At the ACI Conference in Kansas City i attended a session on unvented gas logs. Paul Fransisco, was the fellow who presented, had done a 3 year study on about 30 applicaitons in IL on unvented gas logs and how they effect indoor air quality. I had always known that unvented gas logs effect air quality and I figured it was an issue or CO2 or CO but this study actually found the issue is actually NO2 in elevated levels (beyond industrial standards).
Rather than giving you a full summary here is a link to the full study and his powerpoint presentation.

Both of the downloads are pretty good size, but they are worth the wait.