The ideal water has no minerals in it. Minerals will build up in your humidifier reservoir over time, because each night you are evaporating the water, and leaving what doesn't evaporate (minerals) to build up in the reservoir. For that reason, distilled, demineralized, or reverse osmosis (RO) water is the best to use. I have heard that it is hard to find in the UK however. Just plain bottled water is usually just tap water in a bottle. Mineral water is probably about the same. If you look on the ingredients list, distilled, or demineralized, or RO water will have zero or close to zero calcium (Ca), or Magnesium (Mg). If you can't find water without minerals, just use ordinary tap water, and rinse out the reservoir daily before you refill it. If you see any build up of minerals use vinegar to remove it, and then rinse thoroughly. When we go on holidays and don't have easy access to demineralized water, we just use tap water. Using vinegar to clean the reservoir every few weeks even when you use demineralized water is a good practice.
Thank you so much for your quick response Sierra. I will continue to look for demineralised or RO. Given up on distilled. The info on checking for Ca or Mg is most useful. There is an Active Plus P1000 Brita here - maybe I'll use that if I have no luck in the shops!
I think the Brita filters are just activated carbon and do not remove dissolved minerals. It would be no improvement over tap water. There is another water purification device called Zero Water that does remove dissolved minerals that would be good to use. However, for just a visit or holiday I would not be bothered with the expense of getting one. Some bottled water may be RO water. I have seen it in some south vacation destinations like Mexico.
The other concern, besides mineral build-up, is that plain tap water or bottled spring water can have bacteria in it. It's fine for drinking because your stomach acid will kill anything, but you don't have acid in your nostirls and sinuses. Breathing in water vapor with bacteria may cause a respiratory infection, if the right kinds of bacteria are present. People on this forum have reported using spring water and getting respiratory infections, chest pains, and nasty mucous discharge.
If the municipal water supplier is doing their job, there will be no bacteria in the water. They typically chlorinate it to kill all bacteria, and then leave a residual chloramine in the water to prevent any bacteria growth. Bottled water is probably nearly as good, but the risk might be slightly higher, because bottled water suppliers often filter out the chlorine and chloramine with activated carbon filters. They USUALLY replace the chorines with ozone to prevent bacterial contamination, but not always. Tap water from a reliable municipal water supplier is almost always the safest. There is no good reason to use bottled water over tap water, but if you do, check the label to ensure they have ozone, chlorine, or chloramine in it to prevent contamination.
You breathe in a lot of tap water anyway, whenever you shower, use the hose in the garden, run a tap or do anything else that agitates the water. If you don't get a respiratory infection from this then using it in your humidifier should make no difference. The only real risk with infection from humidifiers occurs when they (and tubing if it gets condensation) are not allowed to dry out every day. When a humidifier chamber or tubing remain damp long term this will eventually lead to mold growth. Mold can cause nasty infections. Drying your equipment every day will prevent growth of pathogens.
I agree. Regular cleaning is the secret. If you use distilled water or demineralized water the risk of contamination is probably even higher. While these products are likely sterile when you buy them, as soon as you open the lid, you are susceptible to contamination. These products are very pure, and usually do not contain chlorine, chloramine, or ozone to prevent growth if contaminated. Similar issue with filtered water from Brita machines. The activated carbon in the filter takes out the chlorine and/or chloramine.