Understanding pepper (OC) spray ratings

Pepper spray has been legal in Massachusetts without any sort of permit required for a few years now. It’s also known as OC spray, from the extracted oleoresin capsicum, being an oleoresin formed from capsicum, the ingredient that makes hot peppers spicy.

I recently picked some up, and would like to share a few things I’ve learned.

Where to buy pepper spray

Finding pepper spray in MA can be hard. I’ve seen it in lots of places across the border in New Hampshire, but it’s hard to find in Massachusetts. Making it worse, shipping it to Massachusetts is either prohibited, or thought by online merchants like Amazon to be prohibited. I ended up picking mine up in a sporting/gun shop.

Why all the numbers are meaningless

But here’s the part that really trips me up: the ratings. The spray I purchased is listed as “10% OC (2 million SHU).” Intuitively, I assumed this meant that it was 10% of the maximum strength possible. (If 10% is 2 million SHU, the maximum must be 20 million SHU.) But this is totally wrong! The SHU rating or the percentage alone are meaningless numbers.

What this actually means is this:

  • The solution in the canister is 10% oleoresin capsicum, and 90% other stuff (inert ingredients). If this number were 100%, I assume it would be a sticky goop that you couldn’t spray, so it’s necessary that a decent percentage of the contents be something like water or propylene glycol.
  • The OC that’s in the spray is rated at 2 million Scoville heat units (SHU).

As an analogy: you have a rum and Coke, which is about 25% rum and 75% Coke. The rum used is 80-proof, or 40% alcohol. If your goal is to get drunk, neither of those metrics is meaningful on its own. 25% rum doesn’t necessarily tell you how strong the alcohol is, and “The rum in this Coke is 100 proof!” isn’t good news if they only put a dollop of it in your drink. What you’d actually want to know is the total amount of alcohol.

The same is true of OC spray. Sabre, one manufacturer of pepper spray, has a blog post explaining that you need to consider the product of the two numbers, known as the percentage of Major Capsaicinoids, or MC. The same math is discussed in a number of places; the Sabre blog post is just the first place I saw the math given a name.

Pure capsaicin is 16 million Scoville heat units. So if the oleoresin capsicum in my canister of pepper spray is 2 million SHU, it’s 2/16 = 12.5% of the maximum strength.

But only 10% of the canister is OC, so it’s 12.5% × 10% = 1.25% MC. (Well, the Sabre article uses the same starting numbers to arrive at 1.33%. I’ll assume that I’m the one that’s off on the math.)

A competing product is marketed as 6% and 3 million SHU. That’s 18.7% × 6% = 1.122% MC. Other sources have higher SHU ratings at lower percentages. This all gets very confusing, because manufacturers seem to just decide whether they want to market their product on having a high-percentage of OC, or on a high SHU number. You need to compare the total percentage of capsicum if you want to do any sort of meaningful comparison.


If you’re going to carry pepper spray, you would probably do well to also carry something like these decontamination wipes to help alleviate the effects, in case you end up getting some of the spray as well. (There are a few other products linked from there as well. Having never used any of them, I can’t recommend one over the other. All I can say is that Sudecon wipes fit in my glovebox nicely.)

Related compounds (science nerds take note!)

As mentioned above, the “active ingredient” in hot peppers is capsaicin. Pure capsaicin is 16 million Scoville heat units. The hottest pepper is the Carolina Reaper, around 1.5 million SHU. (You should definitely not vape it.)

However, capsaicin isn’t the hottest compound. That honor appears to belong to something even tougher to pronounce, Resiniferatoxin. Found naturally in a plant known as Euphorbia poissonii (what could be scary about a plant whose Latin name is poissonii?), Wikipedia writes that it “causes severe burning pain in sub-microgram (less than 1/1,000,000th of a gram) quantities when ingested orally.”

All of these compounds seem to act on a receptor in the body known as TRPV1, which controls body temperature and “provides a sensation of scalding heat and pain.” There’s also a compound known as capsazepine which inhibits the TRPV1 channel, effectively blocking the effects of capsaicin and its ilk. I’m yet to see it being marketed as an antidote to pepper spray, though.

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