Even brief exposure to diesel fumes alters activity in key brain network, study finds

Even brief exposures to diesel fumes may alter the brain's function, messing with how signals zip through a major brain network, a new study suggests.

Past studies suggest that people exposed to air pollution may be more likely to develop mental health conditions and neurodegenerative diseases. Now, the new study, published Jan. 14 in the journal Environmental Health, shows that exposure to diluted diesel exhaust resulted in less "functional connectivity" in a key brain network than exposure to filtered air. 

Functional connectivity is a measure of how well different brain regions communicate with one another; after exercising in clean air, the study's participants showed a temporary boost in connectivity in a specific brain network, but after exposure to diesel-tainted air, the network's connectivity remained stagnant. Although the study didn't examine the cognitive impacts of the exposure, the network is linked to internal thought and introspection, and disruptions in its activity have been linked to various mental health, cognitive and attention-related disorders. 

According to its authors, the study is the first to examine the brain's response to air pollution in such a controlled way. The researchers "have made a significant contribution to what we know about the impact of exposures to pollution," said Hao Yang Tan, a lead investigator at the Lieber Institute for Brain Development in Baltimore, who was not involved in the study.

Related: 10 things you didn't know about the brain 

The researchers took snapshots of brain activity in 25 adults using functional magnetic resonance imaging (fMRI); fMRI works by detecting increased blood flow to active neurons, therefore giving an indirect indication of brain activity. Then, participants were exposed to either filtered air or air mixed with diesel exhaust for two hours while they rode an exercise bike at a relaxed pace. 

Immediately after the exposure, researchers took another fMRI. All participants took part in both the exposure and control scenarios at different times, and neither the participants nor the data collectors ever knew which group anyone was in.

The researchers examined how exposure to diesel impacts the default mode network (DMN), which is involved in self-reflection and a person's internal thoughts, rather than external stimuli, said Dr. Michael Lipton, a professor of radiology at Albert Einstein College of Medicine and medical director of MRI services at Montefiore Medical Center, who was not involved in the study. Research suggests that people with psychiatric disorders, including anxiety and depression, show distinct changes in the DMN, said Tan. 

DMN connectivity briefly increased after exposure to filtered air, which the researchers attributed to the exercise the participants were doing. Previous research has associated light exercise with increased DMN connectivity. DMN connectivity didn't change after diesel exposure. 

The new study was limited by its small size and the fact that people are exposed to higher concentrations of air pollution for longer in the real world. The study also can't show exactly how the diesel may have caused the observed changes in connectivity.

"Diesel exhaust is known to cause systemic inflammation, which could affect the brain and change the DMN," senior study author Dr. Chris Carlsten, professor and head of respiratory medicine at the University of British Columbia, told Live Science in an email. "However, this is speculative." Lipton said he's not sure such a short exposure could cause substantial inflammation in the brain. 

Tan said the study is a good first step towards better understanding how air pollution hurts the brain. It's also important to recognize that, regardless of the biological mechanisms behind this effect, there are other reasons air pollution doesn't impact everyone equally, he added. For example, socioeconomically disadvantaged communities are more likely to be exposed to air pollution, as they're more likely to live and work in polluted areas, Tan said.