The amount of heat in chiles was carefully measured first in the early 1900’s by Wilbur Scoville. Chile pepper extract was diluted with sugar until the heat was no longer detectable. Today the measuring is done with more precise methods. But despite all the technology, the range of heat for each chile still varies tremendously. While the Scoville Unit (SU) for a cayenne chili may register 30,000 today, next week it might register 50,000. This is due to variations in growing conditions, including weather, soil, and neighboring crops.
Luckily, most recipes do not specify Scoville Units, and most cooks just want to know if the chile is hot or not. The following entries are listed as mild (0-2,000 SU), moderate (2,500-23,000 SU), hot (12,000–50,000 SU), and very hot (50,000–325,000 SU). Higher Scoville Units exist for things like police pepper spray, but you won’t be cooking with that stuff, I hope!
They all contain capsicum, which is the compound that creates the sensation of heat on the tongue. This stuff can create some discomfort, especially on tender, sensitive skin. To be safe, use gloves when chopping chiles, and keep your hands away from your eyes (and other tender areas).