You cannot outrun a bear, and screaming may escalate the danger of the situation. Instead, let the bear know you're human. Talk to it in a normal voice. Slowly wave your arms. Help the bear recognize you.
Bears often make bluff charges, sometimes to within ten feet, without making contact.
Continue moving your arms and talking to the bear. Air horns are unproven in their effectiveness. High-pitched noises can arouse curiosity or irritate a bear. The bear may stop to examine what you have dropped, distracting it from you. Never imitate bear sounds.