scatterplots. J Amer Stat Assoc. 1979;74:829–836.
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