Infertility burden among women firefighters: a cross-sectional exploratory analysis

Abstract

Objectives

Despite biological and environmental plausibility, risk factors for infertility have not yet been studied among female firefighters. In this exploratory analysis, we investigated the burden of infertility among a subset of US firefighters enrolled in the Health and Wellness of Women Firefighters Study.

Methods

Women firefighters enrolled in the study responded to surveys administered in 2017 and 2019, reporting on their work environment, reproductive health, infertility history and fertility treatment history among those with infertility. Demographics and reproductive history of firefighters were compared by reported history of infertility. Log binomial regression models were used to estimate the association between occupational factors and risk of infertility.

Results

Of the 562 firefighters in our analysis, 168 of these women (30%) reported a history of infertility. A longer length of employment as a firefighter was associated with a modest, but non-statistically significant, increased relative risk of experiencing infertility (1.39, 95% CI 0.86 to 2.24).

Conclusions

This exploratory analysis highlights an opportunity to further examine infertility and impaired fertility among firefighters. Future studies may focus on comparing the burden of infertility in firefighters to the general population and evaluating the influence of additional occupational factors.

WHAT IS ALREADY KNOWN ON THIS TOPIC

• Female firefighters may have increased risk for certain adverse reproductive outcomes; however, risk factors for infertility have not been directly examined.

WHAT THIS STUDY ADDS

• In this exploratory analysis of 562 women firefighters, 30% reported a history of infertility. After adjusting for age, women with longer employment duration had increased risk for reporting infertility; however, this association was statistically non-significant.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

• Opportunities exist to further investigate infertility among women firefighters, including comparing the burden of infertility to the general population and evaluating the influence of additional occupational factors.

Introduction

Women firefighters may have increased risk for adverse reproductive outcomes such as miscarriage and preterm birth.^{1 2} Some occupational exposures that firefighters may face, including fine particulate matter, polycyclic aromatic hydrocarbons, shift work and certain perfluoroalkyl and polyfluoroalkyl substances, have been reported to be associated with increased risk of infertility, ovarian function or impaired fertility among non-firefighting populations.3,5 Despite environmental and biological plausibility, information on infertility and subfertility among male firefighters is limited,^{6 7} while even less is known among female firefighters.^{8 9} An analysis of anti-Müllerian hormone (AMH), a clinical marker of ovarian reserve, reported that average age-adjusted AMH levels were lower among women firefighters compared with non-firefighters; however, surrogate measures for occupational exposures, such as duration of employment, were not statistically significantly associated with a lower AMH.⁸ To our knowledge, risk factors for infertility among female firefighters remain underexplored. Therefore, in this exploratory analysis, we investigated the burden of infertility among a subset of 562 US firefighters enrolled in the Health and Wellness of Women Firefighters Study.

Methods

Study participants were selected from women enrolled in the Health and Wellness of Women Firefighters Study. Details of study recruitment and methodology have been previously reported.^{1 2} Female firefighters are difficult to recruit using traditional methods; therefore, snowball-sampling recruitment, a non-probability sampling method, was used. Participants were initially identified using affinity group email lists and asked to identify additional participants. A total of 1033 women firefighters in the USA, 18 years of age or older, responded to web-based surveys distributed in 2017 and 2019. The 2017 survey captured lifestyle and pregnancy history data and the 2019 survey captured data on reproductive health, infertility history and fertility treatment of those with infertility.

Firefighter occupational factors were reported during surveys in 2017 (employment status and wildland status) and in 2019 (total years as firefighter). Employment status was categorised as career salaried or volunteer. Wildland status was categorised as wildland/wildland-urban-interface (WUI) firefighter or structural firefighter. Employment status and wildland status have been previously associated with increased miscarriage and preterm birth risk among firefighters.^{1 2} Total years as a firefighter was selected as a surrogate for cumulative firefighter exposures, categorised as ≤5, 6–15 and >15 years.

We compared demographics and reproductive history of firefighters with a history of infertility (ever trying to become pregnant for a year or longer without success) to gravid firefighters without a history of infertility, excluding nulligravid women without infertility (n=471). Gravid status was defined as reporting becoming pregnant at least once, regardless of outcome or duration. Nulligravid firefighters reported never being pregnant. We performed a complete case analysis using log-binomial regression with sandwich variance estimators to estimate firefighter occupational factors and relative risks (RRs) and 95% CIs of risk of infertility adjusted for age and age-squared. Log-binomial regression was selected to directly estimate RRs rather than ORs and sandwich variance estimation was used to calculate robust SEs that can remain valid even when the correlation structure is misspecified.

Results

Among the 562 firefighters in our analysis, most were white and non-Hispanic (>90%), which did not vary by history of infertility (online supplemental table 1). A total of 168 women (30%) reported having experienced infertility (online supplemental figure 1). The reported median age at infertility was 31 years old (IQR 27–35, 21% missing response) (online supplemental table 1). Compared with gravid firefighters with proven fertility, those who had a history of infertility were more likely to be married or in a partnership (72% vs 65%) and hold a college education or higher (57% vs 49%). Age at survey did not vary greatly by infertility status, nor by marital status or education status (data not shown).

Among firefighters with infertility, 80% had visited a clinician to discuss infertility and 55% had used fertility treatments. Of those with clinically investigated infertility, 29% had an unknown reason for their infertility (online supplemental table 1). Other reported infertility diagnoses included male partner factors (20%), polycystic ovary syndrome (18%) and endometriosis (15%).

