Filters that break up the exhaust from diesel engines are intended to decrease the health impacts of heavy roadway traffic, but a new study suggests the opposite might be happening in some cases.
Chris Carlsten, MD, MPH
Filtered diesel exhaust may cause allergy sufferers more distress than unfiltered exhaust, according to new research.
The study suggests policymakers ought to think more carefully about an environmental measure once considered common sense.
“The take-home message is that technologies that remove particulate matter from diesel exhaust cannot be simply assumed to be beneficial to health, especially in susceptible populations,” corresponding author Chris Carlsten, MD, MPH, of the University of British Columbia, said in a statement.
Traffic-related air pollution is a major driver of asthma and other lung distress in patients around the world. One way of dealing with the problem is to filter the exhaust from traffic, including particulate filtering technologies that modify the exhaust coming from diesel engines.
However, it turns out that some filtration technologies lead to an increase in the amount of nitrogen dioxide (NO2) in the exhaust from diesel engines. In the new study, investigators sought to figure out whether those increased NO2 levels might actually lead to negative impacts on human lungs.
To investigate, they recruited 14 non-smoking patients who were sensitized to at least 1 of 3 common allergens. The enrollees were exposed to regular diesel exhaust, particulate-filtered exhaust, or filtered air (which acted as the placebo). Neither the subjects nor the study administrators were aware what each subject was inhaling.
Two hours after exposure to the exhaust or filtered air, the patients were given an allergen challenge test to see whether their responses to the allergen were affected.
As expected, the data confirmed that NO2 levels were increased by the HEPA filtration system. The question then was what impact those levels had.
Analysis showed patients exposed to the filtered exhaust had the greatest level of impairment when tested to see how much air they could forcibly exhale in 1 second. These patients also had the highest levels of white blood cells. Patients who were genetically susceptible to oxidative stress experienced the greatest impacts.
Carlsten told MD Magazine® it’s possible this type of diesel exhaust filtration affects a wider population beyond just allergy sufferers.
“It would be speculative to suggest that the findings would be harmful to those without allergies but there is good reason from the literature to believe that the increased NO2 is a risk even to those without allergies,” he said.
However, Carlsten also noted that the phenomenon of increased NO2 levels is not seen in all types of diesel exhaust filtration.
“So our results are most helpful in reflecting circumstances in which particle-depleting technologies lead to increased oxidative gases such as NO2,” he said.
In addition to calling for additional scrutiny of exhaust filtration and other technologies intended to improve the air we breathe, Carlsten said the study reinforces the message that traffic-related exhaust is a serious health hazard.
“I think the best advice is for everyone to try and distance themselves within reason from traffic corridors,” he said. “This does not so much increase the overall evidence (which is massive) for harm of [diesel exhaust] as give caution to particular technologies.”
The study, “Particle Depletion Does Not Remediate Acute Effects of Traffic-Related Air Pollution and Allergen: A Randomized, Double-Blinded Crossover Study,” was published online in the American Journal of Respiratory and Critical Care Medicine.