A rare March heat wave is pushing San Francisco close to 90 degrees and setting records across the West.

Researchers say smoke from wildfires accounted for up to half of all small particle air pollution in parts of the western U.S. in recent years (AP Photo/Eric Risberg, File)BILLINGS, Mont. – Wildfire smoke accounted for up to half of all health-damaging small particle air pollution in the western U.S. in recent years as warming temperatures fueled more destructive blazes, according to a study released Monday. Nationwide, wildfires were the source of up to 25% of small particle pollution in some years, the researchers said. But it’s difficult to separate how much of the increase in smoke pollution is driven by climate change versus the forest fuel buildup, she added. The new study matches up with previous research documenting the increasing proportion of pollution that comes from wildfire smoke, said Dan Jaffe, a wildfire pollution expert at the University of Washington.