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.

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.

Scientists from NASA and universities are refining satellite imagery to predict where smoke will travel and how intense it will be. Local authorities are using those forecasts to send out real-time alerts encouraging people to stay indoors when conditions turn unhealthy.

Over the past decade as many as 2,500 people annually died prematurely in the U.S. from short-term wildfire smoke exposure, according to Environmental Protection Agency scientists.

The long-term effects have only recently come into focus, with estimates that chronic smoke exposure causes about 20,000 premature deaths per year, said Jeff Pierce, an associate professor of atmospheric science at Colorado State University.

His research team compared known health impacts from air pollution against future climate scenarios to derive its projections. The results suggested smoke will spread to become a dominant pollutant even in areas not typically associated with wildfires, such as the South and Northeast.