Decabromodiphenyl ether BDE-209 acts as a slow-relase reeservoir for lower brominated, more toxic PBDEs, via abiotic and biotic transformation processes. As a result, human populations and biota worldwide are exposed to BDE-209 and other PBDEs in combination. Young children experience the highest exposures of all age groups.While the critical toxicity of BDE-209 to humans is judged to be developmental neurotoxicity, the effects in biota are more varied. Plants and algae do not appear to be sensitive, but molluscs react with DNA damage. In fish and amphibians, effects attributable to thyroid disruption occur.It is plausibel to expect combination effects of BDE-209 and other congeners in terms of developmental neurotoxicity. An assessment of combined human exposures revealed that tolerable exposures are exceeded for all age groups, but particularly for small children, which warrants health concerns. A scoping mixture risk assessment for environmental scenarios revealed concerns for top predators, especially polar bears.An evaluation of BDE-209 in isolation, without taking account of combination effects with other PBDEs would significantly underestimate the toxicity of BDE-209. Combined exposures to BDE-209 and other PBDE pose significant health concerns.