There was some discussion recently on this topic. Here is the latest science based information:
http://www.globalsciencebooks.info/Online/GSBOnline/images/2010/PS_4(1)/PS_4(1)5-17o.pdf
http://www.globalsciencebooks.info/Online/GSBOnline/images/2010/PS_4(1)/PS_4(1)5-17o.pdf