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How the Scoville Scale Works
Now, scientists extract the heat-inducing chemicals and measure their "pungency" by using High-Performance Liquid Chromatography. The relative "heat" is measured in Scoville heat units (SHUs). That stands as the primary means of determining spiciness in food (and the effectiveness, in another use of capsaicin — which is one of the major capsaicinoids — of pepper spray).
A bell pepper, for example, checks in on the Scoville scale at zero SHUs. Chomping down on a bell pepper would not prompt so much as a raised eyebrow or a reach for a glass of water. But that's the absolute bottom of the scale. From there, things get interesting.