Wood burning is one of the oldest ways that humans have produced heat and light and to cook food. But wood burning produces smoke, and smoke produced by burning wood that contains harmful substances such as molds can sometimes be imparted with those same harmful substances, putting anyone around the fire at potential risk. But you can avoid these risks by considering some of the scientific background of why burning moldy wood can be harmful in the first instance.
Molds grow on any surface that provides ideal mold growth conditions, the most important of which is moisture. Therefore moist wood infected with a fungal pathogen can quickly develop mold colonies. Not only can these molds reduce the aesthetic, structural integrity and overall value of the wood, but burning moldy wood can present certain health risks to anyone within close range of the fire and anyone downwind from the smoke produced by the fire.
When you burn moldy wood, microscopic mold spores are released from the wood into the air. These spores can easily create symptoms such as coughing; eye, throat and nose irritation; and sneezing. Those with chronic respiratory conditions such as asthma are believed to be more susceptible to these symptoms. Burning wood also increases the number of airborne mold spores inside your home, which in turn increases the likelihood of problematic and potentially unhealthy indoor mold growth.
Never burn moldy wood. This is sometimes easier said than done, because mold growth tends to be more visible on the inside of wood than the outside. Therefore you should never take firewood from a tree that is diseased, rotting, or visibly moldy or mildewy. Look for uncharacteristic colors under tree bark and mushrooms growing on tree bark as signs of a fungal infection. Only keep as much firewood inside the home as you need for that day to prevent wood in your house from becoming moldy.
There has been much attention devoted of late to toxic black molds -- Stachybotrys chartarum -- that may cause very serious health problems for humans after prolonged exposure. These molds are rare, but it is still possible that a Stachybotrys species can infect your firewood. Toxic molds are nearly impossible to distinguish from non-toxic species by visual inspection alone, so it is best to err on the side of caution by never burning any wood with any mold on it. Even non-toxic molds can cause problems when burned, so it is simply not worth the risk to burn a mold that you believe is non-toxic.