We found that a 10-ppb increase in 7-day average levels of nitrogen dioxide was associated with a 16% increase in the odds of spontaneous pregnancy loss (odds ratio [OR] = 1.16; 95% confidence interval [CI] 1.01–1.33; P=.04). A 10-μg/m 3 increase in 3-day and 7-day averages of fine particulate matter were associated with increased risk of spontaneous pregnancy loss, but the associations did not reach statistical significance (OR 3-day average = 1.09; 95% CI 0.99–1.20; P=.05) (OR 7-day average = 1.11; 95% CI 0.99–1.24; P=.06). We found no evidence of increased risk for any other metrics of nitrogen dioxide or fine particulate matter or any metric for ozone.

Ambient air pollution poses a significant risk to population health, increasing the risk for morbidity and mortality at all ages (). At present, air pollution is associated with multiple adverse obstetric outcomes, including pregnancy induced hypertensive disorders, neonates small for gestational age, preterm birth, low birthweight, and stillbirth (). During early gestation, placental and fetal developments are established, and exposure to deleterious agents can lead to significant damage. The underlying mechanism for adverse pregnancy outcomes is hypothesized to be a result of oxidative stress, systemic inflammation (), and compromised placental growth and function (). Despite biological and epidemiological evidence of such effects, few studies have investigated the relationship between air pollution and spontaneous pregnancy loss. Most (), but not all (), of the available literature has found an increased risk of spontaneous pregnancy loss and air pollutant exposure, and results vary by pollutant and demographic factors. In addition, many studies have been ecological in nature or limited by small sample size. Supplemental Table 1 , available online, shows a summary of results of published studies examining this association. The heterogeneity of results may be due to unobservable personal factors or misclassification of exposure. Further investigation is needed to elucidate the effect of air pollutants on spontaneous pregnancy loss, particularly the effects during acute exposures ().

The University of Utah Emergency Department (UUED) services a large urban area known as the Wasatch Front. This region is an area of unique topography where temperature inversions create high concentrations of air pollutants in the winter for limited periods of time, at levels deemed unhealthy by the United States Environmental Protection Agency (). Physicians practicing in UUED noted anecdotal increases in incidence of spontaneous pregnancy loss during these inversion events. Given these observations, we conducted a case-crossover study to examine the risk of spontaneous pregnancy loss among women who presented to the UUED from 2007 to 2015 with short-term exposures to fine particulate matter (PM, <2.5 μm in aerodynamic diameter), nitrogen dioxide (NO), and ozone (O). Because the underlying mechanisms linking air pollution and spontaneous pregnancy loss have not been identified, we performed an exploratory analysis examining various exposure time windows and pollutant metrics. Due to known differences in pollutant exposure by sociodemographic factors (), we investigated effect modification by Hispanic ethnicity as supplementary analyses.

We estimated the association between a 10-unit change, measured continuously, in pollutant concentrations and spontaneous pregnancy loss using conditional logistic regression clustered at the event level. Effects were estimated as odds ratios (OR). All analyses were controlled for daily average temperature and completed using SAS 9.4.

We then calculated exposure measurements as the average daily concentrations of the day of the spontaneous pregnancy loss and the preceding 2 days of PM, NO, and O. We calculated the average of the 3 days for the case/referent date and 2 days prior (i.e., lag0, lag1, and lag2) 7-day averages for the case/referent date and 6 days prior (i.e., lag0, lag1–lag6), 3-day maximum value, and 7-day maximum value. Descriptive statistics for each pollutant and metric can be found in Table 1 . Pearson correlation coefficients among air pollutant metrics can be found in Supplemental Table 2 , available online.

We obtained air pollution data from the United States Environmental Protection Agency's Air Quality System Data Mart (). We determined population-weighted centroids for every residential zip code based on Census 2010 block group population totals. Using topographic features, we delineated six air basins within the Wasatch Front as areas where lateral air movement would be reduced due to mountain ranges and basin, and assigned each monitoring station to the air basin where it was located using ArcGIS (version 9.3). We estimated daily PM, NO, and Olevels for each zip code centroid using inverse distance weighting of all observations from monitoring stations located in the same air basin as the zip code centroid. The benefit of this method is that we were able to assign values at the zip code level, rather than county-level measurements from the raw data.

We used a time-stratified approach for referent period selection. We selected referent days for each individual as the same day of the week as the event for the calendar month and year, which resulted in three to four referent periods per event day. We selected this strategy to control for any bias associated with time trends, overlap bias, increase efficiency (), and control for season and day of the week by design.

