Exposure to Fine Particulate Matter and Ovarian Reserve Among Women from a Fertility Clinic.

Study participants were women (18 to 45 years) presenting to the Massachusetts General Hospital (MGH) Fertility Center for infertility treatment who enrolled in the Environment and Reproductive Health (EARTH) Study. 15 Approximately 60% of eligible women contacted by the research nurses enrolled. Of the initial 806 women (954 antral follicle scans) available for analysis, we excluded incomplete scans, those done while the woman was on Lupron, those done on women with polycystic ovaries, repeated scans, and scans lacking complete exposure data ( eFigure 1 ). This resulted in a final sample size of 632 women who contributed one unstimulated antral follicle count between 2004 and 2015. The EARTH study was approved by the Human Studies Institutional Review Boards of MGH and the Harvard T.H. Chan School of Public Health. Upon enrollment, all participants provided their residential address and this was geocoded using ArcGIS. We estimated daily residential PM 2.5 exposures starting from three months prior to the antral follicle count date to correspond with the proposed window of antral follicle development (~2–4 months). 14 As a negative control window, we also estimated daily PM 2.5 exposures in the 3 months after the antral follicle count. PM 2.5 exposures were estimated with a validated hybrid model of satellite–derived aerosol optical depth measurements and land-use terms. 16 These daily 1 km 2 spatial resolution values were then averaged to obtain the woman’s 3-month average PM 2.5 exposure preceding the scan. Ovarian antral follicle count was measured using transvaginal ultrasonography by one of the MGH reproductive endocrinologists on the 3 rd day of an unstimulated menstrual cycle or progesterone withdrawal bleed. All follicles above 2 mm were counted. No fertility medications were used in the cycle prior to the count. To reduce the influence of very high counts, we truncated the measure at 30 (15 women, 2% of population). Date of birth was collected at entry, trained study staff measured weight and height to calculate body mass index (BMI) (kg/m 2 ). A detailed take-home questionnaire contained questions on lifestyle factors, reproductive health, and medical history. We defined regular menstrual cycles as being predictable within 10 days, and assessed time spent in leisure time physical and sedentary activities using a validated questionnaire. 17 We abstracted infertility diagnosis from electronic medical records. We calculated descriptive statistics and compared them across quartiles of estimated average PM 2.5 exposure. Differences in demographic and reproductive characteristics were evaluated using Kruskal–Wallis tests for continuous variables and chi-squared tests for categorical variables (or Fisher’s exact test where appropriate). We used Poisson regression models to estimate the association of estimated average PM 2.5 exposure with antral follicle count. Non-linearity was assessed non-parametrically with restricted cubic splines, which used the likelihood ratio test comparing the model with the linear term to the model with the linear and the cubic spline terms. 18 Results are presented as either adjusted % change in antral follicle count per 2 μg/m 3 increase in estimated PM 2.5 exposure (the interquartile range) or population marginal means. We assessed confounding based on biological relevance and descriptive statistics from our study population. Final models were adjusted for age, BMI, smoking status, year of antral follicle count (as a quadratic function), and season of count. We tested effect modification of the relationship between estimated average PM 2.5 exposure and AFC by age, BMI, smoking status, infertility diagnosis, and menstrual cycle characteristics, all well-known predictors of ovarian reserve, by adding a cross-product term to the final multivariable model.

The 632 women had a mean (standard deviation) age of 35.3 (4.2) years and BMI of 24.4 (4.7) kg/m 2 . The majority of women were never smokers (73%) and of Caucasian race (84%) with a college degree or higher (93%). The most common infertility diagnosis at enrollment was unexplained (41%). Women in our cohort resided in Massachusetts (96%), New Hampshire (2%), and Rhode Island (1%), as well as Maine and a few states outside of New England (<1%). The median antral follicle count in our study was 12 (range=1 to 30) ( eFigure 2 ). During the study period (2005–2015), the average estimated PM 2.5 concentration in our population was 9.0 μg/m 3 (range=5.4 to 16.4 μg/m 3 ), which was similar to the average PM 2.5 concentration across the US (mean=10.2 μg/m 3 ) ( eFigure 2 ). 19 Our PM 2.5 concentrations also tended to decrease over time, mirroring national trends. 19 Women in the highest quartile of estimated exposure to PM 2.5 were, on average, heavier and their antral follicle counts tended to be measured during the earlier years of the study and during the summer months compared to women in the lowest quartile ( Table 1 ). All other characteristics were similar. A 2 μg/m 3 increase in estimated average residential exposure to PM 2.5 in the 3 months prior to the antral follicle count was associated with a −7.2% (95% CI −10.4%, −3.8%) lower count adjusting for age, BMI, smoking status, and year and season of count ( Table 2 ). This translated into approximately 1.1 fewer antral follicles for every 2 μg/m 3 increase in estimated average exposure to PM 2.5 ( Figure. 1 ). There was no evidence of a departure from linearity. For comparison, a 2-year increase in age was associated with a −8.0% (95% CI −8.9%, −7.0%) lower antral follicle count. In contrast, a 2 μg/m 3 increase in estimated average residential exposure to PM 2.5 in the 3 months after antral follicle count was associated with an imprecisely measured −1.4% (95% CI −5.0%, 2.2%) lower count after multivariable adjustment. The association between estimated PM 2.5 exposure and antral follicle count was similar across age (<35 vs. ≥35 yrs), BMI (<25 vs. ≥25 kg/m 2 ), and smoking status (never vs. ever) groups; however, the estimated effect of PM 2.5 exposure on antral follicle count was stronger among women whose primary infertility diagnosis was attributable to a female cause (% change= −16.3% 95% CI −21.5, −10.7%) compared to women with an unexplained or male factor diagnosis (% change= −2.8% 95% CI −6.9, 1.6%) (p-for-interaction=<0.001) ( Table 3 ). Moreover, this effect was consistently negative across all diagnostic categories of female factor infertility (e.g. diminished ovarian reserve, endometriosis, ovulation disorders, tubal, and uterine). The association between estimated PM 2.5 exposure and antral follicle count was also more pronounced among women with non-regular menstrual cycles (p-for-interaction=<0.001) and those with short (38 days) cycles (p-for-interaction=0.01) compared to women with regular and normal length menstrual cycles, respectively. While the association between estimated PM 2.5 and antral follicle count was slightly stronger between years 2005–2009 (% change= −7.6% 95% CI −12.7%, −2.4%) versus 2010–2015 (% change= −5.0% 95% CI −9.8%, 0.1%); the difference was imprecise. Results for the main analysis were also consistent after excluding women who resided outside of Massachusetts (% change= −6.8% 95% CI −10.2, −3.3%).

