Toxicology of bromoform, a natural constituent of the seaweed Asparagopsis spp. used to inhibit methanogenesis in cattle, suggests negligible risks to humans

The red seaweeds, Asparagopsis taxiformis and A. armata inhibit methane production in ruminants, considered to be mediated by bromoform. This review examines the toxicology, metabolism, epidemiology and pharmacology of bromoform. IARC concludes bromoform is not classifiable as a carcinogen to humans, whereas the US EPA classifies it as a probable human carcinogen based on a low incidence of intestinal tumours in female rats given high gavage doses chronically. Under the same conditions, mice exhibited no tumours, suggesting that tumour formation is secondary to localised cytotoxicity. While there is some in vitro evidence of mutagenic potential for bromoform, likely via a metabolic pathway involving GSTT11-1, this pathway is unlikely to be relevant at low doses. The human evidence, from drinking water disinfection by-product studies, is inconclusive. While some residue studies find bromoform in milk, the concentrations approach background levels and pose no significant cancer risk. The collective implication of these findings is that bromoform, as an environmental inhibitor, does not pose a significant risk. A precautionary approach would ensure that bromoform intake does not exceed the capacity for rumen degradation such that concentrations of residues in tissues and milk and consequent dietary exposure are minimised and below relevant health-based guidance values.