Dude, it's hydrogen, you need to have different concerns there.
At least they did acknowledge that firing a weapon in a deuterium-rich atmosphere might not be wise, depending on concentration.
Eh, just push the non-chewy bits to the edge of the plate and enjoy the rest of the culinary confection.
Presumably, if you inhale enough dideuterium monoxide vapor, you may experience a similar effect. It would not be acutely toxic, though--more like an occupational exposure level of hazard. Deuterium gas is likely as harmless as light hydrogen gas.
If a retcon is necessary to preserve one's enjoyment of the show, Riker may have already been aware of the hazard level for humans, but was asking for a more informed opinion from the chief engineer, who has presumably read and memorized all the MSDS sheets, for the benefit of any nonhuman organisms on board. Vulcans and Romulans, for instance, experience an anaesthetic effect from TLTDH, similar to the effect of NO2 on humans. It would not be out of character for Star Trek to invent a bizarre species-specific toxicity if the plot demanded one.
How much D2 can you breathe in before you suffocate anyway?
Of whatever amount you do breathe, how little is absorbed?
Of the tiny amount that is in fact absorbed, how much ends up exchanging D atoms with your body?
I don't know the answers to these questions, but my bet is the airborne process is orders of magnitude less efficient than drinking D2O.
Their main concerns should be:
1. Suffocation from lack of O2
2. Risk of explosion if the D2 percentage is in the danger zone
3. If that think was leaking out of a reactor, maybe there were some byproducts leaking out with it? Like, I dunno, tritium? That would be really, really nasty to breathe in.