OP,
You are enduring a bit of ridicule here for what seems like a sincere question. Probably not the responses you expected, but not surprising since relatively few people have deep knowledge in both the details and the big picture regarding toxic body burden (the pollution in people).
I have a Mac Pro (early 2008) purchased the week it was introduced. I don't have sources handy, but I do recall discussion back then about a few (not many) units emitting a strong 'solvent' odor that diminished within a few days or weeks. I also recall one user with an older Mac Pro that emitted a 'sweet' odor that never went away and was eventually replaced by Apple. For what it's worth, my 2008 Mac Pro had no such odors and continues to be an excellent performer.
Regarding the solvent odor, I seem to recall a community consensus that some components in some Mac Pros must have had a thin coating of excess residue left over from manufacturing. A few users had experienced similar symptoms with non-Apple equipment, leading them to believe this was something that occasionally happened (maybe still does) in the mass production of consumer electronics. I also recall some users pinpointing the problem to their graphics card -- the problem went away when they replaced it.
Specifically regarding benzene, it dissipates relatively quickly as will most other VOCs involved with production. If you are in the market for a 2009 Mac Pro I believe any benzene that might have been there is long gone.
As @snberk103 stated, you have a lot more in your home, car, workplace, and community that will affect you more significantly than any toxics emitted by a Mac Pro. A multifunction printer, a cushioned mouse pad, a stack of DVDs/CDs, and a typical desk chair are just a few nearby items that can negatively effect your health more than a Mac Pro.
One of the kids in my family has extreme chemical sensitivity. I have to keep that in mind with every single item brought into the home. When evaluating the Mac Pro I concluded that overall it was the least toxic option for the kind of computing I do. One major factor is the Mac Pro's aluminum case; it is not shedding anywhere near the level of toxics (PBDEs, metals, plasticizers) that typical cases do. Those other cases will never stop emitting, and made more egregious because they become heated and blow their particles into the room.
If you are serious about investing in a Mac Pro, I encourage you not to worry about its potential for increasing your toxic burden. The Mac Pro's contribution to your burden will rank low on the big list of sources in your life.
(If you want more resources about toxic burden let me know, I devoted years researching the subject
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