When horseradish is cut and grated, it produces isothiocyanates, a type of mustard oil that is quite pungent. Your sinuses will clear and your eyes will tear, and if you aren’t careful, you will shed many tears. If grating horseradish on a box grater, I suggest you wear goggles and keep an arm’s distance from the root. (The very cool onion goggles available at gourmet cookware stores really do work!) If using a food processor or blender, stand back and turn away before you remove the cover, to allow the fumes to escape.