JERSEY CITY, NJ – JUNE 7: Smoke continues to obscure the sun as it rises behind the skyline of lower … [+]
On Wednesday, the National Weather Service issued warnings to more than 80 million Americans in the Midwest, Northeast and Appalachians to limit or avoid outdoor physical activity and exposure. The warnings were triggered by renewed shifts in smoke patterns from the Canadian wildfires that hit eastern Canada, similar to the exposure the Northeast experienced a few weeks ago. Just north of the border, Toronto recorded the worst air quality of any major city in the world on Wednesday.
Canada is experiencing its worst wildfire season on record, affecting tens of millions of people in both countries. Aside from the obvious immediate dangers and concerns of individuals in the potential path of the fires themselves, the health effects of exposure to wildfire smoke, even at great distances, are a growing concern around the world. Affected areas are experiencing “unhealthy” air quality levels that pose risks to those at risk, including children and older adults, as well as those with medical conditions such as heart or lung disease.
Effects of wildfire smoke on the brain
The risks and possible consequences for heart and lung health are well known. But what is known about the effects of wildfire smoke on the brain and cognition? What are the consequences of acute exposure to possible long-term effects?
The effects of smoke exposure from wildfires on the brain, even for just a few hours, can have significant implications for brain health and its physiology. It can also affect cognition and memory. One of the main culprits is particulate matter (PM), produced by inefficient combustion, which ends up in the air and eventually ends up in the body.
Despite growing evidence that PM from wildfires can have serious effects on the brain, there is still no complete picture. The number of scientific studies specifically focusing on brain smoke exposure from wildfires is only a handful, but is likely to become an increasing subject of research as the intensity and duration of wildfires continue to increase.
The chemical composition of PM from wildfire smoke is different from that of ambient PM or PM from other pollution sources, e.g. car emissions or industrial pollution. Wildfire PM has a higher proportion of carbon-based pollutants in various chemical forms. The size of PM particles is also important for the effect it has on health. For example, particles of different sizes may reach the lungs in different ways and have different clinical effects. Wildfire PM consists of a mix of particulate matter of different sizes. The size of the PM and the ecological source from which they originate – in other words, where the fire burns – affects the composition of the smoke, how it travels and the health effects it causes.
From a cellular and physiological perspective, wildfire smoke is known to be toxic to cells, pro-inflammatory and can affect how the immune system responds and functions to exposure to PM. Particulate matter within size ranges found in wildfire smoke is likely to reach the brain by inhalation through the lungs or nose (olfactory).
When particles enter the brain from the lungs, they must first cross the blood-brain barrier to gain access to the brain. The blood-brain barrier is a physical barrier formed by tight junctions between the endothelial cells that form the capillaries that carry blood to the brain. Those endothelial cells act as gatekeepers, regulating what can and cannot cross over from the systemic circulation and blood into the cerebrospinal fluid that envelops the brain and spinal cord. But the systemic inflammation that results from smoke exposure can increase the permeability — the “leakage” — of the capillaries that feed the brain, giving PM a pathway to traverse and reach the brain. Inflammation can cause the tight connections between endothelial cells to become compromised and more permeable, resulting in an unregulated flow of particles across the barrier.
Wildfire smoke PM could also access the brain more directly through the olfactory neurons in the nose. These neurons extend from the olfactory bulb, which is part of the brain itself, to the olfactory epithelium. Thus, if PM enters these neurons and travels to other parts of the brain, it is no longer necessary to cross the blood-brain barrier.
Once in the brain, environmental PM has been shown to cause oxidative toxicity that can lead to cell death in neurons and other non-neuronal cells, as well as inflammation. Similar cellular and physiological effects on brain health have been proposed for PM produced by forest fires.
A recent study from 2020 quantified in detail a number of neuroinflammatory and neurometabolomic wildfire smoke inhalation effects in mice, by setting up a mobile lab more than 180 miles from active wildfires in California, Arizona and Washington. After four hours of exposure per day for 20 days, the scientists measured significant and sustained neuroinflammation, even as peripheral immune activity in the lungs and bone marrow resolved itself. The researchers were able to show that PM exposure from wildfires had a direct impact on endothelial cells and the expression of a number of specific molecules in the blood and in the brain, as well as an increase in the infiltration of peripheral immune cells into the brain. In the brain itself, they measured an increase in the activation of neural glial cells, which have homeostatic roles that maintain a healthy environment for neurons to function, including molecular changes and activation of microglia, the brain’s immune cells.
Interestingly, the scientists also measured a decrease in molecules that normally protect the brain against aging, while also observing an increase in amyloid beta protein, which is associated with neurodegenerative disorders such as Alzheimer’s disease.
Another study documented a statistical association between acute short-term exposure to smoke from wildfire PM with a quantified decline in a cognitive task consisting of an attention-oriented brain training game. Exposure for even a few hours was correlated with decreased cognitive scores. In addition to smoke exposure from wildfires, other work is documented the serious mental health impacts a wildfire can have on anxiety, depression, and traumatic stress.
Still unknown are the potential long-term effects of acute exposure to wildfire smoke on brain health and cognition. And as wildfires get bigger and hotter, the effects of the resulting smoke on the brains of affected individuals could potentially grow in severity over the coming decades.