Global climate change is driving an increase in wildfire activity characterized by larger fires and longer fire seasons. Wildfire smoke, which can spread over immense geographical areas, often contains a variety of pollutants and exerts a wide range of adverse effects on human health. Evidence from a new rodent study suggests that particulate matter in wildfires drives neuroinflammation, increasing the risk for neurodegenerative diseases.

Particulate matter in air pollution is a mixture of solid particles and liquid droplets. It is present in fine inhalable particles, with diameters that are generally 2.5 micrograms (PM2.5) or less. Ultrafine particles less than 1 microgram in diameter, referred to as nanoparticles, are often enriched in highly reactive metals such as iron, aluminum, titanium, and others. Exposure to particulate matter in air pollution promotes oxidative stress, increases the risk of developing many chronic diseases, and accelerates aging.

The investigators studied the effects of wildfire smoke on mice that were housed in a mobile lab located roughly 186 miles (300 kilometers) away from naturally occurring wildfires in the western United States. They exposed the mice to the smoke for four hours every day for 20 days and assessed the animals' immune and inflammatory responses.

They found that the animals were exposed to high levels of PM2.5. This exposure switched on the activity of brain microglia (immune cells); promoted the infiltration of pro-inflammatory immune cells and molecules into the brain tissues; and increased accumulation of amyloid-beta 42, a toxic protein associated with Alzheimer’s disease and neurodegeneration. Particulate matter exposure also decreased the production of compounds that protect the brain against aging, such as nicotinamide adenine dinucleotide (commonly known as NAD+) and taurine.

These findings suggest that exposure to PM2.5 in wildfire smoke elicits harmful effects on the brain via activation of immune and inflammatory responses. The investigators noted that the mobile lab used in this study was located a considerable distance from the smoke sources, likely diluting the animals' exposure and reflecting PM2.5 exposures far lower than those experienced by humans living closer to the fires.

Robust evidence demonstrates that HEPA filter air purifiers reduce indoor PM2.5 concentrations and improve health outcomes, and many government agencies and public health authorities recommend the use of indoor HEPA filters to reduce wildfire smoke exposure and its negative health effects. In addition, well-fitting N95 masks and equivalent respirators can reduce PM2.5 exposure. Interestingly, dietary consumption of omega-3 fatty acids may help protect the brain from damage associated with PM2.5 exposure. Learn more about the health effects of omega-3 fatty acids in this episode featuring Dr. Bill Harris.

• Link to study abstract.