Well, the Telegraph is getting better on the subject of thallium:

Thallium is fatal in doses of only one gram and its salts are colourless, odourless, tasteless and soluble in water.

• In its basic form, thallium is a heavy metal with some industrial uses, although it is restricted because it is so toxic.

Still the odd problem though:

Thallium poisoning is normally treatable if it is identified within six hours of injestion.

Prussian Blue is the treatment I think. But shouldn’t that be ingestion?