Horseradish is “hot” because, when the root is cut or grated, it releases a sharp, volatile chemical called allyl isothiocyanate that irritates your nose and sinuses rather than your tongue.

The science of the “heat”

When horseradish cells are broken by grating or chopping, an enzyme (myrosinase) meets a stored compound (sinigrin), and together they form allyl isothiocyanate, a spicy mustard‑like oil. This oil vaporizes easily, so the sensation rushes up into your nasal passages, causing that fast, eye‑watering burn that fades quickly.

Why it feels different from chili

Chili peppers burn mainly because of capsaicin, a heavy, oily molecule that sticks to your mouth and lingers on your tongue. Horseradish’s allyl isothiocyanate is lighter and more volatile, so it hits the sinuses hard but disappears faster, which is why you get a sudden “nose burn” instead of a long mouth burn.

What affects how hot it gets

Freshly grated horseradish is usually the strongest, but its pungency starts to fade once it’s exposed to air as the volatile oils break down. Acid (like vinegar) helps stabilize those isothiocyanates, which is why prepared horseradish is stored in vinegar and can stay hot longer in the jar.

TL;DR: Horseradish is hot because grating the root triggers a reaction that makes allyl isothiocyanate, a pungent, nose‑burning oil that evaporates into your sinuses, hits hard, and then fades quickly.