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

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Air Pollution: Brief Report

Preterm Birth After the Utah Valley Steel Mill Closure

A Natural Experiment

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

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doi: 10.1097/EDE.0b013e3181883d5d
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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.


We compiled birth certificate data for Utah deliveries during 1984–1990 from the National Center for Health Statistics ( 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.


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).

Preterm birth* by first month of pregnancy (month of LMP) in the Utah Valley (solid line) and outside the Utah Valley (dashed line): Utah, January 1985–January 1989. *Smoothed percentages using a LOWESS smoother24–25 with a 10% bandwidth.

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).

Preterm Birth by Period of Pregnancy, Timing Relative to Utah Valley Steel Mill Closure, and Location

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).


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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