Why does smoke from a stove or hearth irritate the eyes?

Question from Sahad Chembra

A: When smoke gets in your eyes, it is not really the small pieces of the ash that make your eyes water up. Instead, the culprits are mostly the chemicals riding along with them.

Smoke from wood, cigarettes, and garbage contains many irritating gases, such as formaldehyde and acrolein. When these gases reach the surface of your eye, they dissolve in the thin layer of water that normally coats it. This forms mild acids and other irritating substances right on the eye’s surface.

Your eye has very sensitive nerve endings in the clear front layer called the cornea. These nerves detect the irritation and immediately send ‘danger’ signals to your brainstem. In response, special glands above your eyes start producing extra tears.

These tears have two jobs: they have to wash away the irritating chemicals and particles, and they have to dilute anything harmful so it becomes less concentrated. At the same time, blood vessels in and around your eyes widen, making your eyes look red.

So watery, red eyes in smoky air are basically your body’s emergency cleaning system switching on to protect a very delicate surface.