In a small town with air heavily affected by residential wood-burning, the use of high-efficiency air (HEPA) filters was associated with improved endothelial function in healthy adults, Canadian researchers found.

The reactive hyperemia index -- used as an indicator of microvascular endothelial function -- improved by an average of 9.4% (95% CI 0.9% to 18%) when the filters were turned on compared with when they were turned off, according to Ryan Allen, PhD, of Simon Fraser University in Burnaby, British Columbia, and colleagues.

At the same time, there was an average drop of 32.6% (95% CI 4.4% to 60.9%) in C-reactive protein concentrations, the researchers reported online in the American Journal of Respiratory and Critical Care Medicine.

"Our results support the hypothesis that systemic inflammation and impaired endothelial function, both predictors of cardiovascular morbidity, can be favorably influenced by a reduction of indoor particle concentrations," they wrote.

In an interview with MedPage Today, Allen said air cleaners might have a role to play in areas of existing air pollution.

"Although the evidence base isn't totally consistent, I do think that there's enough evidence that we can be recommending air cleaners as a possible intervention, particularly for susceptible groups," including pregnant women and individuals with asthma and cardiovascular disease, he said.

Allen and his colleagues performed a randomized crossover study of 45 healthy adults from 25 nonsmoking homes in Smithers, British Columbia. The researchers chose the town because it serves as a pilot setting to assess the feasibility of a government program to exchange old wood-burning stoves for newer, cleaner stoves.

The town's air is heavily affected by residential wood combustion, and about half of the study participants reported using a wood-burning stove in their home.

Each household received two HEPA filters, one for the main activity room and one for the bedroom. The participants were randomized to consecutive seven-day periods in which the filters were either active or not active.

When the filters were active, they reduced particulate matter concentrations by nearly 60% and levels of levoglucosan -- a marker of woodsmoke particulate matter -- by 75% indoors. The filters reduced particulate matter concentrations in all but one of the study homes.

About two-thirds of the indoor concentration of particulate matter was generated indoors, with the rest leaking in from outside.

Although particulate matter concentrations were not associated with changes in the reactive hyperemia index or C-reactive protein levels, filtration was associated with improvements on both endpoints.

Lower concentrations of particulate matter and levoglucosan were, however, associated with reduced band cell counts, a marker of inflammation.

There was no relationship between either filtration or particulate matter and markers of oxidative stress.

Allen described the effects as subtle, but "even though we're looking at fairly subtle effects, they're subtle effects that have been linked with more serious effects over the long term."

He noted that the findings of the study are likely generalizable to more urban environments, citing a similar study that explored the effects of HEPA filters on traffic-related air pollution exposure in Copenhagen. In that study, filtration was associated with an improvement in reactive hyperemia index that was of a similar magnitude as in the current study.

Although the mechanisms by which particulate matter may affect endothelial function are not fully understood, Allen said, there are two main hypotheses.

The first involves very small particles traveling through the lung and into the circulation, where they interact with cells in the blood vessels, he said.

The second, which he said had more supporting evidence, is the induction of an inflammatory response that spills over into the blood vessels when the particles make contact with airway epithelial cells.

The researchers noted in their paper that the reactive hyperemia index has not been used extensively for either research or clinical purposes, although it has been associated with traditional cardiovascular risk factors.

Also, they acknowledged that the measure does not distinguish between endothelium-dependent and endothelium-independent effects.

However, they added, "inhibition of endothelial nitric oxide synthase attenuates the reactive hyperemia index response, suggesting that this measure is indicative of endothelial function."

The study was funded by the British Columbia Lung Association, the British Columbia Ministry of Environment, and Health Canada.

Primary source: American Journal of Respiratory and Critical Care Medicine
Source reference:
Allen R, et al "An air filter intervention study of endothelial function among healthy adults in a woodsmoke-impacted community" Am J Respir Crit Care Med 2011; DOI: 10.1164/rccm.201010-1572OC.