Most use of ethidium bromide in the laboratory (0.25–1 µg/ml) is below the level required for toxicity, and is 3 orders of magnitude lower than the therapeutic dosages used in cattle. The level is high enough that exposure may interfere with replication of mitochondrial DNA in some human cell lines, although the implications of that are not clear. Testing in humans and longer studies in any mammalian system would be required to fully understand the potential risk ethidium bromide poses to lab workers.