Well in the wild there arent any glass walls either are there? That said, it disperses heat quite well, which is needed, provides a substrate that crickets cant easily hide under, which happens with paper towel and carpet. Ever thought about the fumes commercially made carpets emit, what about chemicals in the papertowel? Yes chemicals are used in the manufacturing process. About the only substrate I would not recommend would be silica, or sand. Basically all sand is comprised of is tiny shards of glass. Bottom line is that there are arguements for and against most substrates and I have found a reputable source for the use of crushed walnut shells and I happen to trust this person's opinion based on years of working with Leopard Geckos. Mac, if you or anybody else can provide me with documentable evidence, not just hearsay and opinion, I'll be the first to change our gecko's substrate. Thanks, Tim