How hot is hot? The heat in all chiles, whether hot or mild, is due to the flavorless, odorless, colorless chemical known as capsaicin. In 1912, William Scoville, a Detroit pharmacologist, measured capsaicin by having a panel of hardy souls sip a sweetened solution of dried chile peppers dissolved in alcohol. The concoction had decreasing amounts of capsaicin until it no longer burned. The results were converted into Scoville units -- with no mention made as to what happened to all those tasters.
In recent years, this subjective test was converted to a chemical process, but with results still expressed as Scoville units. Scoville units can range from zero (the good old bell pepper) to more than half a million (the Red Savina habanero chile).
Disagreement is common, and the heat of chiles can even vary on the same plant, but the ratings give a good idea of relative chile heat.