Cedar is a soft wood and contains aromatic oils called phenols. Pine is also a soft wood and also contains phenols, although at much lower levels. Phenols cause respiratory irritation, which leaves the airways open to secondary respiratory infections, and liver damage. This is well documented not only in animals housed on cedar shavings in confinement but also in humans who have worked in sawmills or other woodworking industries where cedar or pine was the main wood. Whether or not pine shavings are "safe" for small animals is still a subject of debate even among veterinarians. The current general consensus is that because pine shavings have so much fewer phenols than cedar they are less likely to cause damage, but that damage can still be caused.
Personally, I will not use pine shavings for small caged pets because I feel that it isn't worth the risk. Yes, pine has lower levels of phenols than cedar. But lower levels of something bad doesn't make it a good thing. I know that I personally get respiratory irritation from digging through pine shavings in an enclosed space. For example, we used to use pine shavings at work for small animals and we stored it in a large plastic barrel, every time I had to lean down into the barrel when the shavings were getting low I would get irritation from it. And for me it wasn't just an allergic reaction, I would get a raw feeling in my throat the way I do when I am battling bronchitis. If pine shavings can do that to me I can only imagine what it would be doing to an animal less than 1/10 my size who has to live on it 24/7.
