PORTLAND, Ore. (AP) ” Safety experts estimate that an average of 40 percent of skiers and snowboarders use helmets.

The issue arose anew after a fatal incident Jan. 5 at Mount Hood Meadows in Oregon. Investigators say the death of Geoffry Bradeen, 45, occurred when he was hit from behind by a snowboarder as he was getting up. A helmet would likely have saved Bradeen, who died of a skull fracture, said Dr. Karen Gunson, Oregon’s medical examiner.

But studies show such collisions are rare and account for only 6.4 percent of reported ski accidents, said Jasper Shealy, who has studied skiing and snowboarding injuries and fatalities for 35 years. He said most skiing and snowboarding deaths are caused by hitting a fixed object at high speed, resulting in chest or torso injuries.

Shealy and others looked at 562 deaths from fall 1991 through spring 2005, finding that 60 percent were the result of a skier or snowboarder hitting a tree. Hitting the snow is the second-biggest killer, with 9.7 percent, and hitting manmade objects, such as lift towers, is third, at 7.6 percent. The researchers also found that helmet use is up by up to 5 percentage points a year but deaths still averages 38 a year.