Horseradish feels “hot” because of a special pungent chemical that is released when the root is cut or grated. This type of heat goes up into the nose rather than burning the tongue like chili peppers do.
Main chemical
When horseradish cells are damaged by grating or chopping, an enzyme called myrosinase breaks down stored compounds (glucosinolates) into isothiocyanates. The main one is allyl isothiocyanate, the same family of compounds that makes mustard and wasabi sharp and sinus-clearing.
Why it hits your sinuses
Allyl isothiocyanate is volatile, meaning it evaporates easily and its vapors travel up the back of the throat into the nasal passages. There it activates pain and heat-sensing nerve endings (nociceptors), which the brain interprets as an intense but short-lived burning sensation.
Difference from chili heat
Chili peppers are hot because of capsaicin, a thick, oily compound that sticks to tissues and lingers on the tongue. Horseradish heat fades faster because its volatile isothiocyanates dissipate quickly in air and saliva, so the burn is brief but powerful, especially in the nose.
Role of air, heat, and vinegar
Once exposed to air, the isothiocyanates start to break down, so freshly grated horseradish is the hottest and then gradually mellows. Cooking destroys much of the enzyme and pungent compounds, while vinegar helps stabilize them, which is why prepared horseradish is usually stored in vinegar to preserve its bite.