- Water, water, everywhere
nor any drop to drink. – from The Rime of the Ancient Mariner
The quality of drinking water varies between countries and regions.
Tap water is often provided by the local government and treated to be at least relatively healthy for the local population. This may include filtering and adding chemicals. Tap water sourced from mountain springs and rivers may require little treatment and be exceptionally high quality. However, water quality varies greatly between regions, and there are many countries where tap water, or at least unboiled tap water that hasn't gone through a reverse-osmosis filter, is dangerous to drink and can make you sick. One basic rule is to look at what the locals do: If they do not drink the tap water or do so only after boiling it, you should not drink water directly from the tap either.
Your body is accustomed to the water from your local environment. If you travel to a region with poor water quality from another with high water quality (or with different pathogens), then you may suffer from illness. Your body may adapt over time, but some water sources are so contaminated that it is unsafe to use them, lest you become infested with parasites or suffer other consequences. See Infectious diseases for details.
Note that drinking the water is not the only hazard. If you rinse your toothbrush in tap water, or have a drink with ice cubes, or order a nice leafy salad in a restaurant that washes vegetables in tap water, then you may be exposed to whatever nasties are in the water.
Short-term ill effects of drinking water are usually down to contamination by bacteria. Boiling and filtering can mitigate the danger of illness, although toxins produced by some bacteria may remain despite the bacteria themselves being gone after the treatment. Another method that will kill most pathogens is UV radiation. In a pinch putting a clear bottle of water into the direct sunlight for a day or two will get it as close to drinkable as boiling would. This is however emphatically not the safest or best way to purify water. There are also other pathogens, e.g. amoebas.
See Travellers' diarrhea for more on several afflictions that can come from contaminated water.
Cyanobacteria in big concentrations ("algal bloom") can make water unhealthy to consume and irritating for the skin. This is mainly a problem at some beaches, usually not a real hazard unless you have children or pets.
In some countries, water supplies are prone to contamination by chemicals from industrial and agricultural sources. Although drinking such contaminated water is not recommended, it will generally not cause any immediate health issues if the contamination is at relatively low levels. If you drink such water over an extended period of time then long term health issues may arise. Note that water filters are unlikely to remove any of these chemicals. Another contaminant that can even occur in high income nations is lead. Plumbing used to be made from lead (plumbum in Latin) and some houses may still have lead tubes on the last few meters. This is usually no problem and those are increasingly being replaced, but if the chemical composition of the water changes, lead can leach into the water. The only way to be 100% sure is to regularly check water at the tap and to remove all lead plumbing.
You may hear locals refer to "hard/soft" water. The "hardness" of the water is determined by its mineral content, and there are stark regional variations owing to the various soils and rocks that water filters through before it reaches the mains supply. Water from different regions will likely vary in taste.
There are no negative health effects associated with drinking "hard" water, but it can cause limescale build-up in appliances such as kettles and dishwashers. The ancient Romans preferred "hard" water, as can be seen by deposits in erstwhile aqueducts that in some cases were large enough to be used by medieval masons for figurines.
Quality by country or region
According to a 2014 report by several health and other organizations, tap water is safe to drink in most of the EU, the United States, Canada, Greenland, Australia, New Zealand and a handful of Asian countries — elsewhere it isn't.
Australia's tap water is always safe to drink in urban areas, although the quality can be rather poor. Remote cities such as Perth and Adelaide rely on ground water supplies that are very poor for drinking, but are by no means unsafe. Many households use water purifier jugs. Australian mineral water is available everywhere. Remoter areas in the outback may not have treated drinking water on tap.
Tap water should be filtered and possibly boiled before drinking. Many restaurants and homes have water filters installed, although it is hard to know whether the filter has been replaced recently. Also note that much river water in China has been contaminated by chemicals that filters can not help much with, although this should only be dangerous if consumed over an extended period of time. People typically buy bottled water for drinking, which is usually a domestic brand for about Y2-6, or a European brand for over Y10, and drink tap water only in the form of tea (using water which has been boiled).
