Why Are Protesters Washing Their Eyes With Milk?

It’s the summer of getting pepper-sprayed for no real reason at a protest. Knowing this, protestors carry dairy as an essential accessory, giving us the haunting images online of people appearing to weep milk.

It’s an odd sight — and a disheartening one — but for people who have been pepper sprayed, milk is a source of instant respite from searing pain. Why?

This stems from the same reason we’re told to drink milk rather than water after biting into a volcanic pepper. The active ingredient in both hot peppers and pepper spray is an oil called oleoresin capsicum. It’s a natural inflammatory agent that causes mucous membranes — the body’s “wet” tissues, like the eyes and respiratory system — to swell, causing extreme pain and temporary blindness.

Thinking back to high school chemistry, remember that like dissolves like. Because the capsicum molecule is nonpolar, meaning it doesn’t carry much of an electronic charge, it can only be dissolved in something that’s equally nonpolar. Because milk contains some nonpolar fats, it’s better at washing the capsicum molecules away than, say, water, which is extremely polar.

It’s effective in theory and, seemingly, in practice, but experts have warned against using milk to wash eyes because it isn’t sterile. Using a lot of water, they say, should do the trick. They also recommend avoiding oily ointments or salve — which are also nonpolar — to soothe mace burns because they could trap capsicum on the skin.

It’s tragic that this is even a point of discussion. But until the state of law enforcement changes enough that 10-year-olds don’t need to think about these kinds of things, we have to think about these kinds of things.