A lot of people offer scrap wood for burning. It is not illegal to offer this, but some caution should be used by the person receiving the wood.
A good explanation and article about pallets and reuse is https://www.1001pallets.com/pallet-safety/
A particular issue that does cause concern is that some treated timber contains CCA (chromated copper arsenate) and should not be burnt as firewood. CCA is a wood preservative that has been used for timber treatment since the mid 1930's, but stopped in 2006. As the name suggests it is a mixture of chromium, copper and arsenic, it imparts a greenish tint to the timber. It is highly effective as wood preservative, often used as a substitute for creosote. It protects against fungal decay, wood eating insects and affords a good degree of weather-resistance. But when burnt, toxic arsenic is released into the atmosphere and the ash from burnt CCA treated timber can contain up to 10 per cent (by weight) arsenic, chromium and copper. Swallowing only a few grams of this ash can be harmful. Symptoms can include nausea and/or vomiting, diarrhoea, and pins and needles feeling in the skin. Common advice is to never burn CCA treated timber as firewood in fireplaces, barbecues, wood stoves or any wood fire.
Any wood designed for indoors use, or which was first used outdoors before 1933 or after 2006 is likely to be safe to burn. CCA was latterly primarily used in fences, sheds, and decks. It was banned initially voluntary, and restricted it's use to external use, but later became encoded in law, and it was no longer available at all.
For safety, it is probably best to take a stance that any pressure treated wood is likely to have chemicals that, once burnt, are harmful to health and the environment.
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