The law is supposed to apply to those foods which are contaminated with cesium and bananas get their radioactivity from potassium (and Brazilnuts from radium).
It can be treated with an adrenaline injection, but to prevent such a reaction susceptible children should avoid legumes including peanuts, brazilnuts, almonds and hazelnuts.
Severino had built up a mass of knowledge on where to find water, which nuts to eat and where to find them and how to avoid predators, the BBC's Brazil correspondent Tom Gibb reports.