In 1912 a chemist by the name of William Scoville developed a way to determine the heat content of the different types of chili peppers, albeit the hot chili peppers or the sweet bell peppers. This heat content is also referred to as the pungency of the capsicum. This pungency is indicative of the amount of capsaicin present in the chili pepper. He did this successfully by using a test that involved the dilution of the pepper extract of these capsicums with sugar water until the heat was no longer detectable. This sugar water solution was made up to a standard (same as a control solution). The number of times that the pepper extract solution had to be diluted was then used as the heat rating. This can be illustrated as follows: the heat rating of a Habanero capsicum, which is a bell pepper, is approximately between 200 000 and 300 000 units. These units are also called Scoville Heat Units. This is just another way of saying that a solution of the pepper extract or capsaicin of the Habanero capsicum has to be diluted approximately between 200 000 and 300 000 times before the heat was no longer detectable. Nowadays the method to determine the heat rating of capsicum is based on a chemical test, but the principle stays the same.
It is rumored that pure capsaicin has a rating of 16 million Scoville Heat Units. The capsicum, at present, that has the highest scoville heat unit rating is the Naga Jolokia capsicum which is a variety of the Capsicum frutescens. The Naga Jolokia capsicum rates 855 000 Scoville units. Tabasco capsicum has a rating of 200 000 scoville heat units. The Jalapeno capsicum weighs in at 4 000 scoville heat units.
All things said, one should however bear in mind that the Scoville Heat Unit can be imprecise since the strength of chilies, even those on the same capsicum plant can differ from one another. There are also many factors that influence the heat rating of capsicum. These factors are the weather, the water schedule, the soil conditions and even plant stress. (Capsicum plants that are stressed will produce fewer fruits and those fruits will tend to be much hotter that the fruits on a similar type of bush that bears many fruits.) Capsaicin in chili peppers also differ from grower to grower. However, the Scoville Heat Unit is still a very useful indicator of heat strength in capsicum
These days' scientists developed the so-called half-life scale, which is based on a specific high top end, and halves the Scoville rating each time until zero is reached. Each step is allocated a number from 0 through to 10 and has a name assigned. This simply means that you can safely say that a chili pepper with a rating of 2, to be roughly twice as hot as a chili with a rating of 1, and likewise 3 is twice as hot as 2.