We used a case-crossover study design to analyze the acute effects of short-term exposure to air pollution. The case-crossover design is characterized by selecting a case or event date and each subject then serving as her own control. In this design, acute exposure before an event is compared with a similar window of exposure on days not associated with the event (). This allows for an increase in efficiency and minimizes time-invariant confounding (e.g., genetic predisposition, age, race/ethnicity, and birth cohort) as well as confounding by time-variant factors that do not change within a single month (e.g., socioeconomic status or chronic health conditions), making it useful for examining acute exposures to high levels of air pollution. We considered case dates as the day of presentation to the UUED. Controls (i.e., other “referent” times), days for the individual were then selected, and risk was estimated by comparing the date of the event to the referent dates.

We identified cases of spontaneous pregnancy loss diagnosed in the UUED by extracting data from the University of Utah Enterprise Data Warehouse using the following diagnosis codes: ICD-9-CM: 634.xx, 632.xx, 637.9; ICD-10: O03.4, O03.6, O03.9. We identified 1,577 events in the Enterprise Data Warehouse from 2007 to 2015. We excluded 73 events that occurred to women residing outside the state of Utah at the time of spontaneous pregnancy loss. For women who experienced multiple pregnancy loss events, we were not able to determine whether a second observation was truly another pregnancy loss event or related to the first event. Therefore, we included only the first event if the two events occurred within 14 weeks; we excluded 106 events that occurred within 14 weeks of a previous event. Our final sample consisted of 1,398 spontaneous pregnancy loss events that occurred before 20 weeks gestation. This study was approved by the Institutional Review Board of the University of Utah (IRB_00104032). Sample inclusion criteria are found in Figure 1 . County-level daily average temperature data was obtained from the United States National Centers for Environmental Information Climate Data Online ().

Discussion

2 , are associated with spontaneous pregnancy loss. There are several possible biologic mechanisms by which air pollution could contribute to spontaneous pregnancy loss including oxidative stress to the developing fetus, maternal endocrine disruption, and systemic maternal inflammation leading to abnormal placentation and growth abnormalities. Approximately 50% of early pregnancy spontaneous loss are attributed to nonchromosomal abnormalities ( 29 Møller P.

Loft S. Oxidative damage to DNA and lipids as biomarkers of exposure to air pollution. 30 Checa Vizcaíno M.A.

González-Comadran M.

Jacquemin B. Outdoor air pollution and human infertility: a systematic review. 31 Vrijheid M.

Martinez D.

Manzanares S.

Dadvand P.

Schembari A.

Rankin J.

et al. Ambient air pollution and risk of congenital anomalies: a systematic review and meta-analysis. 2 . Previous meta-analyses have shown NO 2 exposures were related to an increased risk in cardiac defects including coarctation of the aorta (OR = 1.17; 95% CI 1.00–1.36) and tetralogy of Fallot (OR = 1.20; 95 CI 1.02–1.42) ( 32 Nybo Andersen A.M.

Wohlfahrt J.

Christens P.

Olsen J.

Melbye M. Maternal age and fetal loss: population based register linkage study. 2.5 , although the estimates did not reach statistical significance. Because PM 2.5 and NO 2 are emitted from mobile sources, the results of our study add to a growing body of evidence that primary emissions contribute to spontaneous pregnancy loss. The results of this study provide evidence that acute elevated levels of ambient air pollutants, specifically NO, are associated with spontaneous pregnancy loss. There are several possible biologic mechanisms by which air pollution could contribute to spontaneous pregnancy loss including oxidative stress to the developing fetus, maternal endocrine disruption, and systemic maternal inflammation leading to abnormal placentation and growth abnormalities. Approximately 50% of early pregnancy spontaneous loss are attributed to nonchromosomal abnormalities () and maternal exposure to combustion particles is associated with oxidative damage to DNA and lipids (), which could be detrimental to the growing fetuses. Exposure to air pollution has also been shown to inhibit embryo implantation, which is a risk factor for spontaneous pregnancy loss (). We found the highest risk for spontaneous pregnancy loss occurred with a high 7-day average exposure to NO. Previous meta-analyses have shown NOexposures were related to an increased risk in cardiac defects including coarctation of the aorta (OR = 1.17; 95% CI 1.00–1.36) and tetralogy of Fallot (OR = 1.20; 95 CI 1.02–1.42) (), which supports a DNA damage-mediated pathway of embryonic disruption. We also found an increased risk for spontaneous pregnancy loss with exposure to PM, although the estimates did not reach statistical significance. Because PMand NOare emitted from mobile sources, the results of our study add to a growing body of evidence that primary emissions contribute to spontaneous pregnancy loss.

