The macabre poison we know from crime novels and history books has some surprising modern uses.
Justin Rowlatt travels the Subcontinent - first to India's Forest Research Institute in the Himalayas where Sadhna Tripathi explains why the chemical element ends up in telegraph polls.
We then head to Bangladesh, scene of the "largest mass poisoning in history". Justin speaks to Dr Quazi Quamruzzaman who helped first uncover it, and to Richard Pearshouse of Human Rights Watch, who says the problem still hasn't gone away.
Sanjay Wijesekera of Unicef explains how the road to this particular hell was paved by good intentions, and how his aid agency is helping to guide Bangladesh back out again.
(Picture: Bangladeshi woman's foot showing lesions caused by arsenic poisoning; Credit: Majority World/UIG via Getty Images)
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