Plastic or wood.

Quoted from Friends of the Earth - Good wood guide.

"PVC-u is a common alternative to timber for windows and has been heavily marketed as long lasting and low maintenance. However, its production and disposal involve the release of highly poisonous chemicals that are a threat to human health and the environment. Dioxins, persistent chemicals that are known carcinogens and hormone disrupters,  accumulate in the food chain and are an unavoidable by-product of PVC production. Lead, cadmium and organotins are used as stabilizers in PVC. When PVC products are disposed of some of these chemicals are again released into the environment. Recycling PVC is fraught with difficulties, and, since recycled PVC-u is lower quality and more expensive, recycling is minimal. Contrary in popular beliefs, PVC-u windows do need on going attention and their life expectancy has been put at 20-25 years. Far shorter than timber windows. "