Another ultimate player sent this link, to an article in Environmental Health Perspectives, a peer-reviewed open-access journal published by the NIH: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2265067 Major concerns include: toxicity of turf's rubber pellets as they degrade; leaching of heavy metals directly into municipal storm drains (since there's no vegetation to filter runoff); and turf reaching surface temps of 140-160°F (not a typo) on summer days, causing burns. It's a complex issue, so you should read the whole article yourself, but my own reading left me even more firmly in favor of preserving the grass. Thanks.