The World Health Organization notes that only about 73% of the world's population have access to drinking water that is both nearby and free from contamination, leaving billions of people who aren't afforded such a basic luxury. Even worse, at least 1.7 billion people have to drink from a water source that's contaminated by human waste.  

And while there are notable exceptions such as the Flint water crisis, those of us living in the United States have few reasons to worry about such the safety of our drinking water. "In relationship to the risk of illness associated with other countries around the world, our drinking water ranks fairly highly as we use advanced testing measures to ensure that drinking water is safe and free from both chemical and microbial contaminants," says Rachel Noble, a distinguished professor of marine sciences at the University of North Carolina at Chapel Hill.

Is tap water safe to drink? 

Within the U.S., tap water sources are heavily regulated by the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act of 1974. Because of this bill, the EPA is able to set standards and regulations for public drinking water – protecting people from disease-causing germs, contaminants and harmful chemicals. As a result, "the vast majority of American tap water is clean and safe for drinking and everyday use," says Noble.

In addition to providing standards and rules about what can and can't be in public drinking water, the EPA ensures that drinking water treatment plants within the country "use advanced technologies to remove contaminants to ensure water safety," says Rianna Murray, director of the graduate studies department of Global, Environmental, and Occupational Health at the University of Maryland School of Public Health. 

These treatment facilities filter out more than 90 known contaminants, including pathogens, pollutants, heavy metals such as arsenic, and inorganic and organic chemicals and their harmful byproducts, "so the water is microbially safe to drink," says Susan Richardson, the executive editor of the peer-reviewed journal "Environmental Science & Technology" and a professor of chemistry at the University of South Carolina.

It's worth noting that all these standards and practices only apply to public drinking water, however, and do not apply to bottled water. Bottled water sold within the country is regulated by the U.S. