Aim This study evaluates the effectiveness of preprocedural mouth rinsing when performing non-surgical periodontal prophylaxis to reduce bacterial aerosol contamination.Data sources The authors used the population, intervention, control and outcomes (PICO) question format to perform a systematic online search in Scopus, PubMed, Cochrane Library and Web of Science. All the databases were explored with no time limit until April 2020.Study selection Randomised clinical trials were included in this systematic review. No inclusion or exclusion criteria are mentioned in this study and it is limited to bacterial contamination studies.Data extraction and synthesis The authors extracted the following information from the included studies: authors; article publication year; study design; sampling size and allocation of test and control groups; intervention details; type and description of periodontal prophylaxis procedures; and primary outcomes. They focused on statistically significant findings and the reduction of bacterial aerosol contamination between groups was measured by colony forming units (CFU) using means and percentages. The primary outcome was that bacterial count expressed as CFU on blood agar plates. If permissible, they calculated the mean CFU reduction in the included studies and reported them. The authors also assessed the studies' risk of bias using the revised Cochrane risk of bias tool for randomised trials.Results In total, 30 randomised controlled trials were included, 21 of which assessed the preprocedural rinsing and the remaining nine focused on other interventions. The bacterial incubation protocol differed among studies. The data collection sites differed among the studies in terms of the number of samples obtained, position, direction and distance from the subjects' mouths. There were no marked differences in the CFU reduction regarding the periodontal prophylaxis devices used and the location of aerosol sampling collection from these studies. Nearly half of the studies (52.4%, 11/21) collected the sample at or near the operator and dental assistant. Chlorhexidine (CHX) rinse was mostly tested (80.9%, 17/21) with various concentrations and volumes. Among studies comparing CHX with other agents (71.4%, 15/21), CHX was more effective for the better part of studies, with more than half of the studies (7/15) reporting over a 70% reduction in CFU when using CHX preprocedural mouth rinse.Conclusions Evidence corroborates the effectiveness of preprocedural mouth rinses, especially CHX, in reducing the bacterial contamination of aerosols in periodontal, non-surgical prophylaxis compared with mouth rinsing with water, or with no rinsing. Nevertheless, the low-quality of evidence and the high heterogeneity among the included studies warn that one should be cautious in generalising the findings of this study to other settings and contaminating agents, like viruses. The findings of this study emphasise the need for more studies on the efficacy and effectiveness of preprocedural rinses in reducing the viral load in aerosols (and droplets) during periodontal prophylaxis procedures.