Salmani et al. 2023 — Metals in Herbal Teas: Systematic Review
This systematic review, published in Biological Trace Element Research, synthesized 49 primary research articles (selected from 212 screened, published 2012–2023) reporting essential and toxic metal concentrations in herbal teas including thyme, rosemary, black tea (Camellia sinensis), and chamomile. The review found that all herbal teas contain measurable metals; none fully meet WHO requirements for all elements. Arsenic and lead health risks in black tea, and cadmium in chamomile, were considerably higher than in other herbal teas. Cd in chamomile (Matricaria chamomilla) was reported at a notably elevated mean of 23 µg/g — substantially above WHO limits — while Cd was not detected in rosemary or green tea in some studies.
Key numbers
Mean metal concentrations in studied herbal teas (µg/g dry weight, from Table 2):
Rosemary (Salvia rosmarinus):
- Fe: 63 / ND / 432 (three studies)
- Pb: 1.8 µg/g (one study)
- Cd: ND
Green tea:
- Fe: 154.963 µg/g; Al: 245.7 µg/g
- Pb: ND; Cd: ND
Black tea (Camellia sinensis):
- Pb: 1.7 µg/g and 11.42 µg/g (two studies)
- Cd: 0.2 and 0.67 µg/g (two studies)
- As: 0.06 µg/g; Mn: 22.29 µg/g; Al: 496.8 µg/g
Thyme (Thymus vulgaris L.):
- Pb: 1420 µg/g (Shirazi thyme — anomalous high value from chamomile/Shirazi thyme study); Cd: 23 µg/g
- Note: the 1420/23 values appear to be for Shirazi thyme (a different genus) and chamomile combined in one source [ref 33], not Thymus vulgaris specifically
Chamomile (Matricaria chamomilla):
- Pb: 1420 µg/g (max mean from ref [33])
- Cd: 23 µg/g (max mean from ref [33])
Note on the Pb 1420 µg/g / Cd 23 µg/g values: These are the highest mean values reported across all studies. The review authors flag arsenic and lead in black tea, and cadmium in black tea and Shirazi thyme as the standout risks. The 1420 µg/g Pb figure for chamomile/Shirazi thyme from one Iranian study is anomalous and should be treated with caution; it was reported in a medicinal plant study from Khorramabad, Iran (ref [36]) and may reflect atypical contamination from that specific locality.
Health risk assessment finding: More than 70% of herbal teas reviewed had acceptable health risk (HI < 1). Risks from arsenic and lead in black tea, and cadmium in black tea and Shirazi thyme, exceeded acceptable limits in some studies.
Methods (brief)
Systematic review per PRISMA-informed approach. Search: Google Scholar, PubMed, Scopus; terms: “herbal teas” + “heavy metals, essential metals, thyme, rosemary, chamomile, tea.” Inclusion: primary research articles 2012–2023, with quantitative metal data. 212 found; 49 included after exclusion (duplicates, conference documents, non-quantitative). Analytical methods in underlying studies include ICP-OES and AAS. Data extracted: mean, SD, sample size, geographic location.
Limitations
The review pools studies from geographically diverse settings (Iran, Turkey, China, Italy, Serbia, Brazil, USA) without stratifying results by region or tea origin, which limits interpretation since geographic origin strongly affects metal levels. The anomalously high Pb and Cd values for chamomile/Shirazi thyme from one Iranian study dominate the upper range and should not be generalized. The review does not separately analyze tea infusion versus dried leaf concentrations, which is a significant methodological limitation for dietary exposure estimates.
Implications
- Certification: Herbal teas (thyme, chamomile, rosemary) are not currently in the HMT&C-10 analyte matrix, but this review flags black tea and chamomile as potentially carrying elevated Pb and Cd. If herbal teas enter the program, this review provides the baseline.
- Courses: Useful illustration that “herbal tea” is not a uniform category — geographic origin and species both matter substantially. The Shirazi thyme data from Iran illustrate localized contamination hotspots.
- App: Dried herbal tea matrices are not currently in the app ingredient scope, but the data suggest Al accumulation is high in black tea (496.8 µg/g) — relevant if Al is ever added to the analyte vocabulary.
- Microbiome: Not applicable.