Capsaicin is the chemical that causes the spiciness of chile peppers, and the concentration of capsaicin is measured in Scoville Heat Units (SHUs). SHUs range from 0 (bell peppers), to about 2.2 million (carolina reapers). But there is a lot more that goes into the perception of heat than just the SHU. From an article in 538, these are the 5 elements that contribute to the perception of heat (and only the last point is measured by SHU):
1. How fast does the heat come on?
Asian types of chile peppers produce instantaneous heat, while habaneros are known for a long delay.
2. How long does the heat linger?
Asian varietals tend to punch you in the mouth and diminish, while habaneros linger, the heat creeping up over several minutes. The lingering makes them taste “hotter,” even if they aren’t higher in parts-per-million of capsaicinoids.
3. Is the heat sharp or flat?
Peppers can produce a sharp pins-and-needles feeling or produce the sensation that someone has spread heat inside your mouth with a paintbrush.
4. Where in the mouth do you sense the heat?
Habaneros burn the back of the throat, while New Mexican varietals scorch the mid-palate.
5. What is the amount of the actual heat?
This is what is measured by the HPLC test.
Two people may not agree on the relative heat level of two sauces because everyones palettes react to the above influences in different ways. So next time you disagree with someone over the heat level of a sauce, consider that while there is an objective heat level, the perception of heat is in the tongue of the beholder.