More than 9,000 Californians are dying prematurely every year because of the health effects of the kind of pollution emitted by diesel trucks and heavy equipment, according to a new study by the Air Resources Board, the state’s air quality regulator.
The study is the first released by the state to claim that the microscopic particles emitted by engines burning diesel fuel actually cause early deaths, rather than simply being correlated with them.
The new analysis uses the same methodology as a recent report from the U.S. Environmental Protection Agency, which relied extensively on a study of a half million people in 116 cities across the country.
“The EPA concluded that there was a causal relationship between [diesel pollution] and premature death,” Bart Croes, chief of research for the Air Resources Board, said in an interview. “It’s not just some random statistical association.”
The study doesn’t say how much longer the victims would have lived, on average, if they had not been exposed to diesel pollution. But the people dying early are thought to have been suffering already from heart disease, emphysema and other disorders that left them vulnerable to the effects of the particles, which are about 1/30th the diameter of the average human hair.
Both the EPA and air board studies used epidemiology – the study of disease trends – to conclude that fine particles were causing premature deaths. Researchers examined death rates from all causes and found that they were correlated with increases in detected levels of diesel pollution. They then used statistical modeling to try to account for other potential causes of death, from smoking to diet and job hazards.
They were left with numbers which, they concluded, showed about a 5 percent increase in deaths above the expected rate in places with a heightened exposure to PM 2.5, the kind of particulate matter generated when engines burn diesel fuel and send their exhaust into the air.
Although toxicologists have not established exactly how these fine particles cause disease and death, they are believed to lodge in the lungs or other tissue and lead to illness.
The latest air board study replaces one released two years ago that estimated the number of early deaths at about 18,000 per year. That study was controversial in part because its lead author, Hien Tran, had falsified his credentials.
Although the ARB disciplined and demoted Tran, the agency stood by his study, saying it had been reviewed and found to be sound. But the latest study used different methods, a different data base and had the benefit of the latest EPA numbers.
While its estimate of premature deaths is only half as large as the Tran study concluded, the new report has a smaller error range – plus or minus about 2,000 deaths – and is considered by the board’s staff to be a tighter estimate.
The research also concluded that about 2,700 deaths per year could be avoided by implementation of the air board’s rules forcing trucking companies and construction firms to reduce the pollution from their trucks and equipment.
But James Enstrom, a UCLA researcher and frequent critic of the air board’s work, said in an interview that he doubts the accuracy of the new report. He said his own research and that of several other scientists has shown that there is essentially no measurable risk of premature death from fine particles in California.
“There’s no relationship between total mortality and PM 2.5 in the state of California,” Enstrom, an epidemiologist, said in an interview. “I am saying I think the effect is not present in California.”
ARB researchers, however, point out that Enstrom’s own research found an effect similar to the one they have established. His work focused on people who were part of a group followed for decades as part of a research project originally aimed at tracking the effects of tobacco.
When that group was younger, Enstrom’s data show, there appeared to be an increase in early mortality that was similar to what the most recent ARB study found. Numbers for the older members of the research database did not show the same effect, however, and those are the figures that Enstrom says are more reliable.
But the ARB says the effect declined as the research subjects aged because those who were most vulnerable to the harmful effects of the pollution died early. Those whose bodies were more equipped to weather the pollution lived longer, making it less likely that researchers would see an effect on them in their later years, said Croes, the chief of research.