Particulate matter in the air comes from multiple sources, including cars and trucks, equipment that burns diesel fuel, and coal-burning factories. Each releases particulate matter categorized as fine (less than 2.5 micrometers in diameter, or PM2.5), or coarse (2.5 to 10 micrometers, or PM10), and that varies in composition, including the presence of heavy metals and organic chemicals. People who live near busy roads or industrial plants—often in urban areas—are exposed to more of this pollution. It is generally accepted that particles entering through the lungs or nasal passages can end up in the bloodstream or brain to create a host of complications, such as inflammation of the airways, vascular endothelial disturbances, and more. What has not been clear is the effect that has on cognition. Researchers previously linked coarse particulate matter to mild cognitive impairment in women older than 68 (see Ranft et al., 2009), higher ozone levels to lower cognitive scores (see Chen and Schwartz, 2009), and black carbon from traffic to cognitive decline in older men (see Power et al., 2011). In addition, living in areas of poor air quality for up to a decade correlated with decreased cognitive function in older adults in China (see Zeng et al., 2010). Until now, however, no study specifically looked at cognitive effects of fine particulate matter, or tracked cognition and air pollution longitudinally.

Researchers led by Jennifer Weuve at Rush University Medical Center, Chicago, Illinois, followed 19,409 women, aged 70-81 years, from the Nurses’ Health Study Cognitive Cohort. Beginning in 1995, the researchers assessed cognitive status over the phone, on average about every two years, using the Telephone Interview for Cognitive Status (modeled on the Mini-Mental State Examination) and with tests for delayed paragraph recall, fluency, working memory, and attention. They correlated these scores with each woman's prior estimated fine and coarse particulate matter exposure over the short term (one month), and up to 14 years prior to cognitive testing. Estimates were based on models using data from air monitors of the U.S. Environmental Protection Agency (EPA) and other local factors such as wind speed and proximity to busy roadways.

The investigators found that greater long-term exposure to both coarse and fine particles associated with faster cognitive decline. The researchers controlled for several factors, including age, education, and physical activity. Neither socioeconomic status nor the incidence of respiratory or cardiovascular conditions changed the outcome.

"It seemed that women with those highest levels of exposure experienced the worst cognitive decline over time," said Weuve. For each additional 10 μg/cubic meter exposure to fine (exposures ranged from 1.9 to 33.7 μg/cubic meter) or coarse particulate matter (0 to 69 μg/cubic meter), it looked like women performed cognitively as if they were two years older, she added. Currently, the EPA specifies the safe annual exposure to fine particulate matter to be an average of 15 μg/cubic meter per day, while the daily average for coarse particulate matter should not exceed 150 μg/m³ more than once per year.

Cognitive impairment is often a prelude to dementia. The authors note that delaying the onset of Alzheimer's disease by just two years could decrease the number of cases in the U.S. by two million over 40 years (see Brookmeyer et al., 1998). If the pollutant effect is real, environmental regulations are "a really powerful way to lower the entire population's risk of developing dementia and substantially reduce the number of cases that occur over the next few decades," Weuve said. In an e-mail to ARF, Melinda Power, Harvard School of Public Health, wrote, "Identification of modifiable risk factors is important, since we know very little about how to prevent cognitive decline or impairment, and few treatments are available."

“The EPA is currently reviewing the national ambient air quality standards for PM2.5 and PM10,” wrote Stacy Kika from the EPA Office of Media Relations in an e-mail to Alzforum. But she added, “In October 2011, after careful consideration of the scientific record and analysis by Agency scientists, EPA wrote Congress that it is prepared to propose to keep the current standard for PM10.”

Secondhand smoke may also contribute to dementia, suggest Ruoling Chen, King’s College London, U.K., and Zhi Hu, Anhui Medical University, China. In their study of 1,979 never-smoking Chinese women older than 60, they found that those exposed to secondhand smoke (predominantly from their work environment) had a greater risk of dementia than those who were not exposed (11.7 percent versus 7.4 percent, as assessed by the Geriatric Mental Status). These results are consistent with smoking being a risk factor for dementia (see ARF related news story on Rusanen et al., 2010), although a previous study found no correlation between secondhand smoke alone and dementia (see Barnes et al., 2010).

