Preliminary results from a Montana air quality study suggest that replacing older wood-burning stoves and fireplaces with cleaner-burning, EPA-certified models can substantially reduce airborne fine particulates. A similar study is underway on the Nez Perce Indian Reservation near Lewiston, Idaho, and results are expected by spring 2008, says EPA's Amanda Aldridge,  919-541-5268.
The only other comparable US study that compared air quality before and after replacement of a high percentage of older woodstoves occurred in 1988-89 in Crested Butte, CO.  Switching to cleaner-burning stoves available at that time contributed to a 40% reduction in larger particulates (known as PM10).
Scientists are consistently finding that fine particulates (known as PM2.5) are harmful even at extremely low concentrations, and many US cities and counties are struggling to reduce their fine particulates.
In Libby, MT, the first year of air monitoring shows that a coordinated, heavily-subsidized program to replace approximately 1,000 older wood-burning appliances has resulted in a reduction in fine particulates from an average of about 25-30 micrograms per cubic meter in each winter month to about 19-21 micrograms per cubic meter. Peak concentrations have also tended to drop substantially. The data are limited, represent a transitional period in the replacement process, and won't be official until spring 2008. But researchers say the weather in the winter of 2006-2007 was similar enough to that of the previous half-dozen years that the reduction likely is due to the changeout program, according to a March 15, 2007, presentation  by Montana Dept. of Environmental Quality meteorologist John Coefield,  406-444-5272.
The magnitude of the Libby results won't necessarily be replicated in other communities, because almost 100% of the older, uncertified stoves were replaced (aided by more than $2 million in subsidies from various governments and the Hearth, Patio & Barbecue Assn.), the replaced stoves made up about 50% of all wood-burning appliances in the area, the area is heavily dependent on wood heat, and local stagnant air conditions exacerbate air pollution. For more information on this effort, see a July 19, 2007, Region 8 EPA press release. 
Sizable changeout efforts are underway in other states, including CA, MI, NV, OH, OR, VT, WA, and WI, as well as British Columbia. For more information on changeout programs, certified woodstoves, and more, see EPA's Clean Burning Wood Stoves and Fireplaces  and 2007 Residential Wood Smoke Workshop  in Reno, NV, or contact EPA's Amanda Aldridge for details on the changeout programs not highlighted on these Web sites.