Check these links they seem to have some interesting info. They show bear spray is quite effective but not perfect

http://alaskanalpinetreks.com/ramblings/2008/03/27/bear-spray-effectiveness/


http://www.jacksonholenews.com/Archives/Environmental/2004/040922-enviro.html

http://www.absc.usgs.gov/research/brownbears/pepperspray/pepperspray.htm

Toward the bottom of the last link is a chart showing that a firearm is also not not a perfect solution.

Marcy