from Scientific American:
commercial grade pepper spray leaves even the most painful of natural peppers (the Himalayan ghost pepper)
far behind. It’s listed at between 2 million and 5.3 million Scoville
units. The lower number refers to the kind of pepper spray that you and I
might be able to purchase for self-protective uses. And the higher
number? It’s the kind of spray that police use, the super-high dose
given in the orange-colored spray used at UC-Davis.
The reason pepper-spray ends up on the Scoville chart is that – you
probably guessed this - it’s literally derived from pepper chemistry,
the compounds that make habaneros so much more formidable than the
comparatively wimpy bells. Those compounds are called capsaicins and –
in fact – pepper spray is more formally called Oleoresin Capsicum or OC Spray.
From a 2004 paper: Health Hazards of Pepper Spray, written by health researchers at the University of North Carolina and Duke University
"Depending on brand, an OC spray may contain
water, alcohols, or organic solvents as liquid carriers; and nitrogen,
carbon dioxide, or halogenated hydrocarbons (such as Freon,
tetrachloroethylene, and methylene chloride) as propellants to discharge
the canister contents.(3) Inhalation of high doses of some of these
chemicals can produce adverse cardiac, respiratory, and neurologic
effects, including arrhythmias and sudden death. The health effects of
solvents and propellants are beyond the scope of this article, but they
too need to be considered in evaluating potential hazards and effects of
exposure to specific brands of OC spray."