I don't necessarily disagree with anything you said. However, the manufacturers seem to state otherwise, and post their reasons. For the tiny costs of the distilled water I say what the heck.
Here's what the manufacturer says;
Use distilled water to help keep the humidifier chamber clean and mineral deposit free. Tap water should not be used as it will leave hard white mineral deposits in the chamber as the water evaporates, or it may lead to mold growth. Cases of lung disease have been connected to using contaminated well water in a CPAP humidifier
And they site their source, and this is just but one article of many, so right or wrong I try and use it as often as I can as the cost is so negligible;
Lung disease linked to the use of contaminated well water in a CPAP machine;
Many people rely on continuous positive airway pressure (CPAP) machines as a safe and effective treatment for sleep apnea. But a new case report describes a rare complication—a lingering inflammatory disease of the lungs, apparently related to the use of contaminated well water in a CPAP machine. The report appears in the December Southern Medical Journal, official journal of the Southern Medical Association. The journal is published by Lippincott Williams & Wilkins, a part of Wolters Kluwer Health, a leading provider of information and business intelligence for students, professionals, and institutions in medicine, nursing, allied health, and pharmacy.
Dr. Lawrence W. Raymond and colleagues of Carolinas HealthCare System, Charlotte, describe an unusual case of lung disease related to CPAP, which uses humidified air to keep the airways open while the patient is sleeping. The patient in the case report had used her CPAP machine for several years with no problems—she was careful about cleaning her machine and filling it with distilled water, as recommended.
Symptoms Developed After Using Well Water for CPAP
The problem started when the patient forgot to bring a supply of distilled water on a trip to her North Carolina vacation home. Instead, she used tap water, which came from a well located a few miles away. She awoke the next morning with a severe sore throat, and immediately suspected that her symptoms were caused by using tap water in her CPAP machine.
The patient was ill for several weeks—with "crackles" in the lungs and decreased blood oxygen levels—despite treatment with antibiotics. She finally started getting better after beginning treatment with steroids; her condition gradually improved over several weeks. However, even three years later, she still had minor problems related to a chronic cough.
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An infection from contaminated well water was suspected, but testing of the water showed no bacteria. Instead, there were high levels of a toxic bacterial compound called endotoxin, probably related to repair work done on the pipes a few weeks earlier.
Dr. Raymond and colleagues diagnosed their patient's illness as bronchiolitis: an inflammatory condition affecting the smallest air passages in the lungs (bronchioles). Most often caused by viral infections in infants, bronchiolitis has also been linked to high levels of endotoxin—for example, in dusty rooms. Bronchiolitis and exposure to high levels of endotoxin have both been linked to the development of asthma.
Although the patient's illness was moderately severe and lasting, Dr. Raymond and colleagues point out that it was very unusual—it would not likely result from using normal tap water in a CPAP machine. The recommendation to use distilled water in CPAP machines is related to preventive maintenance, rather than avoiding contamination. Infections related to CPAP machines are rare, and most often related to poor cleaning of the machine.
However, the case report shows that, in unusual circumstances, using contaminated water in CPAP machines has the potential to cause respiratory illness. Dr. Raymond and co-authors conclude, "We do believe that caution is warranted in CPAP humidification using tap water from wells in remote locations such as the North Carolina mountains."