Air pollution and female fertility: a systematic review of literature

Air pollution is a cause of concern for human health. For instance, it is associated with an increased risk for cancer, cardiovascular and respiratory disorders. In vitro and in vivo studies suggested that air pollutants could act as endocrine disruptors, promote oxidative stress and exert genotoxic effect. Whether air pollution affects female infertility is under debate. The aim of the present study was to conduct a systematic review of studies that evaluated the impact of air pollution on female infertility. We systematically searched the MEDLINE (PubMed) and SCOPUS databases to identify all relevant studies published before October 2017. No time or language restrictions were adopted, and queries were limited to human studies. We also hand-searched the reference lists of relevant studies to ensure we did not miss pertinent studies. The risk of bias and quality assessment of the studies identified were performed using the Newcastle-Ottawa Scale. Primary outcomes were conception rate after spontaneous intercourse and live birth rate after in vitro fertilization (IVF) procedures. Secondary outcomes were first trimester miscarriage, stillbirths, infertility, number of oocytes and embryo retrieved. Eleven articles were included in the analysis. We found that in the IVF population, nitrogen dioxide and ozone were associated with a reduced live birth rate while particulate matter of 10 mm was associated with increased miscarriage. Furthermore, in the general population, particulate matter of 2.5 mm and between 2.5 and 10 mm were associated with reduced fecundability, whereas sulfur dioxide, carbon monoxide and nitrogen dioxide might promote miscarriage and stillbirths. The main limitation of our findigns resides in the fact that the desegn of studies included are observational and retrospective. Furthermore, there was a wide heterogenity among studies. Although larger trials are required before drawing definitive conclusions, it seems that air pollution could represent a matter of concern for female infertility. Electronic supplementary material The online version of this article (10.1186/s12958-018-0433-z) contains supplementary material, which is available to authorized users.


Introduction
Female infertility has increased in recent years [1]. It was estimated that this condition affects 1 in seven couples in developed countries [2]. Most cases of female infertility are related to specific disorders, namely, ovulatory disorders, endometriosis, chromosomal abnormalities and male factors [3][4][5][6][7]. There is also evidence that air pollutant could play a role in the pathogenesis of female infertility [8][9][10]. Air pollution appears to be a cause of concern for human health. For instance, it has been associated with an increased risk of cancer [11], and cardiovascular [12] and respiratory disorders in adults and children [13,14]. In addition, air pollutants have been associated with adverse perinatal outcomes [15,16].
Anthropogenic activities, namely traffic, industrial facilities and combustion of fossil fuels, which are particularly intense in large cities and in proximity of farms, are the main sources of health-related air pollutants. Air pollutants are in four main categories: gaseous pollutants (sulfur dioxide [SO 2 ], nitrate oxide [NO 2 ] and carbon monoxide [CO]), organic compounds (organic solvents or dioxins), heavy metals (lead and copper) and particulate matter (PM 10 PM 2.5-10 and PM 2.5 ) [17]. Ingestion and inhalation are the most common routes of exposure [17]. Ingestion is also facilitated by the fact that air pollution contributes to the contamination of food and water [18]. Some air pollutants, namely Cu, Pb and diesel exhaust seem to exert endocrine activity [19] that could affect female reproduction. Moreover, these endocrine "disruptors" exert estrogenic, antiestrogenic and antiandrogenic activity and some could interfere with the thyroid axis and influence metabolic disorders, such as insulin resistance and obesity, which are strictly related to infertility [20][21][22]. The increase in female infertility seems to parallel the increase in toxic emissions, which suggests that the impact of air pollution on human health could increase in the next years [23,24]. In an attempt to summarize current evidence, we carried out a systematic review of studies devoted to the impact of air pollutions on female infertility.

