HEPA Air Filters Show Good Results at Reducing Indoor Wood Smoke

By Diane Peng and John Ackerly*

Alliance for Green Heat
Updated: Jan. 2017

If you smell wood smoke in your house, you should go to the source and fix the problem. But if the source is a neighbor’s stove or outdoor wood boiler, it can be very difficult to solve the problem.  Recent research suggests that a high-efficiency particle air (HEPA) filter can reduce indoor wood smoke by up to 60%. Even small amounts of smoke from backpuffing when you open your stove to reload it can pose a health concern and may warrant the investment in a HEPA air filter. 

A 2016 study by the Department of Energy found in-home HEPA filters to reduce particulate matter (PM) by 16 - 50%. It also found that replacing an old wood stove with a new, certified one could reduce indoor PM by 36 - 53%.  However, studies found that in a third of the homes where stoves old stoves were replaced with new ones, there was no change in indoor PM levels.

This highly rated Whirlpool HEPA air
filter cost under $300.

Wood smoke contains two types of pollutants that that are of major concern, particulate matter and carbon monoxide (CO). Particulates from wood smoke  range in size from 0.01 to 0.1 microns and can penetrate deep in the lungs, causing respiratory ailments. Carbon monoxide is a colorless, odorless, and tasteless gas that can be lethal when concentrations grow high enough in contained spaces. Having a working carbon monoxide meter in your home is vital to your safety. Fortunately, carbon monoxide is unlikely to be a problem if the smoke is from your neighbor’s stove or boiler, because the concentration will be too low.  An air filter will help reduce particulate matter.

Many air filters claim that they are effective at reducing particulates including from wood smoke, but only HEPA filters have been proven to be effective.  The June 2012 issue of Consumer Reports rated HEPA air purifiers and found the Whirlpool AP51030K ($300) and the Hunter 30547 ($260) to the most effective and “Best Buys.”

Fireplaces are much more likely to cause
indoor wood smoke problems than wood
stoves.  Your nose is an excellent instrument
to tell if smoke is leaking into your home.

Indoor air pollution poses a real threat to human health. The air we breathe inside our homes can be filled with dust, pollen, animal dander, smoke, and many other physical and chemical pollutants, sometimes making indoor air more harmful to our health than outdoor air. Simply opening a window and providing ventilation can reduce the amount of pollutants inside our homes, but when this solution is not sufficient, many turn to indoor air filters.

Air filters direct air through a filter that removes any suspended particles. They can be portable units for individual room purification or cartridges that are installed directly into the home’s HVAC (heating, ventilation, and air conditioning) unit for home purification. The filter itself can be made out of a variety of materials: foam, cotton, fiber, and synthetic fibers. Pleated fibers work best because they provide greater surface area for which to catch the pollutants. One major limitation to air filters is its inability to filter larger particles because they settle from the air before reaching the filter. It is also important to note that these types of filters remove particle pollution only; gaseous pollutants such as carbon monoxide will not be removed from the air.

Filter efficiency is measured by the minimum efficiency reporting value (MERV). MERV values range from 1 – 20 with higher scores corresponding to more efficient removal of particles. High-Efficiency Particulate Air (HEPA) filters have MERV values ranging from 17 – 20 and have met the U.S. Department of Energy standard of removing 99.97% of particles greater than 0.03 microns from the air that passes through. HEPA filters are not traditionally installed in a home’s HVAC unit but are found in separate filtration devices.

Both old and new wood stoves can leak
smoke into the home.  Its often the installation
and the chimney, not the stove, responsible
for leakage. A CSIA certified chimney sweep
is an excellent resource to find and help
remedy indoor smoke issues.

Wood particles are between .01 and .1 micrometer.  One source gave the average at 0.07. According to the EPA, HEPA filters can filter particles as small as 0.03 micrometers in size.

Researchers from the University of Montana and other experts published a paper, “Evaluating the Effectiveness of a Commercial Portable Air Purifier in Homes with Wood Burning Stoves” in the Journal of Environmental and Public Health in 2011 and found that a portable HEPA air filter is an effective option for reducing particle concentrations in homes that use wood burning stoves as their primary or secondary heating source. Particle concentrations of the homes in the study decreased 61-85%.. Another research group also found that installing a HEPA filter significantly reduced particulate pollution. In addition this group found that people living in these homes saw a 32% average decrease in their levels of C-reactive protein, a marker of inflammation and cardiovascular disease.

One thing to keep in mind is that it is always better to prevent the problem rather than try to fix it. The best solution to purifying indoor air is to remove the source of contamination. This can be hard to do if the source of wood smoke inside your house is a neighbor’s outdoor wood boiler. Smoke from a neighbor’s outdoor wood boiler could very well end up inside your own home through your ventilation system.

Bottom line: If you have any level of detectable wood smoke in your house, go to the source and fix the problem.  If you can’t fix it, it is probably time to upgrade your stove and have it professionally installed, particularly if you have a stove made prior to 1988 that is not certified by the EPA.  If you can’t replace or fix the leaky stove, an air filter may be an inexpensive way to mitigate the particulate matter problem. Even with an air filter, however, carbon monoxide may still remain a serious pollutant in the home.   Given the potential negative health impacts of any prolonged exposure to wood smoke and other indoor air pollutants, there is a growing body of scientific evidence that HEPA filters may significantly improve air quality in homes effected by wood smoke and that these reductions may have positive health benefits.

Sources:

http://www.epa.gov/iaq/pubs/airclean.html

http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/air-purifiers/room-air-purifier-ratings/ratings-overview.htm

http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/air-purifiers/home-air-purifier-ratings/ratings-overview.htm

An air filter intervention study of endothelial function among healthy adults in a woodsmoke-impacted community,  Am J Respir Crit Care Med. 2011 May 1;183(9).

http://www.ncbi.nlm.nih.gov/pubmed/21257787

Hart JF, Ward TJ, Spear TM, Rossi RJ, Holland NN, Loushin BG. Evaluating the Effectiveness of a Commercial Portable Air Purifier in Homes with Wood Burning Stoves: A Preliminary Study. Journal of Environmental and Public Health. 2011 Jan: 2011: 324809

Ogulei D, Hopke PK, Wallace LA. Analysis of Indoor Particle Size Distributions in an Occupied Townhouse Using Positive Matrix Factorization. Indoor Air. 2006 Jun;16(3):204-15.

INDOOR AIR QUALITY: Wood-Burning Stoves Get Help from HEPA Filters

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3080954/

* Diane Peng was a Research Fellow at the Alliance for Green Heat and is now a medical student.  John Ackerly is the President of the Alliance.  Thanks also to edits from Derek Rogalsky, a medical student at Georgetown Medical School and the lead author of Estimating the Number of Low-Income Americans Exposed to Household Air Pollution from Burning Solid Fuels.