What's The Deal With Outdoor Wood Boilers?
Outdoor Wood Boilers (OWBs) are outdoor heat-transfer based heating systems that generate heat through the combustion of wood. Used incorrectly, OWBs become sources of toxic emissions that endanger the health of anyone living remotely near them.
Though all OWBs produce particulate matter (PM) pollution, the amount of pollution produced by an OWB varies from model to model and increases significantly when the wrong material is burned. Average OWBs have been shown to produce as much PM pollution per hour as 22 EPA certified wood stoves, 205 oil furnaces or 8,000 natural gas furnaces (NESCAUM, 2006). The combustion of unapproved materials such as treated wood, plastic or household waste reduces the efficiency of the OWB and increases the release of a number of different pollutants into the air. Though the burning of unauthorized materials in OWBs exacerbates the pollution problem, even burning approved materials can produce enough pollution to adversely affect human health. The smoke produced by an OWB isn’t just colored air; it contains a number of different harmful pollutants like PM, carbon monoxide (CO), poly aromatic hydrocarbons (PAHs), oxides of nitrogen (NOx), sulfur dioxide (SO2), carbon dioxide (CO2) and many more.
The health and environmental impacts of the types of pollutants produced by OWBs, especially PM, have been extensively studied. PM is a complex mixture of particles, including dust, dirt, soot, smoke and droplets of liquid that are small enough to be inhaled. There are two different types of criteria PM: PM2.5, which includes particulate matter that is 2.5 micrometers and smaller, and PM10, which includes particles that are between 2.5 and 10 micrometers in size. PM2.5 particles are small enough that they can be inhaled deep into the lungs and cause numerous health problems, including increased asthma attacks, increased respiratory symptoms such as wheezing, coughing and difficult or painful breathing, chronic bronchitis, decreased lung function and even premature death.
Of course, there are regulations in place to prevent exposure to these pollutants produced by OWBs, but numerous studies have also shown that current regulations may not be enough to protect members of communities that live near OWBs. As it is inherently an airborne pollutant, smoke containing harmful substances can drift many hundreds of feet downwind from its source. Even when OWBs are in compliance with regulations, fine particulates can be found in neighboring homes in concentrations that exceed the USEPA 24-h PM2.5 limit of 35 micrograms per cubic meter.
Part of the reason that the level of indoor particulates can be found in such high concentrations so far from their source is due to the fact that the stack heights on OWBs that allow smoke to escape the unit aren’t high enough to ensure that the smoke is dispersed adequately. As such, the smoke produced by OWBs is dispersed closer to ground level, meaning that it contributes to ambient air conditions.
Though the pollution produced by an OWB is dangerous, the cost of utility-produced energy is usually more expensive than the energy produced by an OWB. For many who use OWBs, there doesn’t seem to be any reasonable alternative to utilizing them to heat their homes in winter. Unfortunately, many of the viable alternatives are expensive and have high start-up costs that make them unrealistic options for many homeowners.
Studies of Interest
Brown, D., Alderman, N., Weinberger, B., Lewis, C., Bradley, J., Curtis, L. (2014). Outdoor wood furnaces create significant indoor particulate pollution in neighboring homes. Inhalation Toxicology, 26(10), 628-635.
Kinsey, J., Touati, A., Yelverton, T., Aurell, J., Cho, S., Linak, W., Gullett, B. (2012). Emissions characterization of residential wood-fired hydronic heater technologies. Atmospheric Environment, 63, 239-249.