BILLINGS, Mont. — Climate change in the Western U.S. means more intense and frequent wildfires churning out waves of smoke that scientists say will sweep across the continent to affect tens of millions of people and cause a spike in premature deaths.That emerging reality is prompting people in cities and rural areas alike to prepare for another summer of sooty skies along the West Coast and in the Rocky Mountains — the regions widely expected to suffer most from blazes tied to dryer, warmer conditions.“There’s so little we can do. We have air purifiers and masks — otherwise we’re just like ‘Please don’t burn,’” said Sarah Rochelle Montoya of San Francisco, who fled her home with her husband and children last fall to escape thick smoke enveloping the city from a disastrous fire roughly 150 miles (241 kilometers) away.Other sources of air pollution are in decline in the U.S. as coal-fired power plants close and fewer older cars roll down highways. But those air quality gains are being erased in some areas by the ill effects of massive clouds of smoke that can spread hundreds and even thousands of miles on cross-country winds, according to researchers.With the 2019 wildfire season already heating up and fires breaking out from Southern California through Canada to Alaska, authorities are scrambling to better protect the public before smoke again blankets cities and towns. Officials in Seattle recently announced plans to retrofit five public buildings as smoke-free shelters.