There’s little question that air pollution is toxic for the human body. Studies have shown that particulate matter in the air can lead to lung disease, heart disease, strokes, and lung cancer. But researchers thought the brain might be protected due to the blood brain barrier—a natural system that filters out foreign substances and certain neurotransmitters before they circulate in the brain. A new study from researchers at Cedars-Sinai Medical Center in Los Angeles shows that many heavy metals found in the air may make it into brain tissue, and those pollutants are activating genes that may lead to cancers or neurodegenerative disorders. 

To understand how air pollution impacts the brain, doctor Julia Ljubimova, director of the Nanomedicine Research Center at Cedars-Sinai, produced air with the same chemical makeup as that found in Riverside, California, in the Los Angeles Basin. She and her team then subjected rats to the air, with different groups of rats breathing the polluted air for two weeks, one to three months, and 12 months. After examining the rodent brains, the researchers found higher than normal concentrations of heavy metals including cadmium, cobalt, lead, nickel, vanadium, and zinc accumulated in the rats exposed to the pollution for a month or more. Even more disturbing, coarse particles of the pollutants had switched on certain genes. The research appears in the journal Scientific Reports. 

“Initially I was even skeptical we could find anything. For example, a smoker has to smoke 20 years to develop lung cancer,” Ljubimova says, “so I was not sure that in three, six, or 12 months of exposure we would detect changes in these animals’ brains at the genomic level. I was very, very surprised when we found so many changes.”

So how are these heavy metals making it into the brain despite the blood brain barrier? The coarse particulate material gets in through the lungs, which absorb the pollution particles into the bloodstream and may somehow beat the blood brain barrier, which can weaken due to high blood pressure, inflammation, and other stresses. Particulates inhaled through the nose have a more direct route into the brain through the olfactory system and may accumulate through that pathway. Once in the brain, the metals cause inflammation, switching on certain genes including those that cause both benign and malignant tumors, and others that are suspected of causing neurodegenerative disorders like Parkinson’s, ALS, Alzheimer’s, and other types of dementia—something that other recent studies have also found.

There are still many questions the study can’t answer. For instance, do these heavy metals accumulate throughout a lifetime, or can the body flush them out? And most importantly, can a study on rats translate to humans? While Ljubimova says it’s likely that the same systems are at work in humans and rodents, her team is also studying Cedars-Sinai’s archive of human brain tissue to see if there’s evidence of these coarse particles accumulating in the brains of people who lived in areas with air pollution.

Ljubimova says that while the pollutants in her study were based on Los Angeles, she guesses many of the same effects are happening in cities across the world with similar loads of coarse pollutants. The hope, she says, is that the study and future follow-ups will galvanize policymakers to take a closer look at the health impacts of industry, auto emissions, agriculture, and military activities in L.A. and other areas with pollution problems. She points out that more and more people are being exposed to questionable air as urbanization expands, and scientists don’t know all the possible organs and ill effects exposure can cause. 

“We thought that nature protected the brain through the blood brain barrier,” she says. “But now we see that no, air pollution affects even isolated and protected organs such as the brain. This is important information for thinking about new developments and ways to protect the public.”