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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.