Occupiers face off with police during a Portland, Ore., demonstration: To control the crowds, some cops have used pepper spray, which can cause intense skin and eye pain.

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fter police at UC Davis caused a national uproar by using pepper spray on peaceful student protesters, Fox News host Megyn Kelly dismissed the crowd-control weapon as "a food product, essentially." That's not quite right. How dangerous is pepper spray? Here's what you should know:

What's in pepper spray?
Pepper spray relies on varying concentrations of a compound called capsaicin — the same substance that naturally gives peppers their notorious heat, and makes "habaneros so much more formidable than the comparatively wimpy bells," says Deborah Blum at Scientific American.

And how does pepper spray work?
The "oily" capsaicin extract used in pepper spray triggers burning sensations by "binding directly to proteins in the membranes of pain and heat" in the nervous system, says Blum. When capsaicin makes contact with the eyes, it can cause 30 minutes of temporary blindness, and "induces a burning sensation... by damaging cells in the outer layer of the cornea." When pepper spray touches the skin, it can cause tingling, intense burning pain, swelling, redness, and sometimes blistering, says Katherine Hobson at The Wall Street Journal. Contact with the respiratory system is the most dangerous, says Hobson. It can cause swelling and inflammation of the air passages — a serious risk to asthmatics or people with other breathing conditions.

When can police use pepper spray?
Officers are permitted to use pepper spray as a "compliance tool," says Charles J. Kelly, a former Baltimore police lieutenant who wrote his department's use of force guidelines. Indeed, pepper spray is often preferable to using batons or lifting protesters up. "When you start picking up human bodies, you risk hurting them. Bodies don't have handles on them."

What kind of pepper spray did UC Davis police use?
They employed a brand called Defense Technology, says Kyle Wagner at Gizmodo. The intensity of Defense Technology's various pepper sprays is based on "Major Capaicinoid content." The lowest concentration, 0.2 percent, is authorized for tactical deployment. A concentration of 1.3 percent is powerful enough to stop a bear. The type used on the students has a rating of 0.7 percent. The manufacturer recommends the spray be used at a minimum distance of six feet, yet the officers in this case sprayed it on sitting students at near-point blank range.

How does this pepper spray stack up against actual peppers?
Sweet bell peppers have zero Scoville Units (a heat-intensity measure named after American pharmacist Wilbur Scoville, who was the first to chart "the intensity of a pepper's burn," says Blum). Red chiles have a heat range between 500 to 750. Jalapenos sit between 3,500 and 8,000. Spicy-hot habanero peppers range between 200,000 to 350,000. And pepper spray employed by police? It ranges between 2 million and 5.3 million Scoville Units, leaving even "the most painful of natural peppers" in its dust, Blum says.

Sources: CBS, Gizmodo, Scientific American, Wall Street Journal