Association between outdoor air pollution and semen quality Protocol for an updated systematic review and meta-analysis

Background: Various studies have explored the association between outdoor air pollution and semen quality. However, the results were still controversial. The object of the current meta-analysis is to evaluate the role of outdoor air pollution in semen quality. Methods: Databases including PubMed, Web of Science, and Embase will be searched to identify qualified studies. All qualified cross-sectional studies researching the association between outdoor air pollution and sperm parameters will be included. Relative data in participants under higher exposure and lower exposure to air pollution will be extracted by 2 investigators independently. Only participants under the highest and the lowest exposure will be enrolled if the original study contained more than 2 exposure levels. The semen volume, sperm concentration, progressive motility, total motility, and normal morphology rate will be the primary outcomes of the current study. Pooled estimates with corresponding 95% confidence intervals will be calculated to assess the specific effects of outdoor air pollution in semen quality. Moreover, trial sequential analyses will be performed to obtain a more comprehensive assessment of analyses. Results: A high-quality synthesis of the current evidence for the association between sperm parameters and outdoor air pollution will be provided. Conclusions: This meta-analysis and systematic review will generate evidence for judging whether outdoor air pollution can impair semen quality. PROSPERO registration number: PROSPERO CRD 42019126060