The pungent peppers comes from capsaicin. This substance is salivating and active digestion. To reduce the stinging effect, it is best to ingest yogurt, bread, cooked rice or sugar which is much more effective than water because capsaicin dissolves in fat and not water.
In 1912, Wilbur Scoville imagine the Scoville organoleptic test (based on a dilution of spice sugar test) to assess the degree of heat (force) pepper (Scoville unit). It performs a ranking of 0 to 10.
Today, scientists use a more sophisticated method, liquid chromatography to assess the levels of capsaicin, but the power (heat) overall is still evaluated in Scoville units of force.
This is based on taste tests. A group of 3 or 5 testers taste a beverage which was diluted x times the pepper variety tested. There must be at least 60% of the testers can feel the spiciness of chili. A variety of strong chili can be diluted up to 50,000 times. It thus has a score of 50,000 on the Scoville scale.
As you can see, the Espelette pepper is rated at 4. This figure is the same as for the pepper. This is what has contributed to his reputation. It falls flat without really take and hide the other tastes.