Is it safe to burn heat treated wood?
In respect to this, is it safe to burn pressure treated wood?
Treated wood should not be burned in stoves, fireplaces or outdoors because toxic chemicals are produced as part of the smoke and ash and can be harmful if inhaled. It is legal to dispose of treated wood in the landfill, although it's always best to find a way to re-use it.
Consequently, what wood is toxic burning?
Sumac is a known toxic burning wood of North America. You are not to use it with the preparation of food. As noted in a comment, all wood can be considered to release toxins in the smoke when burned. You may be exposed to more toxic smoke if you burn hotter fires.
Generally, pallets are safe to burn in fireplaces, although those that are treated with the fumigant methyl bromide (labeled with the initials MB) are unsafe to burn. Also, pallets may have been exposed to a variety of chemicals while they were in use.