'Tanalised', CCA pressure treated timber contained Arsenic, not recommended for burning on any sort of fire.
'Tanalith', the latest supposedly more eco friendly treatment contains some nasty toxins too.
CCA treatment has now been banned but unless you know which treatment has been used and its impossible to tell by sight, be warned.
However there must be thousands of CCA treated pallets still in circulation which eventually get chipped and you guessed it, burnt.
Food for thought!