To know if your pallet is toxic or no, check for a stamp or marking on the sides of the pallet.

There are two main things to look for on the stamp. Avoid chemically-treated pallets to keep you and yours Safe.

The IPPC stamp: if you don’t see it, use it with caution! A pallet may be perfectly safe without this logo, but you don’t have clear information.
The treatment code : [HT] = Heat treatment / [MB] = Methyl Bromide / [DB] = Debarked / [KD] = Kiln Dried.
The IPPC marking on pallets is for International Plant Protection Convention (IPPC). IPPC marking on pallets are used for international shipping. These pallets are required to be made of a material that will not carry invasive insect species or plant diseases through different countries. IPPC standards require pallets constructed using raw wood to be treated. These pallets are treated with one of the methods listed below. Officially approved agencies supervise pallet treatment methods. A pallet may or may not be safe without the IPPC stamp. Stay safe; use pallets with stamps identifying clear sources!

Choose the Safe treatment method when upcycling pallets

[HT]: Wooden pallets manufactured in Canada or the US undergo a pest control treatment called heat treating (HT) which involves heating the pallet to a minimum core temperature of 56°C for softwoods and 60°C for hardwoods for a minimum of 30 minutes in a kiln. HT pallets are not harmful to your health.

[KD]: The purpose of kiln-dried lumber is to reduce the moisture content of the wood (19% or less). This coding system is a means to control warping, fungal growth, and other quality features. The kilns or ovens used to dry the wood do not necessarily reach the sustained temperature of 133 degrees Fahrenheit (56 Celsius) that would qualify as heat treated. Many lumber mills are processing their timber to meet the heat treatment requirements; thus you will see “KD-HT” incorporated in the wood grade stamp.

[MB]: Methyl bromide fumigation, this is a potent pesticide linked to human health problems and ozone layer depletion. Methyl bromide fumigation kills invasive species like pine beetles.

In 1987, the Montreal Protocol aimed to regulate the use of methyl bromide, along with multiple other chemicals found to impact the ozone. This type of treatment is now banned in Canada and many countries because it poses health risks to workers handling the pallets. However, you can still find it in some places. If you find an MB pallet (likely from Asia or Oceania), please do not use it for your craft projects or as firewood, find a waste-removal company that can dispose of it properly.

In conclusion, never use [MB] Methyl bromide pallets. More information are available here: Is my pallet safe?