<p>The industrial use of monomeric halogenated flame retardants has now gradually been phased out due to their toxicity to humans and ecosystems. Polymeric flame retardants are emerging as a ‘safe’ alternative and so have a high production and consumption volume. However, the environmental fate and toxicity of their derivatives remain unknown, making it difficult to understand and adequately manage the associated risk. We take two tetrabromobisphenol A-based polymers (polyTBBPAs) that are widely used in electronics as model flame-retardant chemicals, and we study their behaviour when they break down in the environment and the toxicity of the derivative products. Our results show that polyTBBPAs break down into smaller products in the environment. Using a non-target screening strategy called BrMiner developed by us, we identified 76 breakdown products of polyTBBPAs with molecular weights in the range 400–2,000 Da. These were detected in environmental samples taken from electronic waste recycling facilities in South China. Toxicity tests with zebrafish embryos showed that when they break down in the environment, polyTBBPAs become more toxic, with mitochondrial dysfunction representing a key toxicity mechanism. This study reveals that there are environmental risks associated with polymeric flame retardants, and therefore, their use should be adequately assessed and regulated.</p>