The heat of chile peppers is mostly in the inner ribs. It’s a myth that the heat is in the seeds: the seeds absorb heat from the ribs.
Pure capsaicin would have a Scoville Unit rating of 16,000,000. Police pepper spray has a rating of 2,000,000.
You may see peppers given different Scoville ratings by different sources. A pepper’s heat level can vary on where it was grown, and what the growing conditions were. If the pepper is stressed (lack of water), etc, it will develop more heat.
Another rating system rates chiles 0 to 10. 0 is no heat, 10 is the hottest. Sometimes people get mixed up and say “level 4 on the Scoville scale”; what they mean is level 4 on this alternative rating system.
The Scoville Scale was first developed by Wilbur Scoville in 1912.
The method required puréeing some chile pepper, and mixing that with water sweetened with sugar. The scale measured the amount of sugar water required for a taster to no longer be able to detect the heat. A sweet pepper needs 0 dilution for the heat not to be tasted, as it has no heat. If the amount of chile pepper extract had to be diluted with 100,000 parts of sugar water, then its heat was 100,000.
Now the Scoville Units are measured by machines rather than by human tasters.