The seemingly endless parade of cool recycled pallet projects makes these near-ubiquitous items look like an irresistible resource -- but beware, not all pallets are created equal. The safety factor of your pallet depends on which country it is from and which procedures have been used on them to make them suitable for international shipping.
Canadian pallets are the safestApparently, pallets from Canada are the safest, since most of them are only pressure- and heat-treated (marked with "HT"), as opposed to being fumigated with the neurotoxin and carcinogen methyl bromide (marked with "MB"), to kill off invasive species like pine beetles.
Methyl bromide has the potential to 'gas off' as elemental bromine, after which it acts as a serious ozone depleter. I'm not sure if methyl bormide or its products enter food, compost or soil but exposure to it is dangerous and the effects are cumulative. During the Montreal Protocol talks, in which substances causing ozone depletion were banned, methyl bromide got an exemption when the pallet undustry argued that it was necessary to their trade and to prevent the spread of harmful species. All except Canada, who still doesn't treat their pallets with anything except pressure and high temperatures.
Repallet gives further details on Canadian pallets:
The best way to find a safe pallet for your home project is to look for this stamp on your pallet. This is the accredited heat treatment stamp for regulated wood packaging in Canada regulated by the CWPCA (Canadian Wood Pallet and Container Organization). The CWPCA represent over 85% of the manufactured pallets and wood packaging in Canada.
Decoding the stampInstructables author minnecrapolis notes that newer American pallets also may be suitable to re-purpose:
More companies are starting to build one-time-use pallets or using heat treatment rather than Methyl Bromide fumigation.
Pallets now require an IPPC logo which certifies that the pallet was heat-treated or fumigated with Methyl Bromide.
The standard is a 2 letter country code (xx), a unique number (000) assigned by the National Plant Protection Organization (NPPO), HT for Heat Treatment or MB for Methyl Bromide, and DB to signify debarked.
The logo in the first image shows that it was produced in the U.S., the material was provided by 11187 (Unique number assigned to the producer), it was heat treated (HT) and was verified by PRL (Package Research Laboratory).
Another good tip: don't use any pallet that seems to have had something spilled on it, and don't burn treated lumber in a fireplace.