From the Kenai National Wildlife Refuge Notebook:
Bear spray really does work. A 2008 article in the Journal of Wildlife Management reviewed 83 bear spray incidents in Alaska during 1985-2006. Threatening behaviors by brown, black and polar bears were stopped more than 90 percent of the time by spray, although in 18 percent of cases, bears had to be sprayed more than once. Only 3 bear-inflicted injuries associated with defensive spraying occurred — all involved brown bears and all were relatively minor, requiring no hospitalization.
But like any weapon or tool, it works better when you know what you’re doing.
John Morton is the supervisory biologist at Kenai National Wildlife Refuge and has some excellent tips here.