<P> The odorless and tasteless thallium sulfate was also used as rat poison and ant killer . Since 1975, this use in the United States and many other countries is prohibited due to safety concerns . </P> <P> Thallium may be quantitated in blood or urine as a diagnostic tool in clinical poisoning situations or to aid in the medicolegal investigation of suspicious deaths . Normal background blood and urine concentrations in healthy persons are usually less than 1 μg / litre, but they are often in the 1--10 mg / litre range in survivors of acute intoxication . </P> <P> There are numerous recorded cases of fatal thallium poisoning . Because of its use for murder, thallium has gained the nicknames "The Poisoner's Poison" and "Inheritance Powder" (alongside arsenic). </P> <P> In Australia, in the early 1950s, there was a notable spate of cases of murder or attempted murder by thallium poisoning . At this time, due to the chronic rat infestation problems in overcrowded inner - city suburbs (notably in Sydney), and thallium's effectiveness as a rat poison, it was still readily available over the counter in New South Wales, where thallium sulphate was marketed as a commercial rat bait, under the brand "Thall - rat". </P>

Why thallium is known as the poisoner's poison