When you hold a pepper in your hand, it probably doesn't feel any hotter than if you were holding an apple or a plum. But when the pepper comes into contact with your lips or your tongue, you can definitely feel the heat. This is because the capsaicin in the pepper reacts with the nerve endings in your body's mucous membranes that are sensitive to heat and cold. This explains why biting into a spicy pepper can actually feel like your mouth is burning.
Because capsaicin is the same component that makes peppers seem hot, its potency is measured in Scoville heat units (SHU) -- a heat scale used to rate the heat level of peppers. The overall heat level is based on the amount of capsaicin present. Typical pepper spray is rated between about 500,000 and 5,000,000 SHU. By comparison, a jalapeño pepper is only around 8,000 SHU, and a habanero is close to 350,000 SHU.
The severity of the effects of pepper spray varies depending upon the amount of pepper spray that's used, the strength of the spray and where it's sprayed. If sprayed directly into a person's face, the effects may be more intense or long-lasting. When pepper spray is sprayed at an attacker, the burning sensation they would feel is just the tip of the iceberg.
If you were sprayed in the face with pepper spray, you would immediately feel a burning sensation in your eyes, nose and mouth, and possibly even your throat and on your skin. This burning feeling, if left untreated, can last anywhere from 45 to 60 minutes. Your eyes would become irritated and probably swell shut, causing temporary blindness that can last from 15 to 30 minutes. You might also begin coughing and find it difficult to breathe as your throat swells -- this can last from 3 to 15 minutes.