Point-of-Purchase Patterns of Cadmium, Lead, and Other Toxic Elements in Houttuynia cordata Thunb. Rhizomes from Retail Markets Across Guizhou, China

Houttuynia cordata Thunb. (HCT) is an edible and medicinal rhizome widely consumed in southwestern China, but its contribution to dietary exposure to toxic trace elements remains insufficiently characterized. Total Hg, As, Cd, Pb, and Cr were quantified in 101 market-level composite samples collected across 76 county-level units of Guizhou Province, and regulatory compliance, point-of-purchase variation, multi-element co-occurrence, and adult dietary exposure were evaluated. Mean fresh-weight concentrations were 4.8, 4.1, 111.5, 291.3, and 6.3 ng/g for Hg, As, Cd, Pb, and Cr, respectively. Pb and Cd exceeded the applicable Chinese maximum limits in 87.1% and 49.5% of samples, respectively, whereas Hg exceeded its limit in 4.0%; no exceedances were observed for As or Cr. FDR-adjusted correlations and principal component analysis indicated limited multi-element co-occurrence rather than a single uniform contamination pattern. The five-element comparative screening HI increased with HCT consumption, with 1, 3, and 17 samples exceeding 1 under the low-, medium-, and high-consumption scenarios, respectively. Cd and Pb were the principal contributors, accounting for mean sample-level contributions of 55.5% and 29.0%, respectively, and together representing 84.5% of the HI. Although the median As contribution was only 4.0%, its 95th percentile reached 49.5%, indicating substantial upper-tail variability. Because Pb was incorporated into the comparative screening HI using a historical normalization benchmark, it was additionally evaluated using an effect-based margin-of-exposure approach. The renal Pb MOE was below 1 in 1.0%, 16.8%, and 32.7% of samples under the three consumption scenarios, respectively. These findings identify Cd and Pb as priority elements in market-sold HCT and support targeted retail surveillance, supply-chain tracing, and refined assessment of samples with elevated As or Pb.