All public supplies are safe to drink and, of course, mineral water from Karlovy Vary tastes great!
Tap water in France is generally both safe and acceptable in taste, but mineral water (French: eau minérale) is generally considered to taste better, except in areas that use mountain water from the Alps for their municipal supply. Volvic and Évian are cheap and available most everywhere, and many locals consider them nothing special. You may find Vittel a more interesting-tasting inexpensive French mineral water, and Badoit, a sparkling water, is quite good.
Tap water is safe and drinkable (in fact, due to tight regulations it might be safer than bottled water), although many look down on it and mineral water is usually offered unless tap water (Leitungswasser) is requested. In some regions there can be a high concentration of nitrates. Most people buy bottled water in crates of 12 glass bottles or packs of 6 plastic bottles. Both the bottles and crates include a returnable deposit (Pfand). While the deposits for reusable plastic (15 cents) or glass bottles (8 cents) are relatively low, the deposit for disposable plastic bottles (marked by a special symbol on the side of the bottle) is relatively high at 25 cents and may be higher than the price of the water itself. Bottled water is usually sold carbonated (sparkling), although regular water (stilles Wasser) is also widely available and slowly gaining popularity among Germans. Sparkling water is usually sold in supermarkets in two degrees of sparkling: one with more CO2 (usually called spritzig or classic) and one with less CO2 (usually called medium).
Most springs and many public restrooms (e.g. on planes or trains) use non-potable water that has to be clearly marked by the words "kein Trinkwasser" or a symbol showing a glass of water with a diagonal line through it. If there is no such sign and the surroundings don't indicate otherwise it is safe to assume that the water is safe for human consumption.
Most of Hong Kong's water comes from mainland China, and the same concerns about chemicals apply. Most people drink water boiled or tea as a method (not completely effective) to remove impurities. Bottled water is readily available everywhere for a few Hong Kong dollars, and comes in mineralized and distilled varieties. Hotel rooms typically provide bottled water which should be used for drinking and the brushing of teeth if you are new to the region.
Tap water is drinkable throughout the country, but except in certain towns that use mountain water for their municipal supplies, such as Spoleto, mineral water (acqua minerale) is universally preferred, and some very good mineral waters are bottled locally. Many towns have fountains with tap water that you can use to refill your container, but do not use water from fountains with an "Acqua non potabile" sign on them.
Tap water is safe and of good quality throughout the country. Domestic and foreign brands of bottled water are available for ¥100-200 everywhere (at least in tourist destinations). Most restaurants serve filtered tap water for free. Unless specifically labeled "mineral water" (ミネラルウォーター), water in Japan has a low mineral concentration in general. Radioactivity levels in the water supply have been closely monitored in some areas since the 2011 earthquake and nuclear disaster, but found negligible as far as water used by civilians is concerned; see also U.S. Embassy's summary and Tokyo government's daily reports.
In Denmark, Finland, Iceland (despite the volcanic smell), Norway and Sweden the tap water is mostly of very good quality, often better than bottled water. Where the tap water is not safe (such as on trains), you can expect there to be a warning. Also, good-looking water from streams is good in many areas. In important ground water areas there may be restrictions on swimming, et al.
The quality of tap water in North Korea is somewhat unknown, although it is typical for tourists to drink bottled water.
South Korea has a good quality of tap water throughout the country. However, bottled mineral water from Jeju Island is very popular. Fresh mountain spring water is available directly in wells around the country (especially Buddhist monasteries), and although these are generally safe you should note that the water has not been treated in any way and is therefore potentially risky.
The tap water in Spain is safe and of a drinkable quality. The water in some southern regions of the country, however, is sometimes sourced from salt water and can have a high mineral content. While it is safe to drink, locals in some areas will often drink bottled water instead as it tastes better. Bottled water is readily available to buy in most areas and in a variety of brands.
