Wildfire smoke now accounts for up to half of all fine-particle pollution in the Western U.S., said a new study that blames climate change for worsening air quality and health risks in both urban and rural communities in recent years.

The study by researchers at Stanford University and UC San Diego found that the concentration of tiny, lung-damaging pollutants known as PM2.5 that are attributable to wildfire smoke roughly doubled between 2006 and 2018, while the share of pollution from other sources like car and truck exhaust declined.

The trend is most pronounced in Western states and highlights the rapidly growing health threat of wildfire smoke. This became shockingly apparent to millions during last year's record-breaking firestorm, which enveloped much of the West Coast in an unhealthy pall for weeks.

Levels of PM2.5 had been improving over the past two decades in which they have been routinely monitored, as a result of regulations that have cut emissions from vehicles and power plants. But those gains started to slow, then reverse, over the past decade or so, according to the study.

"The overall picture is of a stalled and reversed improvement, which is a result of other sources getting cleaner and wildfires getting a lot worse," said Marshall Burke, a professor of Earth system science at Stanford University and lead author of the study published in the Proceedings of the National Academy of Sciences.

The two major factors driving the increase in wildfire smoke are the warming climate and decades of fire suppression that have allowed fuels to build up, researchers said. They made their estimates by developing a statistical model using fire and smoke data from satellites and readings from ground-based air quality monitoring stations.

Nationwide, wildfires are now responsible for up to 25% of fine-particle pollution, the study found.

Francesca Dominici, a professor of biostatistics at Harvard's T.H. Chan School of Public Health who was not involved in the research, said it "provides strong evidence that wildfires are an increasing threat to human health."

Stanford and UC San Diego researchers predicted dramatic health impacts if nothing is done to slow climate change. The study projects an additional nine to 20 smoke-related deaths per 100,000 people by midcentury if emissions continue at their current pace, which is close to the roughly 24 additional deaths per 100,0000 people predicted directly from rising heat — the deadliest effect of climate change on people.

"Wildfires are going to be the way that many of us experience climate change," Burke said.

Of greatest concern are the microscopic particles in smoke that can be inhaled deep into the lungs and enter the bloodstream. Not only do those pollutants irritate the eyes, nose and throat, tighten the chest and cause difficulty breathing, they can trigger asthma attacks, strokes and heart attacks. Wildfire smoke poses serious risks to young children and the elderly, and people with chronic health conditions such as asthma, lung disease and heart disease face increased risk of hospitalization and death.