16 Ha S.

Sundaram R.

Buck Louis G.M.

Nobles C.

Seeni I.

Sherman S.

et al. Ambient air pollution and the risk of pregnancy loss: a prospective cohort study. 3 and PM 2.5 during pregnancy were positively associated with the risk of pregnancy loss (Hazard Ratio O3 = 1.12, 95% CI 1.07–1.17; Hazard Ratio PM2.5 = 1.13, 95% CI 1.13–1.24), but not NO 2 (Hazard Ratio NO2 = 1.03; 95% CI = 0.98–1.08). In two recent ecological studies, Dastoorpoor et al. ( 13 Dastoorpoor M.

Idani E.

Goudarzi G.

Khanjani N. Acute effects of air pollution on spontaneous abortion, premature delivery, and stillbirth in Ahvaz, Iran: a time-series study. 2 and premature birth, but not spontaneous pregnancy loss, and Enkhmaa et al. ( 15 Enkhmaa D.

Warburton N.

Javzandulam B.

Uyanga J.

Khishigsuren Y.

Lodoysamba S.

et al. Seasonal ambient air pollution correlates strongly with spontaneous abortion in Mongolia. 2 and spontaneous pregnancy loss (r > 0.8). The observed differences between studies may be due to underlying differences in the composition of air pollution in different geographic regions or due to methodological differences between our studies. Ha and colleagues ( 16 Ha S.

Sundaram R.

Buck Louis G.M.

Nobles C.

Seeni I.

Sherman S.

et al. Ambient air pollution and the risk of pregnancy loss: a prospective cohort study. 2 , our study benefits from self-matching and thus allowing us to find an effect at the individual patient level. Interestingly, results from previous studies have been mixed. Ha et al. () found chronic exposures to Oand PMduring pregnancy were positively associated with the risk of pregnancy loss (Hazard Ratio= 1.12, 95% CI 1.07–1.17; Hazard Ratio= 1.13, 95% CI 1.13–1.24), but not NO(Hazard Ratio= 1.03; 95% CI = 0.98–1.08). In two recent ecological studies, Dastoorpoor et al. () found a significant relationship between NOand premature birth, but not spontaneous pregnancy loss, and Enkhmaa et al. () found a strong correlation between NOand spontaneous pregnancy loss (r > 0.8). The observed differences between studies may be due to underlying differences in the composition of air pollution in different geographic regions or due to methodological differences between our studies. Ha and colleagues () used a prospective cohort from Michigan and Texas, and whereas their study did examine risk at varying time points, the exposures were measured through the entire pregnancy and thus were largely chronic in nature. In contrast, we tested for associations between spontaneous pregnancy loss and exposures to pollutants during a much shorter duration in time (3 days, 7 days) in an urban area where air pollution is highly varied based on weather patterns and temperature inversions. Because Utah has the lowest smoking rates in the United States and unique topography, our study area provided for a unique place to conduct this natural experiment. Although the two aforementioned ecological studies show a relationship between pregnancy outcomes and NO, our study benefits from self-matching and thus allowing us to find an effect at the individual patient level.

33 United States Census Bureau

American Fact Finder. The present study has some notable limitations and strengths. Air pollution exposure was assigned at zip code of residence and thus we were unable to measure air pollution at a smaller level of aggregation or total exposure across all daily activities. We were not able to determine the exact gestational age of the fetuses in our study and therefore we could not test for differences in the effect by exact gestational age at time of exposure. In addition, this study only captured women who presented to the UUED for care. Many other women may have sought outpatient care through their obstetric or primary care providers. Spontaneous pregnancy loss that occurs within the first several weeks of gestation may not be documented if a woman is unaware of the pregnancy and perceives the event as a normal menstrual cycle or if the nonviable pregnancy is not detected until the patient's first ultrasound. These factors will limit the absolute number of cases documented in our study period. Furthermore, we were able to document time of symptom presentation, but not time of actual embryonic or fetal demise. Because we cannot ascertain the exact time of spontaneous pregnancy loss, we explored 3-day and 7-day exposure windows. Strengths of this study include the large sample size (n = 1,398) and study design. The case-crossover design allows us to control for unobservable personal characteristics that do not change during the period of study including other risk factors for spontaneous pregnancy loss such as maternal age at conception, smoking behaviors, and previous spontaneous pregnancy loss ().