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Numerous Studies Point to
Neurotoxic Effects of Air Pollution
In a recent article, Science highlights the growing body of
evidence suggesting that inhalation of fine and ultrafine particles
commonly found in air pollution can damage the brain and increase the
risk of developing dementia and Alzheimer’s disease. The potential for
cognitive impairments is added to the long list of established health
issues attributable to air quality such as asthma, lung cancer and
heart disease.
Air Pollution and Dementia
One paper cited in the article and published last month in
Translational Psychiatry was an 11-year epidemiological study of
the effects of particulate matter (PM) exposure on women. The class of
PM they studied is PM2.5, specifically particles with an aerodynamic
radius less than 2.5μm. According to Arian Saffari, an author
on the study that came out of the University of Southern California,
“The smaller the particles that cells are exposed to, the higher their
levels of oxidative stress.” Ultimately, the study found that “...
airborne PM exposure promotes pathological brain aging in older women,
with potentially a greater impact in ε4 carriers.” The authors
“estimate that ~21% of accelerated cognitive decline and all-cause
dementia are attributable to residential exposure to high ambient PM2.5.”
The second finding is particularly interesting because Apolipoprotein
E (APOE) ε4 is among the loci implicated in Alzheimer’s disease (AD)
and yet these genetic alterations have so far accounted for less than
half of AD cases. The present finding suggests the need for synergy
between genetic and environmental factors to increase risk.
Another study referenced in the article was published last month in
the Lancet. Based on existing evidence that living in closer
proximity to a major roadway might have negative effects on cognition,
a team from the University of Toronto set out to investigate more
closely the associations between proximity to roadways and Parkinson’s
disease, dementia and multiple sclerosis. Their finding? “In this
large population-based cohort, living close to heavy traffic was
associated with a higher incidence of dementia, but not with
Parkinson's disease or multiple sclerosis.” Interestingly, persons
living in less than 50 meters from a major road showed a 7% increased
risk of developing dementia. That risk disappeared completely for
individuals living greater than 200 meters from a major road. For
those living in major cities at less than 50 meters from a major road,
their risk increased to 12%.
Relating Animal Model Insights to Humans
Many of the other studies linking pollution exposure to damage in the
brain have used animal models and translation to meaningful insights
for humans is still necessary. The Science article notes this
will be difficult as long-term data on pollution are lacking globally.
Some work has been done to date however such as a review of 18 studies
in 6 countries, including the US and China, where all but one showed
correlation between dementia and high exposure to a component of air
pollution. More work needs to be done, however. And it’s clear that
more data are necessary. Only a third of US counties monitor pollution
and PM2.5 has only been monitored since 1997.
Using Simulation to Fill in the Missing Pieces
One solution to the lack of data may be simulation, and one study
underway in Seattle will use modeling data to estimate lifetime
exposures to PM2.5 allowing for correlations with dementia incidence.
Participants in the study have already been monitored for cognitive
changes for 20 years. All that’s missing is the PM2.5 data. Lianne
Sheppard, a biostatistician at the University of Washington, says
that combining the data set with genetic studies will allow their
research group to understand “not just the epidemiology of the
relationship between air pollution and cognition, but start drilling
down to mechanisms” for interactions between pollutants and the brain.
Differing Impacts Across Socioeconomic Groups
On a final note, the Science article highlights the
differential effects across socioeconomic groups. Because they more
commonly live in areas with higher PM2.5 levels, the poor will be
disproportionately affected. Additionally, recent studies have
demonstrated a synergistic effect between pollution levels and other
environmental stresses like litter and crime. This means that policy
changes might be most effective if focused specifically on the most
vulnerable communities. In the end, air pollution may end up an even
bigger villain than originally predicted. According to Caleb Finch,
a neuroscientist who works with the USC team, “I think [air pollution]
will turn out to be just the same as tobacco - there’s no safe
threshold.”
Science News Article
https://tinyurl.com/j9mclkg
Translational Psychiatry Article
https://tinyurl.com/gqlhe2n
Lancet Article
https://tinyurl.com/z6r69u7
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