An assessment of the genotoxicity of 2-hydroxy-1,4-naphthoquinone, the natural dye ingredient of Henna. 
2-Hydroxy-1,4-naphthoquinone (HNQ; Lawsone; CAS 83-72-7) is the principal natural dye ingredient contained in the leaves of Henna (Lawsonia inermis).  Published genotoxicity studies on HNQ suggested it was a weak bacterial mutagen for Salmonella typhimurium strain TA98 or was more clearly mutagenic for strain TA 2637, both in the presence of metabolic activation.  HNQ was unable to induce sex-linked recessive lethal mutations in Drosophila melanogaster.  However, a small increase in micronucleus frequency was reported in the bone marrow of mice at a single mid-range dose level, 24h after intraperitoneal injection.  In view of the wide use of Henna hair dyes it was deemed necessary to conduct a thorough investigation, under Good Laboratory Practice conditions, of the genotoxicity of HNQ.  HNQ was non-mutagenic in bacterial (Ames test) or mammalian (V79 hprt) assays.  It was borderline positive in a mouse lymphoma tk mutation assay and a chromosome aberration test (CHO cells), results that may reflect a similar clastogenic mechanism.  Negative in vivo genotoxicity results were noted in the rat hepatocyte in vivo/in vitro UDS test, in peripheral lymphocytes (chromosome aberrations) of rats receiving repeated oral doses of HNQ at the MTD for 28 days, and in mouse and hamster bone marrow chromosome aberration tests.  However small, but statistically significant increases in the incidence of bone marrow micronuclei were observed in two out of five tests at 72 h after dosing, but not at 24 or 48 h. There was evidence of haematotoxicity at 72 h, which may have been enhanced by the vehicle (DMSO) used in the positive tests.  As erythropoiesis and administration of haematotoxic agents are known to induce small increases in the frequency of bone marrow micronuclei, typically at delayed sampling times, the data suggest that the positive 72 h response produced by HNQ is consistent with stimulation of haematopoiesis subsequent to haematological toxicity of HNQ, and not due to a DNA-reactive mechanism.  Overall, the weight of evidence suggests that Henna and HNQ pose no genotoxic risk to the consumer.