Turmeric, a flowering plant of the ginger family, has long been prized in Ayurvedic medicine for its anti-inflammatory properties and in Asian cuisines for its earthy flavour and vibrant hue. Haldi, the spice’s Hindi name, is derived from the Sanskrit for “golden coloured”. But for the millions of South Asians who habitually consume it, turmeric’s skin-staining yellowness can be deceptive and deadly. To heighten their colour, the rhizomes from which the spice is extracted are routinely dusted with lead chromate, a neurotoxin. The practice helps explain why South Asia has the highest rates of lead poisoning in the world. The...