I found this on this website:
http://www.cat.org.uk/information/catinfo.tmpl ?command=search&db=catinfo.db&eqSKUdatarq=Info Sheet_Glazing
Timber, uPVC or aluminium frames?
If you are replacing your windows, then wood is definitely the option to go for. There are lots of environmental problems associated with the manufacture of PVC. High levels of dioxins and other organochlorines are found around PVC plants and are released to the atmosphere on combustion. They've been linked with cancer, immune system damage and hormone disruption. Although manufacturing techniques are improving, disposal at end-of-life is still a problem.
PVC windows will use large amounts of energy in manufacture, whereas wood windows will not. Wooden windows can last for more than 50 years, and beyond that they can be renovated rather then replaced. PVC windows will usually need replacing after a much shorter period. And although PVC window manufacturers claim their products are maintenance free, Dulux recently brought out a PVC window paint, recommended for use every six years!
Aluminium windows will have involved a lot of energy use and pollution during manufacture. The coatings used on some frames make their recycling difficult - which is not ideal for such a high-value material, so look for anodized frames if you do choose aluminium.
FSC-certified temperate (rather than tropical) hardwood or a durable softwood should be the first choice. This will last longer than a cheaper less durable softwood window, and the preservatives needed to protect the latter will cause some pollution when the wood eventually needs to be disposed of. Softwood, or sustainable (e.g. FSC) timber would still be preferable to uPVC.