The science of air pollution is very complex and technical. Below is a "Technical
Primer" giving the basics to help to intrepret the research and technical
documents. See the Links
below for documents on the science and technology of OWBs.
Technical
Primer
Particulate Matter
(PM): PM is simply particles that get
into the air and can get into our lungs when we breathe. Particles can be
composed of sand, dust, flyash or organic chemicals that have condensed together.
PM can be larger particles or very fine microscopic particles. The finest
PM is called PM2.5 or fine particulate that is 2.5 microns and smaller. These
fine particles are most able to penetrate deeply into lungs causing various
health problems. The visible smoke from wood burning is mostly PM, the vast
majority being PM2.5.
Air Toxics: All the air we
breathe contains some toxins in the form of various chemicals including volatiles
(like solvents), heavy metals and particulate in various forms. All wood
smoke contains toxins, the type and amounts depend on how completely and
efficiently the wood is burned. In general, the hotter the fire, the less
toxins are produced assuming an adequate oxygen supply.
Efficiency: A much
used term with many interpretations. In the wood heating world, thermal
efficiency is probably the most important because it is a measure of the
how much of the potential heat in the fuel is transferred into useable heat.
But even this term is more complicated than this simplistic definition because
it can be calculated in different ways. The term "combustion efficiency"
that you may see in ads for some wood combustion devices is vague and probably
meaningless. It may mean that some stated percentage of wood is burned but
it has no relationship to how much useable heat is produced. Be wary of all
efficiency claims.
Emissions
Standards: Emissions standards are limits
put on the amount of pollution emitted by an air pollution source. Standards
can be expressed in different ways and are rarely understood by those outside
the air pollution field.
-
Mass per unit time: Generally expressed in units such as grams per
hour or pounds per hour of some pollutant such as PM. US EPA standards for
indoor woodstoves are expressed in grams per hour.
-
Mass per energy input: This is the weight or mass of the pollutant
per unit of energy in the fuel expressed as grams or pounds per BTU, mmBTU
(million BTU), Joules or Megajoules. This type of standard does not consider
thermal efficiency.
-
Mass per energy output: This type of standard considers the efficiency
of the device and is expressed in units of mass per unit of useable energy
produced by the device.
-
Concentration Standards: The most difficult to understand and often
expressed as mass (lbs, grams or kilograms) per unit volume (cubic feet or
cubic meters) or mass of exhaust gases. Chemical or toxic emission standards
may be expressed in parts per million (ppm). These standards often require
correction to a percentage oxygen or carbon dioxide in the exhaust gases.
Testing: "Stack testing"
is done by drawing a sample of the exhaust gases out of the stack to determine
the amount of contamination in the gases. The details of testing are extremely
complicated and technical and there is a diversity of formal test methods
for testing the various pollutants from a variety of devices. An explanation
of stack testing is beyond the scope of this website but how air pollution
testing is done and how the results are expressed are critical factors when
trying to evaluate wood burning devices.
Ambient Monitoring: Ambient
monitoring is testing the general outdoor air to determine the levels of
air contamination. This type of testing, although equally technical, is done
differently from stack testing. Ambient concentrations of pollutants are
generally far less concentrated than the pollutants measured in the exhaust
gases of stacks. Ambient monitoring may be done in an area away from specific
sources of air pollution just to determine general air quality or it may
be done near an air pollution source to try to determine the impacts of a
specific source's emissions on ambient air quality. See Vermont's ambient
monitoring website at http://www.anr.state.vt.us/air/Monitoring/index.htm
for more information.
Air Pollution Modeling:
Computer models can be used to estimate the impacts of
pollutants emitted from sources of air pollution. Such information can be
used to estimate whether or not pollutants from sources will cause pollutant
levels that are a health concern or that exceed certain set standards. Detailed
protocols have been established for the use of the various types of air pollution
computer models. Predictions by such models are useful but are dependent
on the data input to the model, such as stack height, emission rate and stack
gas temperature. Weather conditions greatly influence the dispersion of
pollutants and weather data can be input to many of the model programs to
predict pollutant impacts under different weather conditions.
