You may be asking this question while considering to purchase a pepper spray. And if you think that 15% spray is better than 10%, it's not always true.

The actual factor you need to look for is heat-generating capacity of OC which is measured in Scoville Heat Units (SHU).

Oleoresin Capsicum (OC) is derived from a variety of hot peppers. Peppers range from 0 SHUs for bell peppers, up to 5000 for jalapeno peppers, and up to 300,000 SHUs for habaneros peppers. Pure OC has a heat rating of about 15 million. As you can see, a 10% concentration of jalapeno peppers is not as affective as a 1% concentration of habaneros peppers. In actual spray formulations, OC from the peppers is processed until it reaches a heat rating about 2 million SHUs.

Currently, pepper spray manufacturers offer three most common concentration: 500,000 SHUs, 1 million SHUs and 2 million SHUs. For example, Mace Triple Action sprays have 500,000 SHUs. Almost all other Mace sprays have 1 million SHUs, including Mace pepper foam. And the most resent innovation from Mace - Pepper Gel - has 2 million SHUs.

One company was going to shoot for 3 million SHUs, but its chemists warned that this could cause tissue damage. When you consider the SHU rating of a spray, it's easy to see why the percentage of OC is not necessarily the best indicator of effectiveness.

In any case, 10% OC pepper sprays have sufficient strength to stop an attacker. And I doubt the assailant in the street you just spayed is going to care about hte OC percentage or heat rating of the spray. He'll just want to get away from the pain, blindness and nausea as quickly as possible.