Just one ten-thousandth of a milligram of stinging nettle fluid is enough to unleash that unpleasant tingling beneath the skin and the accompanying red rash. When disturbed, the stinging hairs on the nettles’ leaves act like needles and inject a chemical cocktail into the skin. Scientists aren’t entirely sure what chemicals the liquid contains, but they include formic acid and acetylcholine, both of which are responsible for the burning sensation under the skin.
The histamine in the ‘cocktail’ causes the swelling and itching. Special needles modelled on the nettles’ stinging hairs are now being designed for medical use. Tip: if you want to pick nettles without being stung, wrap your fingers round the bottom of the stem and pull. Don’t touch the upper side of the leaves where most of the stinging hairs are located.