Microplastic contamination of salt intended for human consumption: a systematic review and meta-analysis

Microplastics (MPs) are an emerging contaminant ubiquitous in the environment. There is growing concern regarding potential human health effects, a major human exposure route being dietary uptake. We have undertaken a systematic review (SR) and meta-analysis to identify all relevant research on MP contamination of salt intended for human consumption. Three thousand nine hundred and nineteen papers were identified, with ten fitting the inclusion criteria. A search of the databases MEDLINE, EMBASE and Web of Science, from launch date to September 2020, was conducted. MP contamination of salt varied significantly between four origins, sea salt 0–1674 MPs/kg, lake salt 8–462 MPs/kg, rock and well salt 0–204 MPs/kg. The majority of samples were found to be contaminated by MPs. Corresponding potential human exposures are estimated to be 0–6110 MPs per year (for all origins), confirming salt as a carrier of MPs. A bespoke risk of bias (RoB) assessment tool was used to appraise the quality of the studies, with studies demonstrating moderate to low RoB. These results suggest that a series of recurring issues need to be addressed in future research regarding sampling, analysis and reporting to improve confidence in research findings.