There are the obvious reasons to avoid a pallet; odors, excessive weight, stains where something has been spilled or too many twisted or rusted nails. Beyond that, there are some major safety issues that may require closer examination.
The first is the process used to treat the wood to fumigate it from insects. The wood will be marked either HT or MB. Heat treating (HT), is a process by which the wood is heated for 30 minutes to a minimum core temperature of 132.8 °F /56° C. Wood marked MB has been chemically treated using and methyl bromide. The use of methyl bromide was banned in 2005 by The International Plant Protection Convention (IPPC), which is a division of the Food and Drug Administration. However, there is still pallet wood out there that has been treated in this manner
If inhaled methyl bromide may case abdominal pain, dizziness, vomiting or loss of speech or coordination. If exposed to the skin, redness and blistering may occur. Always check for the markings determining the process used and DO NOT use wood marked MB.
Bacteria is another issue that must be dealt with. Always scrub each piece of wood with warm soapy water and bleach, rise, repeat and then allow to dry for at least 24 hours.
Some bacteria can become embedded in pallet wood. As a precautionary measure, never use the wood for anything food related like a picnic table or kitchen island. Raised herb and vegetable gardens are growing in popularity, but again be careful using pallet wood. Soil can soak up chemicals leaching from the wood causing a possible health hazard to your family. Never use pallet wood to build forts, playground equipment, furniture, sandboxes and other items for children as even projects that have been painted can still be dangerous. It is common for people to use whatever scrap wood is available for fire pits and bonfires. Under no circumstances should you burn pallet wood in your outdoor fires or in your fireplace.