1996 850 GLT CO or CO2 Leaking Into Cabin???

[fuzzeternal]

Full Disclosure: I know very little about cars, let alone this car specifically, and I'm seeking any insight this community can offer into the problem that our family's Volvo has been alleged (by my mom and brother) to have, so that we don't get taken when we bring it to a mechanic.
Both my mother and brother claim they become dizzy and lightheaded whenever they drive our 850 lately, and they have therefore decided that carbon monoxide must be leaking into the cabin. I'm a little skeptical of their conviction, considering they know even less about cars than I do, but I don't want to downplay a potentially harmful, or even lethal, mechanical issue. I have driven it since and didn't feel anything, but I might just be less sensitive; plus I drove it with the windows down (whereas my mom and brother kept them rolled up), so if there are fumes leaking into the cabin then I assume I would have gotten enough air flowing to mitigate the effects.
Has anyone had this or a similar issue before?
Anything helps.

I've seen an old thread on this before but wanted to reopen the topic to fresh discussion. Please forgive any faux pas, as I'm new to this.

Thanks

[misha]

Check exhaust,it's flexi pipe between manifold and cat and exhaust manifold gaskets.
Also,if the car have sas system,i would check sas valve and hoses for leaks.When that valve fails in open position,exhaust fumes goes back into engine bay,melting it's hoses.
It could also be a clogged pcv system which can cause oil vapors to escape from dipstick pipe.