This California county has the highest death rate for climate change-driven wildfire smoke

Climate change fueled devastating California wildfires between 2006 and 2020, contributing more than 5,200 deaths statewide from harmful wildfire smoke particles, according to a new study.

The tally is greater than any other state over the 15-year study period. A rural Northern California county — Trinity County — had the country’s highest rate of deaths from climate change-driven wildfires and microscopic smoke and soot particles, also known as PM2.5.

“Climate change is here,” said author Bev Law, an emeritus professor of global change biology at Oregon State University.

The study is the first to quantify how much climate change has impacted rates of death from wildfire PM2.5 at the state and county levels, according to the authors. They reported the biggest increases in the western United States.

The scientists developed models that linked weather data to acres of land burned across the country. These results then fed into calculations of annual wildfire PM2.5 concentrations and estimates for human health impacts.

“I call it a daisy chain of analysis,” Law said.

The authors estimated that from 2006 to 2020, there were 164,000 deaths due to wildfire PM2.5 across the contiguous United States. Of that, 15,000 were attributable to climate change, based on simulations using climate models.

Among counties in the contiguous United States, Trinity County stood out. Climate change contributed to an annual average of 1.8 deaths from wildfire PM2.5 in the Northern California county during the 2006-2020 timeframe. That may not sound like a lot, but the county is one of the least populous in California, with just 15,600 residents in 2024. That makes for an annual rate of over 17 deaths per 100,000 people — the highest rate of any U.S. county.

Besides Trinity, Siskiyou, Shasta and Del Norte counties rank among the 8 counties with the highest average annual mortality rates, according to the analysis. The other four counties were in Oregon. These two areas are highly susceptible to wildfire, with heavily forested regions that experience dry summers and the potential for extreme, flame-fanning winds, Law said.

The scientists calculated that from 2006 to 2020, climate change contributed to 25% of deaths due to wildfire PM2.5 in California. But the number fluctuated from year to year: There were an estimated 2,300 deaths in 2020, compared with 58 the year before. 

“One of the key takeaways here is there’s a lot of interannual variability,” said study author Nicholas Nassikas, a pulmonologist at Beth Israel Deaconess Medical Center. “Every year is different.”

Over 4 million acres burned across California in 2020, while the previous year, less than 280,000 acres burned. California wildfires have become more extreme in the past decade, with the state’s nine largest blazes on record all occurring since 2015.

Overall the work is “a very solid study,” said Minghao Qiu, an assistant professor at Stony Brook University in the school of marine and atmospheric sciences and the program in public health, who was not involved with the new work. Qiu said the findings are similar to what he and colleagues reported in a 2024 preprint estimating wildfire PM2.5 in the future.

Qiu said that there are still areas for additional study. Wildfire isn’t just driven by climate change, so other factors, like changes in land management practices, may influence events. The study treated wildfire PM2.5 as having similar mortality effects to particles from other manmade sources such as cars and factories, but scientists are still investigating if that’s the case. 

Nassikas said that areas of further investigation also can include compounding effects of wildfire smoke and heat waves, as well as other health impacts due to wildfire PM2.5, like asthma.