RESUMEN
Nitrous oxide (N 2 O) is widely used as inhalation analgesic and anaesthetic in medical, paramedical, and veterinary practice. Previous evaluations resulted in classification of N 2 O as a possible risk factor for adverse reproductive health outcomes based on evidence from animal data. Available human data were considered inadequate, partly due to the possibility that other risk factors, such as co-exposures to other inhalation anaesthetics may have contributed to the adverse outcomes. As no substantial new human evidence has emerged since previous evaluations, this protocol describes a planned systematic review of the evidence obtained from animal studies. The aim is to assess the available evidence on the effects of N 2 O on reproductive and developmental outcomes in animals to inform a health-based recommended occupational exposure limit (OEL) for N 2 O. Comprehensive search strategies were designed to retrieve animal studies addressing N 2 O exposure from PubMed, EMBASE, and Web of Science. Screening of the studies retrieved will be performed by at least two independent reviewers, while discrepancies will be resolved by reaching consensus through repeated review and discussions. Articles will be included according to criteria specified in this protocol. Outcome data relevant for reproduction and development will be extracted and risk of bias will be assessed by two independent reviewers using the SYRCLE's risk of bias tool. Primary reproductive and developmental outcomes of interest will be the number of resorptions, malformations, and birth weight. We will focus on dose-response studies that allow to derive an OEL with the benchmark dose (BMD) approach. Adverse outcomes occurring at doses that are equivalent to the exposures occurring in human occupational settings will be particularly relevant for dose-response modelling. The proposed review has not been performed before. We will follow the procedures specified in this protocol. We will adhere to guidelines of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA), adapted for animal studies. Ethical approval will not be required, as the review will use existing data available in the public domain.