Variation and Uncertainty of Microplastics in Commercial Table Salts: Critical Review and Validation

Abstract

Increasing concern of human exposure to microplastics (MPs) necessitates an assessment of the quality of MP data relevant to human exposure. In this literature review for table salt, we addressed the variability and uncertainty of MP data caused by different analytical methods among studies. Additionally, validation experiment was conducted to identify and correct uncertainties related to MP size. When combined without validation, salt data in literature (n = 150) showed a wide range of 0-39800 (1386 ± 5477) MPs kg^-1. All procedures, including sample treatment, MP identification, and quality assurance were related to this variability. Most serious variability originated from the MP identification methods associated with minimum cut-off size of targeted/measured MPs and the selection of particles identified. When not corrected by size, MP content differed by 10-600 times among MP identification methods, with greatest value from visual observation, followed by FTIR and Raman methods. Meanwhile, there was a significant correlation-regardless of identification method-between logarithmic mean abundances and minimum cut-off sizes. The size-corrected values showed that adults intake up to 19000 MPs ≥10 μm annually via table salt, compared with 5100 MPs that was estimated from uncorrected mean abundance. Our validation experiment also showed the possibility of serious errors being caused by arbitrary selection of "MP-like particles" in spectroscopic analysis, specifically for smaller-sized particles. A combination of unverified data originated from different methods might have failed to adequately produce reliable human health-relevant results, thereby undermining the ability to quantify human risk.

Keywords: Human intake; Literature review; Minimum cut-off size; Polymer identification method; Small-sized microplastics.