In this study, we developed a simple and cost-effective protocol to isolate MPs from salt samples. The presence of insoluble particles quickly blocked the filter papers with pore sizes of 2.7, 8, and 22 μm. Initially, we hypothesized that the blockage was due to the presence of organic materials. However, lack of changes on the filtration after KOH digestion showed the presence of digestion-resistant organic or inorganic materials. Therefore, dilutions with deionized water, followed by filtration through a membrane with a larger pore size (149 μm), was the simplest and quickest method to isolate MPs from the salt samples. Our recent study demonstrated a high efficcincy of 4.4 M NaI solution to separate (recovery rate >95%) plastic polymers from high-density particles like shell fragments from bivalves and sand grains3. To minimize the chance of overestimating or underestimating MP prevalence in the salt samples, in addition to NaI extraction, we implemented microscopical examination and Raman spectroscopy. Despite processing the samples through NaI extraction and visual identification, 5.50% of the particles were amorphous carbon (Supplementary Information Figure 1e), which underscores the necessity of using spectroscopic techniques to identify the chemical composition of the isolated particles. Micro-Raman spectroscopy is a highly specific technique used to identify the composition of biological, mineral or polymer samples. It offers a number of advantages such as analysis of microscopic particles while being non-invasive towards the samples23,25. In addition, Raman measurements do not depend on the transmission of light through the particle, which consequently allows an accurate analysis of thicker or pigmented particles23. Some of the earlier studies solely relied on the morphological characteristics of MPs, like shape and color to identify MPs in environmental samples26,27 while others partially confirmed the particle composition through a random selection of the isolated particles21,28. Although observation is an indispensable part of polymer identification, it cannot be employed as a stand-alone technique for particle characterization because it is unlikely to be sufficient to identify the polymer type through morphological features.

Consistent with the findings of this study, fragments and filaments have been reported as the main form of MPs29,30. The absence of microbeads in the salt samples may indicate their low prevalence in aquatic environments. Polypropylene (PP) and polyethylene (PE) were the most abundant plastic polymers in the salt samples (40.0% and 33.3%, respectively) which is consistent with reports on their wide distribution in the marine environment22. The presence of these polymers in salt samples could be due to the low density of PP (0.90–0.91 g/cm3) and PE (0.91–0.96 g/cm3) allowing these to float on the water surface and be readily directed into saltpans. In addition, their low density may facilitates their spread by becoming airborne. Polyisoprene/polystyrene (styrene-isoprene-styrene block copolymer) were the other detected synthetic polymers in the salt samples. These polymers are used when elasticity and easy processing is required, such as in adhesives and sealants31. A few isolated particles from the salt samples had a similar composition to their packaging. This might indicate degradation of the packaging materials leading to the contamination of the salt product. Nevertheless, this hypothesis was rejected since all the fragments or films were highly corroded, indicating their long-term presence in the environment.

Almost one fourth of the isolated particles were identified as pigments (phthalocyanine, chromate yellow, and hostaperm blue) because the strong Raman signal of these pigments hindered the identification of plastic polymers. Phthalocyanine is a synthetic pigment and is extensively used in the plastics industry32 and was the main pigment isolated from the salt samples (Fig. 2c). Hostaperm blue falls under the copper phthalocyanine chemical class and is an industrial dye that is mainly used in the plastics industry32. Victoria blue is commonly used as a coloring agent in polyacrylic fibers (Supplementary Information Figure 1a)30 that are mainly introduced to the marine environment through the washing of clothes after passing through sewage treatment plants34. Meanwhile, lead chromate (yellow) pigment is a toxic compound that has extensive applications in paints and plastic industries owing to its excellent light-fastness and low cost33,35. Earlier studies have attributed exposure to lead chromate pigment with incidents of bronchial carcinoma36, cerebrovascular disease37, and nephritis38 in humans. However, the occurrence of only one particle of lead chromate pigment per Kg of salt # South Africa-Q, poses a negligible threat to the health of consumers.

