A New Study Reveals Diesel Filters' Unintended Effects

The study shows that the chemical reaction used in diesel filters can increase the presence of nitrogen dioxide—and worsen common allergic reactions.
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A car mechanic fits a particle filter to a diesel-engined car.

A car mechanic fits a particle filter to a diesel-engined car.

Filters, which have been attached to diesel cars for roughly the two decades, were invented and widely implemented as a way to mitigate some of the damaging health effects of diesel. But a new study suggests that diesel filters have some unintended consequences.

Filters were introduced as a way to decrease fine particulate matter, which is made of dust, dirt, soot, smoke, or other organic compounds. And because of their small size, these particles can get lodged in the lungs or enter the bloodstream.

Inhaling fine particulate matter—and fossil fuel pollution generally—can be deadly. Air pollution currently causes up to 8.8 million premature deaths worldwide each year, surpassing global deaths from smoking.

The study, published last Friday in the American Journal of Respiratory and Critical Care Medicine, found that, while the filters were successfully filtering fine particulate matter, filtered exhaust fumes had higher levels of nitrogen dioxide, which has the potential to exacerbate common allergic reactions.

For the study, the researchers tested the lung capacity of non-smoking adults who had one of three common air allergies: grass, birch, or household dust mites. They also measured participants' white blood cell counts, to see if their immune systems were "overreacting" to allergens, which can trigger an allergic response.

Participants inhaled four different mixtures, each for two hours. The mixtures included air and saline (the control), air and an allergen, filtered diesel with an allergen, and finally the regular diesel exhaust with an allergen.

Researchers found that breathing in filtered diesel and the allergen decreased lung capacity 7.5 percent more than breathing the regular (unfiltered) diesel exhaust with the allergen. Lung function when breathing in regular diesel exhaust with the allergen was the same as normal air with an allergen.

But this doesn't mean that we should just write off diesel filters, the authors say. Pollution from fine particulate matter is a risk worldwide, and the removal of small particles is still important, even if filters have some drawbacks.

"There are decades of research demonstrating the overall harmful effects of particulate matter, so [this study] shouldn't be taken as a suggestion that contradicts that long history of work," Chris Carlsten, the study's senior author and professor of respiratory medicine at the University of British Columbia, told Popular Science.

In the future, regulatory agencies could require the use of "after-treatment methods," such as selective catalytic reduction technologies that capture nitrogen emissions before they are released into the air, in addition to a diesel filter to catch fine particulate matter. However, the authors note that these technologies have only been introduced recently, and still face challenges for implementation.

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