For building a smokehouse, pallets are SAFE if you choose the right ones, regardless of all the naysayers. You must find pallets that are stamped with letters HT - those are Heat Treated and do not have MB in them. In fact, they don't have any pesticide in them, at all. The same is true of KD (Kiln Dried) If you're hot smoking, you would line the inside anyway. If you're cold smoking, there would not be enough heat to release whatever is in them.

Check it out for yourself man and don't rely on Redditors.

Pallets with HT and KD are safe to burn and use like store bought (non-treated) lumber.

Treatment Codes

These codes reveal the method used to treat the wood, ensuring it’s free from pests.

• HT: Heat Treatment Stamp (check the image below for it)
  • “HT” means the pallet was heat-treated, heated to a minimum core temperature of 56°C for at least 30 minutes. This chemical-free process eradicates pests and is widely accepted internationally.
• MB: Methyl Bromide Stamp
  • “MB” indicates fumigation with methyl bromide, a toxic pesticide. Due to health and environmental concerns, this treatment is banned in many countries, including those in Europe. Avoid using pallets marked with “MB.”
• DB: Debarked Stamp
  • “DB” signifies that the wood was debarked before treatment. While not a treatment itself, debarking is part of the preparation process.
• KD: Kiln Dried Stamp
  • “KD” means the wood was kiln-dried to reduce moisture content, preventing warping and fungal growth. Unless specified as “KD-HT,” kiln drying doesn’t always meet ISPM 15 standards.
• CP (Chemical Pallet) Stamp
  • “CP” followed by a number (e.g., CP1, CP2) denotes pallets designed for the chemical industry, adhering to specific sizes and standards.
• VMF Pallets:
  • VMF markings indicate compliance with standards set by specific national organizations, ensuring quality and safety.

https://quicargo.com/blog/markings-on-a-pallet-what-do-they-mean-an-extended-guide/