Police need reasons to believe a suspect is dangerous before firing a Taser and can't use their stun gun simply because the person is disobeying orders or acting erratically, a federal appeals court in San Francisco ruled Monday.

The decision by the Ninth U.S. Circuit Court of Appeals sets judicial standards for police and for people who claim they were victims of excessive force after police hit them with a Taser dart.

"The objective facts must indicate that the suspect poses an immediate threat to the officer or a member of the public," Judge Kim Wardlaw said in the 3-0 ruling.

Though stun guns may offer a valuable, nonlethal alternative to deadly force in defusing dangerous situations, Wardlaw said, they inflict a "painful and frightening blow" and must be used only when substantial force is necessary and other options are unavailable.

"It's a significant use of force, not like cuffing someone or using pain compliance or pepper spray," said Eugene Iredale, a lawyer for a San Diego-area man who was Tasered by a police officer who had stopped him for not wearing a seat belt. "It's not to be used promiscuously or lightly."

The ruling allows Iredale's client Carl Bryan to go to trial in his damage suit against Brian McPherson, a policeman in Bryan's hometown of Coronado. McPherson's lawyers were unavailable for comment.

Tasers enjoy wide support among law enforcement officials, including George Gascón, San Francisco's new police chief, who is considering recommending the devices for his officers and has ordered a study of past police shootings to see whether stun guns would have made a difference. On the other hand, Amnesty International says 334 people died in the United States from 2001 to August 2008 after being hit by Tasers.

McPherson stopped Bryan's car on a summer morning in 2005 as the 21-year-old was driving home. Wearing only boxer shorts and tennis shoes, and upset at himself for forgetting to fasten his seat belt, Bryan swore at himself as he stepped out of the car, and was shouting gibberish and banging his thighs as he stood 15 to 25 feet away from the officer, the court said.

McPherson said Bryan then took one step toward him. Bryan denied it, and the court said the evidence indicated that Bryan was facing away from McPherson when the officer fired his Taser. Bryan fell on his face, breaking four front teeth, and needed a hospital visit to remove the electronic dart, the court said. He was charged with misdemeanors of resisting and opposing an officer, but prosecutors dropped the charges after the jury deadlocked.

Upholding a judge's refusal to dismiss Bryan's civil suit, the appeals court said a jury should decide whether the officer had used too much force to subdue someone who was not threatening him.

Bryan was clearly unarmed and did not challenge McPherson verbally or make any menacing gestures, Wardlaw said. She said McPherson's claim that Bryan had ignored an order to stay in the car - an order that Bryan denied hearing - would not justify a Taser shooting, nor would the officer's concern that Bryan might be mentally disturbed.

Other factors that could support a claim of excessive force, Wardlaw said, were the minor nature of the traffic offense, McPherson's failure to warn Bryan that he might be Tasered and the fact that other officers were on the way to the scene.