depression. Mice said to express depressive-like symptoms
give up swimming more quickly in a forced swim test and stop
sipping sugar water that they normally find attractive. Both
behaviors can be reversed with antidepressants. Nelson found
that mice exposed to the polluted air scored higher on tests of
To find out more about the underlying cause of those
behavioral changes, Nelson compared the brains of mice
that had been exposed to dirty air with brains of mice that
hadn’t. He found a number of striking differences. For starters,
mice exposed to particulate matter had increased levels of
cytokines in the brain. (Cytokines are cell-signaling molecules
that regulate the body’s inflammatory response.) That wasn’t
entirely surprising, since previous studies investigating the
cardiovascular effects of air pollution on mice had found
widespread bodily inflammation in mice exposed to the
More surprisingly, Nelson also discovered physical changes
to the nerve cells in the mouse hippocampus, a region known
to play a role in spatial memory. Exposed mice had fewer
spines on the tips of the neurons in this brain region. “Those
[spines] form the connections to other cells,” Nelson says. “So
you have less dendritic complexity, and that’s usually correlated
with a poorer memory.”
The changes are alarming and surprising, he says. “I never
thought we’d actually see changes in brain structure.”
Nelson’s mice experienced quite high levels of pollution, on
par with those seen in places such as Mexico City and Beijing,
which rank higher on the pollution scale than U.S. cities. It’s
not yet clear whether the same changes would occur in mice
exposed to pollution levels more typical of American cities.
Another limitation, he notes, is that the animals in his study
were genetically identical. Nelson says he’d like to see similar
studies of wild-type mice to help tease out whether genetic
differences might make some people more or less vulnerable to
the effects of pollution. “I would suspect there are people who
are wildly susceptible to this and people who are less so, or not
at all,” he says.
Few studies have investigated connections between
depression and air pollution, but Nelson’s wasn’t the first. A
study by Portuguese researchers explored the relationship
between psychological health and living in industrial areas.
They found that people who lived in areas associated with
greater levels of air pollution scored higher on tests of anxiety
and depression (Journal of Environmental Psychology, 2011).
Back in Ohio, Nelson plans to study how much — or how
little — pollution is necessary to cause changes in the brain
and behavior. He’s also beginning to look at the effects of air
pollution on pregnant mice and their offspring. Though more
research is needed to fully understand how dirty air impairs
the brain, he says, the picture that’s emerging suggests reason
In the United States, the Environmental Protection Agency
JULY/AUGUST 2012 • MONITOR ON PSYCHOLOGY
reviews the scientific basis for particulate matter standards
every five years or so, and completed its last review in 2009.
To date, the EPA hasn’t factored psychological research
into their standards assessments, but that could change,
according to a statement the EPA provided to the Monitor.
“Additional research is necessary to assess the impact of
ambient air pollutants on central nervous system function,
such as cognitive processes, especially during critical windows
of brain development. To this end, as the number of …
studies continue to increase and add to the weight of overall
evidence, future National Ambient Air Quality Standards
assessments will again assess and address the adequacy of
In the meantime, says Weuve, there’s not much people
can do to protect themselves, short of wearing special masks,
installing special filtration systems in their homes and offices
or moving to cities with less airborne pollution. “Ultimately,
we’re at the mercy of policy,” she says.
The good news, Nelson says, is that the mental and
cognitive effects of air pollution are finally beginning to receive
attention from the mental health research community. “We
sort of forget about these environmental insults,” says Nelson.
“Maybe we shouldn’t.” n
Kirsten Weir is a writer in Minneapolis.
Polluting your mind
How does air pollution affect our brains? It depends on
the size of the particle we inhale, says Jennifer Weuve,
MPH, ScD, of Rush Medical College. Fine particulate
matter, which includes smoke, car exhaust and pollen,
can interact directly with the brain. Coarse particulate
matter, however, is more of a mystery that researchers
are only now beginning to study.
Watch Dr. Weuve discuss these findings