I think it depends on who you talk too lol. I think most people agree that Ceder and air dried pine is bad. Kiln dried pine..its depends entirly on who you talk too, I have used it safely for many years and if anything my rodents have lived exeedingly long lives, far exeeding the average life expectancy(average is 3-5 for a Gerbil, average in my house is 6-8), however many will say its just as bad at air dried and ceder. I have never heard a bad thing about Aspen aside from its price lol. my Rabbit isnt on any type of shavings, she is litter trained ad I use Swheat Scoop, which is 100% natural, flushable, biodegradable, clumping, dust free, edible litter made from wheat millings.