Do not use any wood from conifer trees: pine, fir, spruce, redwood, cedar, cypress, or they will make you sick! Elm, eucalyptus, sycamore, sweet gum trees and liquid amber wood is also unsuitable for smoking.
More woods that you should not to use for smoking: Never use lumber scraps, either new or used. First, you cannot know for sure what kind of wood it is; second, the wood may have been chemically treated; third, you have no idea where the wood may have been or how it was used.
Never use any wood that has been painted or stained. Do not use wood scraps from a furniture manufacturer as this wood is often chemically treated.
Never use wood from old pallets. Many pallets are treated with chemicals that can be hazardous to your health and the pallet may have been used to carry chemicals or poison. Avoid old wood that is covered with mold and fungus that can impart a bad taste to your meat.