Protocol and eligibility criteria
The present study was exempt from institutional and ethics board approval because it did not involve human intervention. We adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [25]. The selection criteria are described according to PICO (Patients, Intervention, Comparison, and Outcomes). In detail, we evaluated fertility outcomes in women on reproductive age (in the general and IVF populations) in relation to exposure to air pollutants (Additional file 1: Table S1).

Search strategy
We conducted a systematic search using MEDLINE (PubMed) and SCOPUS databases to identify all relevant studies published before October 2017. Combinations of the following keywords and MESH search terms were used: "air pollutants" AND ("miscarriage" OR "embryo" OR "pregnancy" OR "IVF OR "fecundability" OR "infertility" OR "menstrual disorders"). No time or language restrictions were adopted, and queries were limited to human studies. We also hand-searched reference lists of relevant studies to ensure we did not miss pertinent studies.

Selection of studies
Four reviewers (G.C., M.M., G.CO and P.D.) independently evaluated titles and abstracts. Duplications were removed using Endnote online software and manually. Disagreements were resolved by discussion with a third authors (A.C. and C.D.), and if required, with the involvement of the most experienced authors (R.P.,C.A., G.D.). Articles were included only if they appeared in peer-reviewed journals. Case series, case reports, book chapters, congress abstracts and grey literature [26], which includes a range of documents not controlled by commercial publishing organization, were not included.

Data extraction
Data were extracted independently by four reviewers (G.C., M.M., G.CO and P.D.) using predefined data fields, including study quality indicators. Discrepancies were resolved by discussion with the senior authors (R.P., C.A. and G.D.).

Risk of bias, summary measures and synthesis of the results
The risk of bias and quality assessment of the included studies were performed adopting the Newcastle-Ottawa Scale (NOS) [27]. Four authors (A.C, C.D., G.C. and P.D.) independently assessed the risk bias for each study. The senior authors (R.P., C.A. and G.D.) resolved conflicts. The NOS score was used to evaluate the studies included, and judgment on each one was passed according to three issues: selection of the study group, comparability between groups, and ascertainment of exposed/not exposed cohorts. Primary outcomes were conception rate after spontaneous intercourse and live birth rate after IVF procedures. Secondary outcomes were first trimester miscarriage, stillbirths, infertility, number of oocytes retrieved and embryos transferred.

Study selection and characteristics
A total of 4687 items were identified (Pubmed 2834 and Scopus 1853). A total of 2013 duplicates were removed manually and using the EndNote online library. The titles and abstracts of 2674 papers were scrutinized and 21 full papers were assessed for eligibility. Ten papers were excluded because they did not fulfill the inclusion criteria. Eleven articles were included in the analysis (Fig. 1). The characteristics of the studies included in the present study are reported in Table 1.

Risk of bias
The risk of bias was evaluated with the NOS score and is reported in Table 1.

Summary of results
We summarized our findings considering per each pollutant both IVF women and reproductive age women in general population (Table 2).

NO 2 IVF cycles
Increases in NO 2 concentrations were significantly associated with a lower live birth rate especially from embryo transfer to pregnancy test (OR 0.76, 95% CI 0.66-0.86, per 0.01 ppm increase) [28]. No effect on the number of oocytes retrieved or embryo transferred was observed [28].

General population
In a cross-sectional study involving women of reproductive age between 15 and 40 years, the fertility rate was not significantly associated with NO 2 exposure (OR 0.97, 95% CI 0.94-1.003) [9]. In contrast, another retrospective cohort study, showed that there was a significant decreased fecundability ratio per each increase of 10 μg/m 3 NO 2 exposure (OR 0.72, 95% CI 0.53-0.97) [29]. Miscarriage rate was significantly increased in women exposed to NO 2 compared to not exposed group (OR 1.16, 95% CI 1.01-1.28, per each 10-ppb increase in NO 2 concentration) [30].

O 3 IVF-cycles
A detrimental effect was observed in terms of live birth rate in women exposed to O 3 from embryo transfer to date of live birth (OR 0.62, 95% CI 0.48-0.81, per 0.02 ppm increase) [28]. No effect on the number of oocytes retrieved or embryo transferred was observed [28].