In our prospective study of women seeking infertility treatment, we found that higher residential exposure to PM 2.5 was inversely associated with antral follicle count, a well-accepted marker of ovarian reserve. Moreover, the magnitude of this association was approximately equivalent to a 2-year increase in female age. Our results also suggest that the effects of PM 2.5 on antral follicle count may be more pronounced among women with a female factor infertility diagnosis and abnormal menstrual cycles, whose counts are generally lower than those of women presenting with other diagnoses or regular menstrual cycles. To our knowledge, this is the first study in humans to investigate a link between air pollution exposure and a biomarker of ovarian aging. In the laboratory setting, three previous studies have examined the association between air pollution exposure and ovarian reserve in mice. In the first study, mice exposed to nonfiltered ambient air sampled close to a highly trafficked street (average PM 2.5 of 27.5 μg/m 3 per day) prior to and during pregnancy had reduced numbers of antral follicles compared to mice who only received filtered air. 13 In the second study, there was a reduction in the proportional area occupied by primordial follicles in mice exposed to diesel exhaust (average PM 2.5 of 21.5 μg/m 3 per day) during pregnancy, postnatally, or both periods compared to mice exposed to only clean air. 12 In the third study, mice treated with a PM 2.5 suspension (10 mg/kg), every 2 days, for 22 days had serum anti-Müllerian hormone levels that were decreased by more than half compared to the control group. 11 Furthermore, IL-6 and TNF-α concentrations and the number of apoptotic cells were increased in ovarian tissue and ovarian histologic structures showed evidence of hemorrhage and vascular congestion in mice exposed to PM 2.5 compared to the control group. 11 Taken together with our findings, these data suggest that exposure to PM 2.5 throughout adult life may enhance follicular atresia through effects on ovarian inflammation, oxidative stress, and apoptosis, even at exposure levels within the World Health Organization guidelines (e.g. <25 μg/m 3 24-hour mean). Limitations of our study are worth noting. Due to the sole inclusion of women undergoing infertility treatment, it may not be possible to generalize our findings to all women of reproductive age. However, previous work has shown that infertile women <40 years have similar antral follicle counts compared with women of the same age with no history of infertility. 20 We also used residence-based PM 2.5 exposure as a proxy for personal exposure, potentially leading to exposure misclassification particularly due to lack of information on the women’s work addresses or their time–activity patterns. However, the spatio-temporal models we used are validated and the use of outdoor ambient exposures can be valuable because regulation typically focuses on these concentrations. We also lacked information on other spatial variables such as noise or light pollution, which tend to correlate with PM 2.5 exposure and may still confound our association between PM 2.5 and antral follicle counts. Strengths of our study include its prospective design, large sample size, gold-standard assessment of ovarian reserve, 14 and our comprehensive adjustment for other reproductive and lifestyle factors that enhanced our ability to adjust for confounding. The null results we observed in our sensitivity analysis using the average estimated PM 2.5 concentrations in the 3 months after antral follicle assessment (our negative control window) also further strengthen our argument for a causal association between PM 2.5 in the 3 months prior to antral follicle count and assessment of ovarian reserve. In conclusion, our study’s findings are consistent with the hypothesis that exposure to relatively low concentrations of PM 2.5 may decrease human fertility by accelerating ovarian aging. Moreover, this association may be particularly pronounced among women with an existing female-specific cause of infertility and women with abnormal menstrual cycles.

Air pollution, specifically exposure to fine particulate matter (PM 2.5 ), is a substantial global health concern, 1 responsible for a growing number of adverse health effects such as cardiovascular disease, 2 stroke, 3 and cancer. 4 In the past decade, there have also been an increasing number of reports linking air pollution to diminished fertility. 5 , 6 Specifically, live birth rates were lower among women residing in census tracts with higher exposure to PM 2.5 and other traffic-related air pollutants 7 and women with higher PM 2.5 exposure had slightly longer time to pregnancy. 8 Adverse effects of PM 2.5 on fertility may be due to perturbations in semen quality 9 and reproductive hormones 10 among men; however, mechanisms underlying the potential effects of PM 2.5 on markers of female fertility have been understudied. Animal studies suggest that increased exposure to PM 2.5 may compromise female reproductive potential by accelerating reproductive aging. 11 – 13 Therefore, to address this question in women, we investigated the association between estimated residential exposure to PM 2.5 and antral follicle counts, the gold standard measure of ovarian reserve, 14 among women presenting to an infertility clinic.