Initially, we hypothesized that the pigment particles might be paint particles. However, since none of the extracted particles shared similar mechanical properties as paint particles like brittleness39, we suggest the absence of paint particle in the salt samples. Other than plastics, pigments are widely used in other materials like textile, rubber, and fiberglass40,41. Van Cauwenberghe et al. suspected that the particles identified as copper phthalocyanine, polychloro copper phthalocyanine, and permanent red in deep-sea sediments42, as well as copper phthalocyanines and haematite in bivalves14 to be plastic materials. Similarly, in this study we could confirm that the pigmented particles had an anthropogenic origin but could not ensure they were MPs.

In the present study, a significant portion (29.1%) of the particles was not identified by Raman spectroscopy. Photo-degradation and weathering are the two major factors suggested as the causes for variation in the spectroscopic spectra of polymers such as PVC23. Moreover, the presence of additives could alter the polymer spectra and hinder comparisons with the reference library23,42. Another reason for having unidentified samples is the lack of a comprehensive spectra library to identify mixed samples23.

Lake salt from Malaysia was excluded from MP analysis because this contained a large volume of sediments, which were later identified as CaCO 3 . Calcareous sedimentation is a common process occurring in lakes mainly due to the assimilation of carbon dioxide by photosynthesizing plants and/or seasonal temperature effects on the solubility of carbon dioxide and calcite43. It should be noted that the other lake salt (# Iran-I) did not contain calcareous sedimentation, which shows the variation in calcium contents among different lake salt brands.

In 2010, the global daily sodium consumption was 3.95 g/day (equivalent to 9.88–10.2 g salt/day44) corresponding to 3.6 to 3.7 Kg salt per annum. The number of anthropogenic particles (MPs and pigments) detected in the salt samples ranged from 0 (sample # France-F) to 10 (sample # Portugal-N) MPs/Kg. Based on this data, humans could ingest a maximum of 37 plastic particles annually. This should take into account that this maximum value is based on the assumption that sea salt is the sole source of sodium intake. Other sources of sodium supply are food additives like monosodium glutamate and preservatives. Therefore, in real scenarios, the maximum MPs intake is probably even less than 37 particles.

Microplastics may cause adverse effects to organisms through causing micro injuries (mainly in the case of fragments)4 or the release of pollutants that had been sorbed during their prolonged incubation in the water. In the case of the latter, MPs have shown the ability to sorb persistent organic pollutants (POPs)45 and subsequently desorb them under simulated gut conditions46. However, recent studies have argued the lower importance of MPs as a vector for translocation of POPs to aquatic biota as compared to other routes like food and water15,16. Despite a potentially high concentration of POPs and other contaminants in MPs, the combination of their small particle size and low prevalence indicate that the consumption of sea salt does not appear to be a major route for the contaminant transfer into the human body as compared to other sources like water and food. Due to technical limitations, however, we could only quantify the level of particles larger than 149 μm. The prevalence of smaller particles in the salt samples, however, might be higher than the larger ones. Smaller sizes could facilitate their translocation into other organs and, therefore, cause a higher degree of toxicity. For example, in a study by Lu et al.47, 20 μm polystyrene (PS) microspheres accumulated in the gills and gut of zebrafish (Danio rerio), while 5 μm microbeads were incorporated into the gills and gut as well as liver. Further advances in isolation techniques are needed to quantify smaller MP particles before making a more accurate justification on the health impacts of sea salt consumption. However, it should be taken into account that salt is not the only edible item that has been shown to contain MPs. These have been previously detected in clams12, mussels14, fish13 and unexpectedly in honey48 as well as beer49. Therefore, the long-term consumption of various products containing MPs might become a concern.

Due to their low density and slow degradation, plastics are becoming the chief cross-border contaminant that often travels far from their original source. Hence, MPs found in the salt samples of one country could have been produced by another country thousands of miles away. A potential solution to this global dilemma requires a strong commitment from all the countries to make a substantial improvement in plastic disposal and recycling.