By not using the proper, safe and simple dried softwoods in our fires we could be putting our health and that of our families at risk. When what we burn in the fireplace or wood stove isn't appropriate, it could vent harmful fumes either inside or outside of our home. These fumes could cause immediate damage or long-term either to us or the environment outside of the home. Be careful not to burn pressure-treated wood which, according to the US government, is illegal.
Some other types of wood to definitely stay away from while making fires are plywood, painted wood, or driftwood. Usually wood from pallets, and from construction sites that don't fall under the aforementioned categories, and fallen tree limbs are good sources to take the wood from. Other inappropriate fuel sources would be trash, dryer lint, and allergenic plants.
These are things you may already know, but it would be good to double check before burning different types of wood. We can be all too eager to stoke the fire, but it is always good to be cautious.