CPAP humidifier water
There are many opinions for what type of water to use in a CPAP humidifier. The important thing to realize is that water is never just pure water, there is always something dissolved in it, and it's the dissolved stuff that can be the cause of concern or possible problems.
When there are germs in the water, we boil it to make it safe; and municipal water supplies add chlorine to kill and prevent the growth of germs. When there are too many hard water minerals in the water, we soften it which makes it far less likely that a residue will form in the humidifier tank. Filters are used to remove sediment and minerals that are dissolved in water. UV light is often used to kill germs in some water supplies.
Manufacturers often recommend that you use distilled water in your CPAP machine because distilling the water addresses all of these concerns. Theoretically, distillation removes all of the stuff dissolved in the water making it pure, but as any chemist will tell you, pure water will quickly start to dissolve many things in its environment, so it won't stay pure! It'll absorb germs and other contaminants in the air, and biologists tell us that standing water provides a medium for the growth of bacteria.
This is one of the reasons that the debate over which type of water to use in your CPAP machine will never be settled. There is no 100% safe way to put water in your CPAP machine, just as there's no 100% safe way to do anything. We make choices as to what to do based on what we think is reasonable. Putting water in your CPAP machine's humidifier reservoir is another activity where we try to make informed and reasonable choices.
The safety and quality of your water supply may be the key consideration in deciding whether to use distilled, boiled, filtered or tap water in the humidifier. Boiling water will kill microbes, but it will not remove minerals or chemical contaminants. Filtering water may remove some of the minerals but may not remove living organisms or other chemicals. Bottled water that has been distilled might be the safest option.
As for whether you're required to use distilled water in the CPAP humidifier: most manufacturers do recommend the use of distilled water. According to ResMed’s product website, the use of distilled water "will maximize the life of the water tub and reduce mineral deposits." The site also points out that it's okay to occasionally use tap water to clean the equipment. But since the water tub is typically replaced on a regular basis — perhaps every 6 months — maximizing the product life may not be a major concern. The concentration of minerals, that is the water hardness, will depend on your water supply.
If you're traveling to a part of the world where you don't trust the water supply, you should use either distilled water or bottled drinking water. If the water's not safe for you to drink, it's probably not safe to put it in your CPAP machine. Using distilled water will minimize your exposure to potentially harmful substances. Ultimately, you must be aware that you'll be exposing your lungs to the water placed in the CPAP humidifier, which may motivate you to clean the water tank on a more regular basis, and change the water in the tank every day. Moreover, it's not advisable to place any substances in the tank that would be harmful to breathe. Perfumes, cologne, or scented oils are not a good idea. Exposure to fumes from bleach, alcohol, chlorine or ammonia may damage your lungs. The use of moisturizing, antibacterial, and glycerine-based soaps is also discouraged. Water softeners and descaling agents should also not be used in the reservoir. If you question the safety or quality of your water supply, err on the side of caution and use distilled water in your CPAP humidifier. This is especially true if you are traveling in regions of the world with unsafe water supplies. If distilled water is not available or you don't want to be bothered with finding or carrying it, just use bottled drinking water. Tap water may lead to mineral deposits accumulating in your CPAP humidifier tank. It may contain chlorine (at a maximum concentration of 4 parts per million in the U.S.) which makes fumes that may bother you, and may be a concern for potentially undesirable exposures or other effects.
Your own experience with your water supply, your own water tank, and your own concern for germs will lead you to develop the water-usage and tank-cleaning habits that work best for you. It's a personal choice and there is no one consensus on the best way to do it.