Washington - Water standards vary around the world. About 71 percent of the global population uses a safely managed drinking water service, according to the World Health Organisation.
But 785 million people lack clean drinking water, and at least 2 billion people rely on a drinking water source contaminated with feces.
But finding out whether the water is potable at your destination might be difficult, and most tap water looks fine to drink. So travellers have developed strategies to cope with uncertainty.
There's no easy-to-use, universally trusted reference source for international travellers concerned about water safety. Keeping such a list is impractical, because water quality can vary by country, region or city.
Generally speaking, you can trust the water in Australia, Japan, New Zealand, North America and Western Europe, experts say. Everywhere else, you should assume the water is unsafe until you know for sure.
How can you determine whether the water is safe to drink? Evan Lorendo, a business development consultant who lives in Tanzania, says you should look around at a traffic stop. If the street vendors are selling bottled water to the tourists, there's probably an issue with the drinking water.
"Also look at the menus at restaurants," he adds. "If there's a mention of ice made from bottled water, then you know that it is an issue."
Of course, there are more scientific ways of determining whether the water is safe to drink. For example, you can buy a drinking water test kit like First Alert's (about $17/R250), which identifies bacteria, lead, pesticides, nitrates and nitrites, and chlorine in the water. But most commercially available test kits don't cover every contaminant.
"Drinkable water should be clear, odorless, and it shouldn't feel yucky on your hands after washing them with soap," says Nikola Djordjevic, a physician who co-founded the company MedAlertHelp.org, a medical alert service.
But he cautions that the sniff test and common test kits are not always enough to determine whether the water is drinkable. Other water safety experts agree that when it comes to water safety, a visual inspection is insufficient.
"Obviously bad water is easy to spot," says Chris Davis, an emergency medicine physician at the UCHealth University of Colorado Hospital. "But water contaminated with the bacteria that causes diarrhea often has no odor, and the water can appear clean."
Davis says the most comprehensive and accurate source of information on safe drinking water is the Centers for Disease Control and Prevention's country health database. Look for the Travelers' Health section, and search by destination. You'll find all sorts of useful tips, including whether the local tap water is safe.
For example, consider Tanzania. The CDC recommends avoiding tap or well water, ice made with tap or well water, drinks made with tap or well water (such as reconstituted juice), and unpasteurized milk.
Fred Gaines, a retired engineer who spent more than 45 years working in the sanitation and wastewater business, says it's impossible to be too careful.
"When you're travelling, use extreme caution," he says. "Even water that is considered safe to drink may contain microorganisms that might disagree with travelers accustomed to the water distributed in their community."
In other words, you could get the green light to drink the water and still get sick.
So what to do? Experts say you should err on the side of caution.