What could be the mediating factors that tie air pollution to dementia? Previous studies connected air pollution with inflammation (see Calderón-Garcidueñas et al., 2004) and cardiovascular factors (see Miller et. al., 2007), both of which have been linked to cognitive decline and dementia (see Wyss-Coray, 2006 and Breteler, 2000). Ischemic stroke could be among those cardiovascular risk factors. In their paper, Gregory Wellenius, Brown University, Providence, Rhode Island, and colleagues linked acute ischemic stroke to pollution. Several studies linked air pollution in general to stroke risk (see Villeneuve et al., 2006), but few have looked specifically at fine particulate matter and imminent risk.

Wellenius and colleagues examined the medical records of 1,705 Boston residents hospitalized for stroke at the Beth Israel Deaconess Medical Center. They estimated when strokes started and correlated those times with hourly fine particulate matter levels measured at an air monitoring station at the nearby Harvard School of Public Health. Only "good" and "moderate" days, as defined by the EPA's Air Quality Index, were analyzed. The risk of stroke was 35 percent higher during the 24-hour period following "moderate" days compared with "good" days. In addition, the number of strokes peaked 12 to 14 hours after high levels of particulate matter were registered, and correlated best with markers of traffic pollution (black carbon and nitrogen dioxide).

"Both moderate days and good days are within the current federal standards for air quality," Wellenius told ARF. "If these findings were to be replicated by other groups in other cities, the EPA should consider stricter standards, or at the very least, acknowledging that ... further reductions in air pollution are likely to have further benefits in terms of reducing adverse health effects." He and his team plan to collaborate with others to repeat the study in up to 40 other cities.

Weuve thinks the Wellenius study may explain one link between air pollution and cognitive decline. "We can imagine that these [short-term exposures to high levels of particulate matter] are just repeated small insults to the brain, even if you don't have a stroke. Over time that could contribute to cognitive decline and eventually dementia risk," she said. Mini-strokes with non-focal symptoms are a possible risk factor for AD (see ARF related news story on Bos et al., 2007), though no study has yet correlated these with air pollution. She and her team next plan to see if certain chemicals or metals in particulate matter are associated with decline.

"With the availability of improving pollutant data, future studies should aim to elucidate the health effects of sub-daily time windows of exposure," wrote Robert Brook, University of Michigan, Ann Arbor, and Sanjay Rajagopalan, Ohio State University Medical Center, Columbus, in an accompanying editorial. "If similar findings are corroborated and/or found for other diseases, those who create air quality standards might need to consider the difficult task of curbing sub-daily [particulate matter] concentrations," they added.—Gwyneth Dickey Zakaib.

References:
Weuve J, Puett RC, Schwartz J, Yanosky JD, Laden F, Grodstein F. Exposure to particulate air pollution and cognitive decline in older women. Arch Intern Med. 2012 Feb 13;172(3):219-27. Abstract

Wellenius GA, Burger MR, Coull BA, Schwartz J, Suh HH, Koutrakis P, Schlaug G, Gold DR, Mittleman MA. Ambient air pollution and the risk of acute ischemic stroke. Arch Intern Med. 2012 Feb 13;172(3):229-34. Abstract

Chen R, Zhang D, Chen Y, Hu Z, Wilson K. Passive smoking and risk of cognitive impairment in women who never smoke. Arch Intern Med. 2012 Feb 13;172(3):271-3. Abstract

Brook RD, Rajagopalan S. Can what you breathe trigger a stroke within hours? Comment on "ambient air pollution and the risk of acute ischemic stroke". Arch Intern Med. 2012 Feb 13;172(3):235-6. Abstract

Bhatia R. Policy and regulatory action can reduce harms from particulate pollution: comment on "exposure to particulate air pollution and cognitive decline in older women". Arch Intern Med. 2012 Feb 13;172(3):227-8. Abstract