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Old wooden pallets tend to be very rough and splintery. It's important to use heavy work gloves when handling them. You may like to sand them down before using them. If you do this, use a face mask and eye protection. These are a bit harder to find. This means the wood was heat treated, not chemically treated, in order to preserve the wood when it was made. It's important to use a new, unused HT pallet if growing edible plants. This is important, as wood can absorb chemicals after manufacture, for example if the pallet came into contact with chemicals such as bitumen or petrol when in use. That's why you want to use a new pallet for growing edible plants. When the wood gets wet it's also a good breeding ground for bacteria such as salmonella, E. coli and Listeria. For any kind of use, it may be best to avoid old pallets that you suspect may be contaminated with chemicals. Avoid bringing old pallets into the house. You can mitigate the bacterial risk by scrubbing your pallet clean with soapy water and weakened bleach, or soaking in a disinfectant solution. Over time the bacteria may build up again though. Beware of protruding nails and the wood shattering into sharp pieces when you work with it. If you notice any obvious signs of damage, you should work with a different pallet instead.
Protect yourself from splinters. Look for pallets stamped with 'HT". Avoid using older pallets, as they may be contaminated. Check pallets carefully for signs of damage to the wood.