SAN JOSE, Calif. - The heavy smoke from wildfires that choked much of California in recent weeks was more than an inconvenience.
It was deadly. And it almost certainly killed more people than the flames from the massive fires themselves, health experts say.
Between Aug. 1 and Sept. 10, the historically bad concentrations of wildfire smoke were responsible for at least 1,200 and possibly up to 3,000 deaths in California that otherwise would not have occurred, according to an estimate by researchers at Stanford University. Those fatalities were among people age 65 and over, most of whom were living with pre-existing medical conditions like heart disease, diabetes and respiratory ailments.
By comparison, through Wednesday, 26 people have died directly in wildfires this year statewide.
"Clean air is much more important than we realize," said Marshall Burke, an associate professor of earth system science at Stanford who calculated the impacts. "When you look at it on a population level, you can see very clearly that breathing clean air has huge public health benefits, and breathing dirty air has disastrous consequences."
Decades of medical research has shown that soot is among the most dangerous types of air pollution to human health. Known as "PM 2.5," for particulate matter that is smaller than 2.5 microns in size, the microscopic soot particles are so small that 30 or more of them can line up along the width of a human hair.
Coming from diesel trucks, wildfires, power plants, fireplaces and other sources, the tiny particles can travel deep into the lungs, even entering the bloodstream, when people breathe them in high concentrations.
In mild levels they can cause itchy eyes and sore throats, coughing and a tight feeling in the chest. In more severe instances, they can trigger asthma attacks, heart attacks, strokes or respiratory failure, particularly in the elderly, infants and people with existing heart and lung problems.
Burke and Sam Heft-Neal, a research scholar at Stanford's Center on Food Security and the Environment, looked at a study published last year that used Medicare data to show when levels of particulate pollution increased in communities around the United States, the death rate of people 65 and over also increased, as did emergency room visits.
That study, by researchers at the University of Illinois and Georgia State University, found that for each day particulate air pollution increased by about 10% over typical levels - or 1 microgram per cubic meter - there was an increase in deaths over the next three days of 0.7 per 1 million people over 65, and a jump in emergency room visits among the elderly by 2.7 per 1 million people.