General population
Only one study assessed the fecundability rate in the general population but no difference was reported between exposed and unexposed group [29].

PM 2.5 IVF cycles
Exposure to PM 2.5 during embryo culture was associated with a decreased conception rate (OR 0.90, 95% CI 0.82-0.99, per 8 μg/m 3 increase) but not with live birth rates [28]. No effect on the number of oocytes retrieved or embryo transferred was observed [28].

PM 2.5-10 General population
Multivariate HR analysis did not reveal any association between infertility and PM 2.  [8]. Conversely, another study reported a significant reduction of spontaneous fertility rate in women exposed to PM 2.5-10 (incidence risk ratio: 0.88, 95% CI 0.84, 0.94) [9].

PM 10 IVF cycles
No significant effect was observed in terms of live birth rate, number of oocytes retrieved or embryos transferred in exposed women undergoing their first IVF cycle [28]. Furthermore, no significant effect was observed in the amount of gonadotropin used, number of oocytes retrieved, number of MII oocytes, embryo quality, clinical and live birth rate [26,31]. A higher risk of miscarriage was observed in women with a higher exposure to PM 10 (> 56.72 μg/m 3 ) comparing with those exposed to lower amount of PM 10 (≤ 56.72 μg/m 3 ) (OR 5.05 95% CI 1.04-25-51) [31].

SO 2 IVF cycles
Exposure to SO 2 did not significantly affect birth rate, number of oocytes retrieved or embryos transferred in women undergoing their first IVF cycle [28].

General population
No differences in terms of adjusted fecundability rate was observed per an increase of 10 units in the SO 2 pollutant levels [29]. Conversely, in another study, the fecundability in the first unprotected menstrual cycle was significantly reduced only in couples exposed in the second month before conception to the following SO2 levels: 40-80 μg/m 3 (OR 0.57, 95% CI 0.37-0.88); ≥ 80 μg/m 3 (OR 0.49, 95% CI 0.29-0.81) [33]. The adjusted odds of miscarriage were significantly associated to SO 2 exposure (OR 1.13, 95% CI 1.01-1.28 per each 3 ppb increase in concentration) [30].

Traffic pollutants General population
In a large cohort study involving 4979 women, traffic pollutants were associated with an increased but not with significant risk of miscarriage rate among women exposed to a maximum annual average of traffic pollutants within  [35]. In another large cohort study, women living closer to a major road had a higher risk of infertility than did women living far from a major road (HR, 1.11 95% CI: 1.02-1.20) [8].

Coal combustion pollutants General population
In a small prospective study of 260 women, the miscarriage rate was higher, albeit not significantly, in women exposed to coal combustion pollutants than in non-exposed women (OR 2.99, 95% CI 0.91-9.80) [36] .

