The ingredient in things like pine shavings that can cause harm to a reptile is Phenol(C6H6O). It is present in pine shavings, but not to the degree that it is present in cedar shavings, especially if the pine has been kiln dried. Kiln drying the wood releases most of the aromatic hydrocarbons and phenols.
Cedar is dangerous to reptiles (and other animals for that matter) because of the high levels of these compounds present in cedar. With pine shavings, most of the danger would be from inhaling the dust.
Phenol is a very toxic chemical. It is carcenogenic and can cause respiratory symptoms in reptiles as well as a host of other problems.
1g of Phenol orally ingested is a lethal dose for a human. Extended exposure to smaller amounts leads to liver failure, kidney failure, respiratory failure and deteriorated CNS function.
While pine shavings do not present the same level of risk as cedar shavings, why take the chance? Aspen shavings perform the same function and have none of the same potential for risk.
Phenol does have it's uses. It is used to manufacture certain resins, for manufacturing phenolic, as a cleaner and disinfectant, and in certain medications. It's just not a good thing to have in your snake's cage.
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We do not inherit the Earth from our ancestors; we borrow it from our children. Ralph Waldo Emerson