Unless your cooktop is malfunctioning, you don't have to worry about CO. The main purpose of a vent or downdraft is to exhaust heat, smoke, grease and odors. Unless you have respiratory problems, gas combustion by-products are really not a big issue. On the other hand, overhead vents are much more effective than downdrafts if you have that option.

I'd love to have a proper hood, but the kitchen orientation precludes it. That's good news to me though - it opens up the possibility of a gas cooktop, which I thought would be problematic. Thanks!

Why does your "kitchen orientation" preclude a hood?

Is it b/c your cooktop is on an interior wall? If so, it doesn't necessarily preclude a hood & exterior venting...I speak from experience. My cooktop is in the center of the house...the duct work simply goes up to the ceiling and across the kitchen to the outside. I am so glad I did not believe my KD & Contractor when they said it "couldn't be done" (for different reasons...they always agreed it's not that big a deal to vent from an interior wall).

I would argue that it's not CO or CO2 that is your problem. Rather, it's the fundamental dilemma that the more effective your downdraft vent is, the less effective your range will be. As a prior owner of a (cheap) gas range with a downdraft vent, the biggest problem is that the minute I turned the vent on, about half of my BTU's got sucked out the vent. You could literally see pots come off a boil when I turned the vent on. And the times you would most desire to have a vent working--searing meat, for example, with all the smoke it produces--are the very times when you can't afford to lose BTU's down the vent.

Yes, some higher end downdrafts, especially the ones that rise up 6'' or more above the range surface are less prone to this, but it's still a pretty poor compromise to me.

I agree (as usual) with buehl that you'd be better served looking for creative ways to do a real hood, if you're really serious about cooking well.