I'm another CPAP user. The CPAP machine includes an air
moisturizer. I'd estimate that the water reservoir holds
2/3 of a pint. The CPAP airflow passes through the
reservoir and picks up moisture from the mildly-heated
distilled water therein.

Each night about half of that 2/3 pint of distilled water
is picked up by the air being pushed to the mask for me to
breathe. Some of that moisture, I'm sure, escapes to the
ambient air in the mask's exhaust, which is partly my
exhaled air and partly CPAP oversupply. I don't know how much
of the supplied moisture is actually applied to my breathing
organs each night.

I'm never impressed in the morning with a feeling of
being particularly dry, nor am I plagued with thirst
through my waking hours. Indeed, my Beautiful Wife, (who
drinks so much water that she sloshes when she walks, I
think) never tires of urging me to drink more water,
drink more water, drink more water.

Dave Oesterreich