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volume 121 | number 9 | September 2013 • Environmental Health Perspectives
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Particulate Matter and
Researchers Turn an Eye toward Microvascular
Particulate matter (PM) has been consistently associated with car-
diovascular disease development and progression,1,2 and is believed
to contribute to development either indirectly through the auto-
nomic nervous system or inflammatory responses, or directly via entry
into systemic circulation and subsequent damage to blood vessels.3
However, it’s unclear whether changes in the microcirculation—the
small veins (venules) and arteries (arterioles) that compose the major-
ity of the circulatory system—might also contribute.4 A new study in
EHP explores the impact of PM on small blood vessels by studying
“Researchers suspect that air pollution may cause heart disease,
in part, by limiting the blood vessels’ ability to bring blood to the
heart. This hypothesis has been difficult to test since looking at the
very small blood vessels in people’s hearts is challenging,” says Sara
Adar, an assistant professor of epidemiology at the University of
Michigan, Ann Arbor, who was not involved in the current study. “By
using photographs of the tiny, hair-like blood vessels in people’s eyes,
researchers are able to get a direct look at how air pollution may affect
other very small blood vessels in the body like those that bring blood
to our hearts.”
Adar and her colleagues used this approach in a previous analysis
of data from the Multi-Ethnic Study of Atherosclerosis (MESA), a
multicenter prospective investigation of cardiovascular disease.6 They
found that both short- and long-term exposure to elevated levels of
fine PM was associated with narrowing of the arterioles and widening
of the venules, measured as central retinal arteriolar equivalents
(CRAE) and central retinal venular equivalents (CRVE), respectively.5
Extending that approach to a younger and healthier cohort,
investigators in the current study recruited 84 individuals aged
22–63 years old with no history of cardiovascular disease or diabetes.
The participants, all of whom worked at the Flemish Institute for
Technological Research (VITO) in Mol, Belgium, completed up to
three clinical visits and answered questionnaires about current health,
lifestyle factors, and time spent in traffic in the preceding 24 hours.
Study visits included photography of the fundus (interior surface) of
the right eye for each participant as well as blood pressure and heart
rate measurements for participants who completed two or three visits.
An air monitoring station within 10 km of the institute provided
coarse PM and black carbon exposure data at 2, 4, 6, 24, and up to 48
hours prior to each visit.
During the course of the study (January to May 2012), observed
CRVE did not change significantly, but decreases in CRAE were
measured in association with higher exposures to coarse PM10 and
black carbon. Associations remained significant in multiple statistical
analyses, “no matter how we looked at the data,” says coauthor Luc
Int Panis, research program coordinator at VITO. “We are convinced
that this is a robust conclusion.” CRAE and CRVE were both associ-
ated with cardiovascular disease in other studies, although it is unclear
whether they trigger the disease process or simply arise from it.
Int Panis notes the authors are not implying that the observed
association has any immediate clinical implications. But he adds that
the finding is consistent with downstream effects of air pollution that
are already known to lead to atherosclerosis.
Both Int Panis and Adar note the “repeated measurements”
design as a strength of the study. By collecting multiple (i.e., repeated)
measurements on the same people over time, the authors were able
to estimate the impacts of day-to-day fluctuations in pollution on
individuals free of confounding by characteristics that vary among
people, Adar explains.
The estimated changes in CRAE were about three times larger
than those associated with similar levels of air pollution in the MESA
analysis. However, the authors of the current study suggest that the
younger and healthier study population may have had blood vessels
that were better able to adapt to changing pollution conditions.5 The
current study also looked solely at short-term exposures (2–24 hours)
versus the short- and long-term exposures (24 hours and 2 years)
evaluated in the MESA analysis. The researchers found no evidence
of a threshold below which changes were not seen, consistent with
the MESA analysis and other studies.2,6 “This well-conducted study
confirms our previously published findings from the MESA study,
which has indicated that air pollution may affect the very small blood
vessels in our body,” says Adar.
As in other studies, individual exposure data were not available,
raising the possibility of exposure misclassification. Further, the
participants were not representative of the general population, so the
results may not be broadly applicable. However, the investigators
have already begun followup research with wearable air-monitoring
devices, Global Positioning System devices, and a more diverse study
Julia R. Barrett, MS, ELS, a Madison, WI–based science writer and editor, has written for EHP
since 1996. She is a member of the National Association of Science Writers and the Board of
Editors in the Life Sciences.
1. Miller MR, et al. From particles to patients: oxidative stress and the cardiovascular effects of air
pollution. Future Cardiol 8(4):577–602 (2012); http://dx.doi.org/10.2217/fca.12.43.
2. Brook RD, et al. Particulate matter air pollution and cardiovascular disease: an update to the scientific
statement from the American Heart Association. Circulation 121(21):2331–2378 (2010); http://dx.doi.
3. Nelin TD, et al. Direct and indirect effects of particulate matter on the cardiovascular system. Toxicol Lett
208(3):293–299 (2012); http://dx.doi.org/10.1016/j.toxlet.2011.11.008.
4. Johnson JC. Overview of the microcirculation. In: Microcirculation, 2nd ed. (Tuma RF, et al., eds.).
Oxford, UK:Elsevier (2008).
5. Louwies T, et al. Retinal microvascular responses to short-term changes in particulate air pollution
in healthy adults. Environ Health Perspect 121(9):1011–1016 (2013); http://dx.doi.org/10.1289/
6. Adar SD, et al. Air pollution and the microvasculature: a cross-sectional assessment of the in vivo
retinal images in the population-based Multi-Ethnic Study of Atherosclerosis (MESA). PLoS Med 7(11):
e1000372 (2010); http://dx.doi.org/10.1371/journal.pmed.1000372.
Pollution-related changes in the tiny blood vessels of the retina could
offer insights into potential impacts on cardiovascular disease risk.
© Tijs Louwies