Alex really hit on the crux of the issue...it's been touched on before but no one has gone into any real depth on it.
Aspen and cypress wood contains phenols and oils. While they tend to be less aromatic, they still exist and in fresh form, can be fairly potent respiratory irritants.
It's just in the last 7 years or so that pine has become so darned controversial and has become the scourge of wood beddings.
The truth is that treated pine is no more risky than any other commercially available wood bedding. There has been zero published evidence that it is any more harmful than any other commonly used wood bedding - and yes I mean aspen
People who should stick to eating Pringles and guzzling diet coke while watching Oprah have done a lot of laughable misinterpretation of some select studies that could arguably apply to any wood bedding. The time spent excoriating pine could be better used by these people to work out or to decrease the point loading on their sofas.
The most pathetic of these misconceptions is that the phenols that the tree uses to protect itself from parasites are unique to pine. They are not. Most trees produce the same phenols. The process of drying wood bedding reduces both phenols, oils and the other compounds that the trees use as defense mechanisms.
Animals and people alike can have contact and inhalant allergies to any wood shaving product as they all contain an irritant of one type or another. The trick is to get a bedding material that is properly dried and screened - which most are.
I find it amazing that members of our community are so quick to point out the ignorance and fear that many people direct at snakes and their owners. We accuse these people of looking at statistics wrong, misinterpreting data and vilifying something out of pure ignorance.
Then a large portion of our community is guilty of the same thing....pine substrate bad. No glass tanks. Ball pythons do not climb. You can't keep two snakes of any species together, etc.