Thallium sulphate crystal, computer model. Atoms are represented as spheres and are colour-coded; thallium (blue), sulphur (yellow) and oxygen (red). Thallium sulphate, which is odourless and tasteless, has been used in insecticides and rat poisons, but has been banned in many countries due its high toxicity to humans. A dose of only 500mg can be fatal. It accumulates in the organs and disrupts many cellular processes. Symptoms include hair loss, peripheral nerve damage and organ failure.