Chile peppers like jalapeños, serranos, and habaneros all have different degrees of heat. How hot they are is determined by how much of a chemical called capsaicin they contain, and their heat is measured using a system called the Scoville Scale.
The Scoville Scale is divided into units called Scoville Heat Units, which are a way of describing how many drops of sugar water it would take to dilute the heat of a given pepper.
These numbers are expressed in the thousands for milder peppers all the way up to hundreds of thousands or even millions for the hottest peppers.
Scoville Heat Units
The Scoville Scale was developed in 1912 and the way it originally worked was that a panel of human tasters would taste a sample of a pepper diluted in a sugar solution, and report what level of dilution was required so that the pepper no longer tasted hot. This subjective and not very reliable system has since been replaced by a method called High-Performance Liquid Chromatography, which reads the chemical fingerprint of capsaicin in a pepper and measures exactly how much capsaicin it contains.
Even using this technology, the Scoville ratings are not absolutely precise. The same species of pepper grown in different conditions can have different heat levels. Variables such as how much sunlight a pepper gets while growing can affect the heat (more sunlight produces a hotter pepper), and variations in the soil can also affect a pepper's heat.
Some varieties of chili pepper are just ridiculously hot — so much so that, from a culinary standpoint, they are no more than a curiosity; no one would ever eat one. Below is a table that shows the Scoville heat units of some of the more common chili peppers used in the culinary arts, as well as some other examples:
Scoville Heat Unit Ratings
|16 million||Pure capsaicin|
|5 million||Law enforcement pepper spray|
|1 million||Ghost pepper (Bhut jolokia chili)|
|100,00-350,000||Habanero pepper, Scotch bonnet pepper|
|2,500-8,000||Jalapeño pepper, Paprika, Tabasco sauce|
|500-2,500||Anaheim pepper, Poblano pepper|
|100-500||Pimento, Banana pepper, Peperoncini|