A down-side of living in Los Angeles is its infamous smog, which is a factor in respiratory conditions such as asthma, bronchitis, and COPD. Now, a new study has reported a link between exposure to air pollution and infertility. The findings were published online on January 2 in the journal Human Reproduction by researchers at Boston University School of Medicine, Boston, MA; and Harvard Medical School, Boston, MA.
The study authors note that exposure to air pollution has been associated with lower conception and fertility rates; however, its impact on infertility incidence is unknown. Therefore, they conducted a prospective (forward-looking) study using data extracted from 116,430 female nurses from September 1989 through December 2003 who were enrolled in the Nurses’ Health Study II cohort.
Infertility was defined by a report of attempted conception for 12 or more months without success. The women were able to report if evaluation was sought and if so, offer multiple clinical reasons for infertility. After exclusion, 36,294 women were included in the analysis. Proximity to major roadways and ambient exposures to particulate matter less than 10 microns (PM10), between 2.5 and 10 microns (PM2.5–10), and less than 2.5 microns (PM2.5) were determined for home addresses for the 36,294 women from 1993 through 2003. The data was subjected to statistical analysis.
The investigators found that over 213,416 person-years, there were 2,508 reports of infertility and results for overall infertility were inconsistent across the different types of exposure. They found a small increased risk for those living closer to compared to farther from a major road. This risk was similar for women reporting primary or secondary infertility. (Women with primary infertility have never conceive, while secondary infertility refers to women who have delivered a child but were unable to conceive subsequently.) Furthermore, the increased risk for every 10 µg/m3 increase in cumulative PM2.5–10 among women with primary infertility was a 10% increase for both women with primary and secondary infertility.
The authors note a limitation of the study was that within the two-year window of infertility diagnosis, they did not have the exact date of diagnosis or the exact timing of the start of attempting conception. Because infertility status and subtypes of infertility were prospectively collected biennially (every other year), they were unable to closely examine the timing of exposures on the incidence of infertility. In terms of quantifying exposure, we used ambient air pollution exposures as a proxy for personal exposures, potentially leading to exposure misclassification. (Ambient exposure refers to exposure in the immediate surroundings.) However, several studies suggest that ambient measurements are an acceptable surrogate for individual level exposures in most populations.
The authors concluded that they found an association between all size fractions of PM exposure, as well as traffic-related air pollution, and incidence of infertility. They stressed that the strongest association was observed between cumulative average exposures over the course of follow-up and the risk of infertility. This finding suggested that chronic exposures may be of greater importance than short-term exposures.