Is your husband taking any medications? Some meds cause dry mouth. I'm just asking to rule out that possibility.
But as you're indicating, it's likely the air from the bipap. I use a bipap also, with my pressure at 18 and I too get dry throat, though I'm not a mouth breather and have the nose-only mask. Most humidifiers go up to settings of 5 or 6, has he tried the maximum setting? Some people when using the maximum setting will sometimes experience rainout, or the condition where the moisture condenses in the tubing and rolls back into the mask. That usually happens when the room air is cool, like in winter or sometimes air conditioning. If that happens and the high setting is needed and works, you can get hose covers from the supplier, or make cloth covers for the hose. There are even heated hoses for this purpose.
If these fail, there are over-the-counter products for dry mouth which might help, like Biotene and others. And there are stand-alone humidifiers that can be used in conjunction with his machine. Fisher & Paykel are known for their humidification, and a different one might work better. And another suggestion was offered earlier that might be possible if the doc would order switching machines so that his bipap is an auto bipap, meaning the pressures vary based on pressure needs at any given time.
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