Position: The American Lung Association of Maine strongly cautions
against the use of outdoor wood boilers for residential heating purposes.
The design and operation of most outdoor wood boilers (OWBs) have been found
to create air quality impacts far greater than what is currently acceptable
from indoor wood burning devices. (1,2) Until such time as these boilers
can be designed to meet acceptable public health criteria , we do not support
their use and view them as a potentially important lung health hazard. We
further advocate for the development of risk assessment methods to better
characterize the health risks associated with these devices and the degree
of hazard posed by these units relative to other kinds of wood combustion.
Rationale: All combustion devices release pollutants that are harmful
to lung health. For certain classes of these pollutants, such as fine particles
and toxic air contaminants, there is no safe level of exposure. Consequently,
these devices should be operated to ensure maximum combustion efficiency
by having an optimal temperature, air supply, and residence time in the
combustion chamber. Furthermore, the stack heights should be high enough
to permit adequate dispersion of the pollutants that are released. In addition,
the exit plumes should be at hot enough temperatures to assure buoyancy and
mixing with clean air.
The outdoor wood boiler design is inconsistent with the designs used to reduce
emissions from wood burning devices to acceptable levels for public health.
According to the Michigan Department of Environmental Quality (3):
The firebox is surrounded by a large water jacket. This is good for heating
water, but it cools the escaping gases before combustion is complete.
There are long periods of time when the wood just smolders. During these
periods of low air flow, creosote collects on the water jacket walls. When
the fire is rekindled, the creosote burns off and creates black soot.
The stacks of these units are very short. Smoke and soot are released close
to the ground.
In addition, OWBs can and are used to burn all year round, thus not limiting
the health concern to just being seasonal problem. The large firebox capacity
of many OWBs also turns them into functional incinerators, enabling owners
to burn railroad ties, trash, commercial waste, animal carcasses, and other
dirty fuels unsuitable for residential combustion.
Unlike wood stoves used inside, there are no federal regulations to limit
the amount of pollution outdoor wood boilers generate. They emit over ten
times the amount of air pollution than an EPA certified wood burning stove.
The State of Washington regulations limit the amount of emissions from outdoor
wood boilers to less than four and one-half grams per hour (4). Based on
current emissions data, this restriction would limit the amount of fine
particulate air pollution coming from outdoor wood boilers by almost 95%.
This ALA-ME advisory will remain in effect, therefore, until outdoor wood
boilers can be designed to limit the health and air quality impacts to at
least a level commensurate with the State of Washington standards.
1. Smoke Gets in Your Lungs: Outdoor Wood Boilers in New York State, Attorney
General of New York State, Environmental Protection Bureau, August, 2005
2. Philip Johnson, "In Field Ambient Fine Particle Monitoring of an Outdoor
Wood Boiler: Public Health Concerns," Journal of Human and Ecological Risk
Assessment, in press.
3. Michigan Department of Environmental Quality, Outdoor Wood Boiler and
Air Quality Fact Sheet, DRAFT, November, 2004.
4. State of Washington, Solid Fuel Burning Devices, Chapter 173-433, Washington
Administrative Code, 2/3/93.