As a nitpick, deuterium actually is toxic. The hydrogen bonding in molecules that substitute deuterium for light hydrogen is affected, such that if you drink enough heavy water, you will experience symptoms similar to radiation poisoning. It interferes with how your chromosomes line up during cell division.

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.