Thallium sulphate crystal, computer model. Thallium atoms are green and sulphur atoms are yellow. Thallium sulphate is colourless, odourless and tasteless. It has been used in rat poison and insecticides, 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.