From an article by the NEW Air Coalition of Fond du Lac County in the Fond du Lac Reporter:

As cold weather sweeps in, more and more Wisconsinites turn to outdoor wood boilers to heat their homes.

Wood is a renewable fuel, and the operating costs of wood burners often seem lower than natural gas or electric heat. But we often overlook the cost to air quality.

Because outdoor wood boilers burn over long periods of time and can use green or partially dried wood, they can produce 10 times the smoke of other wood-burning heat sources. Burning wood produces more fine-particle pollution than burning coal, says UW-Extension Pollution Prevention Specialist David Liebl. Inhaling fine particles, even over short periods of time, can aggravate lung conditions like asthma and bring on heart attacks or arrhythmia.

At this time, there aren’t many regulations governing residential wood burning. The amount of emissions can vary by the type of appliance, type of wood, moisture content, air damper setting and weather conditions.

Some municipalities have rules on where outdoor wood boilers can be located, restrictions on when they can be used (such as a ban during air quality alerts), or prohibit them outright.

The Wisconsin Department of Health Services has determined that neighboring residents are at risk of adverse health effects if they can see visible plumes or smell the wood smoke.

To minimize the risks, only clean, dry wood should be used as heating fuel. Because price and performance of wood as fuel can vary, residents do not always see the cost savings they hoped for.