Kate Grenville had always associated perfume with elegance and beauty. Then the headaches started. Like perhaps a quarter of the population, Grenville reacts badly to the artificial fragrances around us: other people's perfumes, and all those scented cosmetics, cleaning products and air fresheners. On a book tour in 2015, dogged by ill health, she started wondering: what's in fragrance? Who tests it for safety? What does it do to people? The more Grenville investigated, the more she felt this was a story that should be told.