Findings and scope
The study grew Cretan dittany (Origanum dictamnus), a Cretan endemic herb used for tea, medicine and food flavouring, in containers at two sites on an Athens university campus beside a moderate-traffic road: an extensive green roof about 12 m from the street, and ground level beside the street (PDF p.11 to 12). Plants were grown in a soilless substrate or in a substrate containing 20% soil, and leaves and flowers were analysed both washed and unwashed. Dittany is a different species from culinary oregano (Origanum vulgare and its subspecies), so these results describe dittany only.
Lead in dried leaves was 15.8 to 19.8 mg/kg dry matter on the roof and 26.5 to 31.0 mg/kg at street level, and nickel in leaves was 3.8 to 5.4 mg/kg on the roof and 5.4 to 7.1 mg/kg at street level (Table 5, PDF p.8). In flowers, lead (16.5 to 22.5 mg/kg) and nickel (4.1 to 7.6 mg/kg) did not differ significantly between sites in the one-way analysis. The authors reported that lead and nickel exceeded their reference limits in every tissue at both sites, including the roof (PDF p.11). Cadmium, arsenic, mercury and chromium were not measured.
Key numbers
Lead in Origanum dictamnus tissue, means of three replicates as printed, mg/kg dry matter (Tables 5 and 6, PDF p.8). W, washed; UW, unwashed.
| Site and substrate | Leaves W / UW | Flowers W / UW | Roots |
|---|---|---|---|
| Roof, soilless | 16.9 / 15.8 | 17.6 / 20.0 | 20.8 |
| Roof, soil | 19.0 / 19.8 | 20.8 / 19.0 | 26.1 |
| Street, soilless | 26.5 / 31.0 | 17.4 / 20.9 | 23.8 |
| Street, soil | 27.7 / 30.3 | 16.5 / 22.5 | 24.9 |
Nickel in the same tissues, mg/kg dry matter (Tables 5 and 6, PDF p.8).
| Site and substrate | Leaves W / UW | Flowers W / UW | Roots |
|---|---|---|---|
| Roof, soilless | 4.0 / 3.8 | 6.5 / 4.6 | 10.0 |
| Roof, soil | 4.0 / 5.4 | 4.1 / 4.9 | 14.3 |
| Street, soilless | 5.5 / 6.9 | 6.7 / 7.6 | 10.1 |
| Street, soil | 5.4 / 7.1 | 4.5 / 5.4 | 16.2 |
DTPA-extractable lead was 0.459 mg/kg in the soil and 0.759 mg/kg in the grape marc compost; DTPA-extractable nickel was 0.038 and 0.045 mg/kg (Table 3, PDF p.7).
Methods and Evidence Fitness
Plants raised from cuttings were planted in December 2016 in 40 cm by 60 cm containers fitted with green-roof layers and 10 cm of substrate, six containers per treatment in a two-by-two factorial design (PDF p.11 to 12). Leaves and flowers were harvested in early September 2017; half of each sample was immersed in distilled water for 1 minute and rinsed under tap water, and roots were rinsed in tap water. Tissues were oven-dried at 60 °C for 7 days, milled, dry-ashed at 550 °C for 3 hours, taken up in nitric acid and analysed by atomic absorption spectrophotometry (PDF p.13 to 14). No detection limit, certified reference material, recovery or blank is reported for the metals.
Evidence Fitness is Modeled or limited evidence. The data support a within-study comparison showing higher lead and nickel in leaves grown beside the road than on the roof, with partial reduction after washing. They describe one campus, one growing season and experimental substrates rather than commercial dittany, and the analytical quality control is not documented. Root values and substrate extracts are context only.
Assessment limits
Both sites are close to the same road, so the roof is not an unexposed reference; the authors attribute high values at both sites to traffic and urban air (PDF p.11). The authors compared tissues with limits of 5.0 mg/kg for lead and 2.0 mg/kg for nickel in dry matter, which they attribute to an FAO/WHO Codex committee document (PDF p.14); these figures have not been checked against that document here. The substrate analysis reports only the DTPA-extractable fraction, which is not total metal content.
Some statements in the text differ from the tables. The results say flower lead did not differ between treatments, but Table 5 marks a significant site effect for unwashed flowers (PDF p.7 to 8). The text says root lead did not differ between treatments, while Table 6 marks a significant substrate effect for root lead with a non-significant one-way analysis (PDF p.8). The methods refer to substrate composition in Table 2, which is Table 3. The Table 4 marginal means for lead were consistent with the Table 5 cells.
Provenance notes
The PDF (17 pages) was delivered in batch 04 of order HMI-GAP-INGEST-1 on 22 October 2026. The SHA-256 matches the batch manifest. The PDF prints DOI 10.3390/plants12020336 and the title in the manifest. The manifest filed the paper under oregano; the species studied is Origanum dictamnus (Cretan dittany), not culinary oregano, and it is routed here to the dried herbs and herbal botanicals pages.
Related evidence
Update history
22 October 2026: Ingested from the batch-04 PDF. 44 tissue and substrate records for lead and nickel were added to the evidence register.
Update history
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