I use a CPAP machine for sleep apnea, which requires distilled H2O. While it's a cinch to find at any grocery store back home in the US, it's exceedingly difficult in Bath, where we're staying for a few days. After searching 7-8 grocery stores & pharmacies, we finally found some deionized H2O at M & K Hardware, in Guildhall Market. My understanding is that deionized H2O may have organic contaminants and is not as pure as distilled water. Distilled H2O has all the minerals and impurities removed, and is free from bacteria.
I've read and heard that distilled H2O is hard to find in England. If someone's found any in Bath, The Cotswalds, or London, please let me know exactly where. In the meantime, I'll give it a try for a few days.
Update:
Tried it for a night, seemed fine and noticed lots of condensation inside the tank, which I've never seen before. Was able to find purified water today however, @ A. H. Hale Ltd. Pharmacy, 8 Argyle St., Bath, BA2 4BQ. (Note: just got home and forgot to mention the price charged was 9£ for 2 litres, roughly $11 USD!)
Also did some more reading @ SleepFoundation.org, which has medical and scientific advisory boards, and found "Should You Use Distilled Water for a CPAP Machine?" (https://www.sleepfoundation.org/cpap/should-you-use-distilled-water-for-a-cpap-machine):
"Most bottled water is purified, meaning it is filtered to remove chemicals, though it may still contain some mineral content. Most purified drinking water sold in stores originates as municipal tap water. Some brands employ a reverse osmosis process, which removes impurities and microbes by using pressure to force water through a semipermeable membrane."
"Distilled water is the best choice for CPAP humidifiers because it’s pure water that is free of minerals and bacteria. Hospitals, labs, and factories often use distilled water instead of tap water because it prevents limescale buildup on devices. Using the wrong type of water in your CPAP humidifier can cause mineral buildup, scaling, or bacterial growth. It may also irritate your airways or cause your machine or its components to break down."
So distilled is best, and I have no idea how long it takes using other types of water to create negative, unwanted effects.