Tap water is of a high drinkable quality, with non-drinkable water supplies clearly marked in practically all cases. In most regions, fluoride is added to the water. In pubs, bars and restaurants, it is best to ask specifically for tap water if you do not wish to pay for mineral water, as some venues will assume that mineral water has been requested. There are numerous mineral water brands on sale in the UK, including imports of French brands.
Mains water supply is practically universal in most of the UK, the exceptions being isolated outlying settlements in remote rural areas.
Mountain water sources in upland areas (such as Snowdonia, the Pennines, the Lake District, and Scotland) are of variable quality, and local advice should be sought, owing to mineral contaminants.
Generally, the mains water in the southeast and east of the United Kingdom (including London) is considered "hard", whereas the water in the north, middle and west of the country is considered "soft".
Tap water is generally safe throughout the United States and of good quality in many, including New York City; however, it tastes bad in certain low-rainfall areas, such as Santa Barbara, so you might prefer to drink bottled water or seltzer in those places. Bottled water is near ubiquitous and available for a variety of prices. Quality tends to be high and the chemical difference between a 50 cent bottle of water and a 5 dollar bottle of water is oftentimes negligible. Lead plumbing is still a problem in some places, as became evident in the high profile case of Flint, Michigan, but hotels and public water dispensers are unlikely to be affected.
Hot (boiling) water
One sometimes wants to have hot (near-boiling) drinking water, e.g. to make a cup of tea or other beverage. Availability of hot water at typical places of lodging and transportation varies around the world; it somewhat correlates with the nation's proclivity for tea-drinking.
- China: very good. These days, hotel rooms tend to have electric kettles; in earlier times, the reception would have a battery of thermos bottles of hot boiled water (开水 kai shui), and give them out to guests. Hot drinking water dispensers are nearly universally available at train stations, in airport terminals, on the trains.
- Vietnam: medium. At many hotels / guest houses one can ask for boiling water (nước sôi), and the staff will get you a thermos bottle of it, or will let you use their electric kettle; but this is by no means guaranteed.
- Russia: medium. At a hotel / guest house, ask for boiling water (кипяток kipyatok), and the staff may lend you an electric kettle, or show how to use the kitchen, but this is not guaranteed. On long distance trains, there is a water boiler in every car, but it may be in use only at certain times (when the car attendant desides to serve tea to passengers, for a fee).
- Latin America: usually, poor. If you ask for boiling water, the staff is more likely to offer to sell you a (small) cup of coffee. Bring your own portable heater (immersion coils) and use it in a suitable container!
If you find yourself somewhere where a trustworthy supply of water does not exist, you can:
- Use an international brand of bottled water (in some areas you should check that the bottle has not been refilled)
- Use a domestic brand of bottled water (cheaper, with quality as good as, or possibly much lower than the international brands)
- Boil the water before drinking (several minutes, depending on what you want to kill)
- Use Iodine tablets (will kill bacteria, but make the water taste bad)
- Use a survival straw (probably best for extremely remote areas)
There are different ways of purifying water, some more effective against specific threats. In some areas boiling water for a minute is enough, in others several minutes are needed. Filters vary in effectiveness, and should you have a concern, then you should consider buying your water in a sealed bottle from a reputable company.
Boiling the water or using iodine kills organisms but does not make chemically polluted water safe. You are unlikely to suffer in the short term from mildly chemically polluted water, but drinking it over an extended period could be harmful to your long-term health.
Remember that polluted water can affect your health without your drinking it directly, such as if the water is used for washing vegetables or brushing teeth. See Precautions against disease in the Stay healthy article.
Even in areas that normally have safe drinking water, flooding and other incidents of contamination can take place. Anyplace with decent news and/or public health announcement coverage and a reasonably responsible attitude on the part of relevant authorities is likely to give warnings at times when it's necessary to boil or completely avoid drinking water that comes from sources that are safe under normal conditions.
Water safety issues that are not related to drinking or other household use usually have to do with either flooding or injuries or deaths as a result of accidents, such as can happen while involved in water sports such as swimming, sunbathing, diving and boating. See the relevant articles for specifics on these issues.