The active ingredient in pepper spray – capsaicin – comes from the chilli plant. Produced to protect the plant’s seeds from being crushed by the teeth of grazing animals, this molecule binds to TRPV1 receptors on the endings of pain nerves, causing them to fire and inducing a feeling similar to burning. Pepper spray is a concentrated capsaicin solution that is designed to irritate the eyes, nose and mouth. It is used as a means of self-defence, in police crowd control, by the military and even as a deterrent against aggressive animals like bears in the wild.
Capsaicin does not dissolve in water, so it cannot be easily washed away, but relief from the pain can be achieved using gentle shampoo, which acts as an emulsifier, drawing the chemical into solution and away from the eyes and skin. Capsaicin will dissolve in fat, and milk or oil-based products can also help to soothe the burning sensation.
Turning chillies into a weapon – Capsaicin is a chemical deterrent contained in the fruits of the Capsicum genus, including peppers and chillies, to protect their seeds. It is not water soluble, so in order to extract it from the fruit it is dissolved in ethanol. The capsaicin-infused alcohol is removed and the fluid evaporated off, leaving a resin. To get the resin to mix with water for use as a spray, it is emulsified using propylene glycol, which helps suspend the fat-soluble particles in the watery solution. The concentration of capsaicin is then measured and adjusted before being pressurised in aerosol cans, ready for use.