No statistically significant associations were observed between employment status (volunteer vs career firefighter: RR 1.09, 95% CI 0.80 to 1.48) or wildland status (wildland/WUI vs structural firefighter: RR 1.01, 95% CI 0.73 to 1.39) and risk of infertility (table 1). Working for >15 years as a firefighter was associated with a modest increased risk of infertility compared with ≤5 years, in models adjusted for age; however, this finding was not statistically significant (RR=1.39; 95% CI 0.86 to 2.24) (table 1). Sensitivity analyses excluding women who reported only male partner factors as an infertility diagnosis (n=25) yielded similar results (data not shown).

Table 1. Associations between occupational factors and risk of infertility among 562 women firefighters^*.

	History of infertility n (%)		RR (95% CI) Unadjusted model	RR (95% CI) Adjusted model†
Employment status	No	Yes
Career	319 (71)	133 (29)	1.00 (Ref.)	1.00 (Ref.)
Volunteer	75 (68)	35 (32)	1.08 (0.79 to 1.47)	1.09 (0.80 to 1.48)
Wildland firefighter status
Wildland/WUI	76 (70)	32 (30)	1.00 (Ref.)	1.00 (Ref.)
Structural	318 (70)	136 (30)	1.01 (0.73 to 1.40)	1.01 (0.73 to 1.39)
Total years of service‡
≤5 years	55 (72)	21 (28)	1.00 (Ref.)	1.00 (Ref.)
6–15 years	157 (69)	69 (31)	1.10 (0.73 to 1.67)	1.17 (0.77 to 1.79)
>15 years	177 (70)	75 (30)	1.08 (0.71 to 1.62)	1.39 (0.86 to 2.24)

^*Log binomial regression models and sandwich variance estimators were used to estimate RRs and 95% CIs. Firefighters who were missing information for exposures of interest (wildland (n=3) and total years of service (n=8)) and were not included in these models.

^†Adjusted for age at survey in 2019 (modelled as the sum of age and age-squared).

^‡Years of service reported at time of survey in 2019, not at time of infertility.

RR, relative risk; WUI, wildland urban interface.

Discussion

Approximately 30% of women firefighters in our sample reported history of infertility. Data from the US National Health and Nutrition Examination Survey (NHANES) estimated prevalence of infertility in US women aged 20–44 years to be 12.5%.¹⁰ It is challenging to directly compare infertility prevalence across studies given methodological differences. For example, participants in the current analysis were between the ages of 22–69 years at the time of survey; however, the prevalence of infertility among those of reproductive age (22–44 years) (n=302) was similar to the prevalence among the whole study sample (31% vs 30%). Future research should directly compare infertility burden between firefighters and non-firefighters. Among firefighters with infertility, 80% accessed infertility care, the same prevalence as among insured women with infertility in NHANES.¹⁰ We observed that usage of fertility treatments among firefighters with infertility was 55%, compared with 7%–15% among all women firefighters.¹¹

To our knowledge, this is the first study to investigate infertility among female firefighters; however, prior work has suggested that occupational exposures related to firefighting, such as exposure to both active and passive smoke, shift work and stress, may influence infertility risk.⁵ A recent study reported that self-reported clinically diagnosed anxiety or post-traumatic stress disorder (PTSD) was associated with statistically significantly lower AMH levels (−33% and −66%, respectively) among female firefighters,⁹ supporting potential biological mechanisms through which mental health conditions may influence firefighter reproductive health. Unfortunately, the current study lacked the data to examine this potential association with mental health and infertility, representing an important gap for future studies to address.

Selection bias that may have occurred due to the use of snowball sampling recruitment has been discussed in previous publications.^{1 2} However, there is also the potential for collider bias by factors that may be related to employment as a firefighter and risk of infertility, such as physical activity. Achieving and maintaining a certain level of physical fitness is a key requirement for firefighters. The role of exercise and physical activity in fertility is complex; however, some research suggests that regular vigorous exercise may adversely impact fertility in healthy women attempting to conceive.¹² Importantly, increasing caloric intake to reduce the resulting energy deficit may play a role in countering this potential risk.¹³ Future studies might consider the potential impact of this by assessing measures of physical activity on infertility among firefighters and recruiting non-firefighting controls.

Our data were based on self-report and potentially collected many years after incident infertility, which may introduce misclassification. We would expect this misclassification to be non-differential, which would likely attenuate findings but may be differential if infertility influenced career choices within the fire service. We observed a modest, non-statistically significant association between years as a firefighter and infertility that may be clinically meaningful. Given qualitative findings that report that reproductive health concerns adversely impact female recruitment and retention in the fire service,¹⁴ future research to identify occupational risk factors of infertility could be used to help address some of these concerns, and support the development of policies meant to improve female firefighter recruitment and retention. Among newly hired firefighters, proxies for fire exposure have been associated with meaningful epigenetic changes,¹⁵ a potential mechanism by which firefighter exposures could be associated with risk of disease. Future studies should focus on comparing the burden of infertility in firefighters compared with the general population and evaluating the influence of additional occupational factors, including psychosocial stressors.

Importantly, the small number of female firefighters with reported infertility limits our ability to detect potential associations and adjust for additional confounders. However, since the current study was conducted, larger prospective firefighter health cohorts in the US, such as the Firefighter Cancer Cohort Study (FFCCS) and the National Firefighter Registry for Cancer, have been established to follow health outcomes among firefighters throughout their career. In the case of the FFCCS, researchers have been able to expand their cohort to include hard-to-recruit subgroups of firefighters, including women. These prospective cohorts represent an important opportunity for future studies of firefighters to continue to explore potential associations with infertility and other reproductive health outcomes.

Data availability statement

Data are available on reasonable request.