Chromated copper arsenate is an interesting one. It is a mixture of the copper salts of chromic acid and arsenic acid.
CCA provides robust protection against the sun, fungus, and bacteria. Unfortunately, chromium (VI) and arsenic are quite toxic, and can leach from the wood.
One problem with wood is that it's essentially tree meat - once it's separated from its host, loads of protection against microbial action and sunlight are absent. CCA provides this, and quite well. You've seen lots of CCA wood - most lumber with a greenish tint is CCA-treated. During wood's regular life, CCA doesn't leach out that much (but some still does, which worries some people). The real problems start when you compost or burn it; then it's in the air and soil.
CCA wood is still very, very common (at least in the States) due to its efficacy - in a country where so many homes and buildings are built with light-frame construction, we need lumber that is robust to decay for at least decades. However, the tide is starting to change. Sodium silicate ("water glass") is an alternative that is making some inroads.
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Actually, the chromium in the initial formulation is in the Cr(VI) oxidation state; when it is used to treat wood, it gets reduced in situ to Cr(III), which is much less toxic. Cr(IV) isn't a common oxidation state of chromium.
Oops, IV was a typo, I meant VI, as you can see by the image. I had no idea it was reduced in situ, though, but the green color should have tipped me off to spent Cr(VI). Thanks for the correction.
No problem. There's a nice paper on the chemical state of CCA in wood here.
http://www-ssrl.slac.stanford.edu/research/highlights_archive/cca.pdf
You've got a great blog, BTW. Of course, as a chemist, I'm biased :-)
I recall case reports of families being poisoned by burning treated lumber in a wood stove, trying to heat their homes. I'd have though it would be obvious that one should not do that, but then, not everyone takes chemistry classes.