Discussion
Only 11 studies have evaluated the potential effect of air pollutants on female reproduction.. In the IVF context, NO 2 and O 3 were associated with impaired live-birth rates. In addition, exposure to high levels of PM 10 (> 56.72 μg/m 3 ) resulted in an increased miscarriage rate after IVF procedures. Consistently, no study reported a significant effect on other quantitative (i.e. number of oocytes retrieved, number of embryos transferred, and consumption of gonadotropin) and qualitative (embryo quality, and number of MII oocytes) IVF outcomes [28,31,32]. In natural conception, reduced fecundability was associated with solvents and SO 2 [33,34].
Notably while abortion rate was associated with traffic pollutants [8,35], and in particular SO 2 and NO 2 [30], no clear relation to coal combustion pollutants emerged [36]. Contrasting findings between infertility and PM 2.5-10 were reported [8,9]. Only three retrospective studies evaluated the effects of air pollution on IVF [28,31,32]. Although Legro and colleagues studied a large IVF population, the heterogeneity of IVF protocols and the lack of information about male partners represent two important limitation factors [28]. Moreover, the two studies conducted by Perin et al., are limited by the fact that only one pollutant was investigated and by the low number of cases enrolled [31,32].
Eight studies have been conducted on the general population. Of the three prospective studies, the one by Mahalingaiah et al. is the largest (more than 36,000 patients) and has the highest qualitative NOS score [8]. The quality of evidence was lowest in the study by Mohorovic et al. as was the number of observations, and the authors did not report effect size for each air pollutant separately [36]. The same weakness emerges in the Green et al. paper, which however analyzed such important factors as work exposure, residential history and employment status of the population studied [35]. Of the five retrospective studies conducted to-date, the quality of evidence is highest in two large studies conducted by Faiz and colleagues [30] and by Slama and colleagues [29] demonstrating that air pollutants significantly affect fertility and stillbirths rates. The remaining three retrospective studies have several limitations, namely a paucity of data regarding the population studied [9], a low number of pollutant analyzed [33] and the methods adopted to assess exposure [34].
The relationship between air pollutants and spontaneous fertility was first observed in an animal model [37]. In detail, Mohallem et al. observed an increased implantation failure rate and a significant reduction of births in mice exposed to polluted city air compared to non-exposed mice [38]. Similarly, Veras et al. found significantly fewer antral follicles and a lower fertility index in mice exposed to traffic pollutants versus non-exposed mice [39].
The effect of air pollutants on human spermatogenesis has also been investigated [40][41][42][43]. The largest study, conducted by Hammoud et al., reported that PM 2.5 exposure negatively correlated with sperm morphology and motility [40]. The negative effect of particulate matter was confirmed in a recent prospective cohort study that identified a significant association between PM 10 and PM 2.5 and sperm chromosomal abnormalities (i.e. disomy Y and disomy chromosome 21) [44].
The mechanism underlying the effect of air pollutants on female fertility is still a matter of debate. Several pathogenetic mechanisms have been proposed. Firstly, it was hypothesized that air pollutants could mimic the effect of androgens and estrogens in humans [45]. These endocrine-disrupting properties could exert their effect by interacting with nuclear receptor, the estrogen or androgen repertory or by interacting with specific targets in cytosol thus resulting in activation of the /Ras/Erk pathway [46]. Others have suggested that air pollutants could promote oxidative stress and inflammatory processes [17]. In this sense, we recently demonstrated that the addition of anti-oxidant factors to ovarian stimulation could improve reproductive outcome in women with polycystic ovarian syndrome [47]. However, whether antioxidant products could mitigate the effect of air pollutants on IVF outcomes remains to be determined. Finally, it has been suggested that air pollutants could exert a genotoxic effect. For instance, increased sperm DNA fragmentation was associated with exposure to elevated air pollution levels (at or above the upper limit of US air quality standards) [48]. Furthermore, DNA methylation seems to be significantly influenced by air pollutants [49]. Indeed, in a recent study of 777 men, an increase in air pollutant concentrations was significantly associated with F3, ICAM-1, and TLR-2 hypomethylation, and IFN-γ and IL-6 hypermethylation [50].
Our review has several limitations. First, most of the studies included in our analysis are observational and retrospective, and hence more prone to bias. Second, exposure ascertainment was heterogeneous among studies.
Most of the trials assessed air quality using a specific air monitoring station, others estimated exposure according to proximity to the potential source [8,35,36]. In addition, the reference levels of each pollutant varied significantly among studies. Lastly, the populations investigated as well as the definitions used to assess infertility and miscarriage were also heterogeneous. These factors render a meta-analytic and quantitative approach to this issue challenging.
In conclusion, our meta-analysis suggests there is a close association between female infertility and air pollution. However, a more robust meta-analytic approach is required before any definitive conclusion can be reached.

Additional file
Additional file 1: Table S1.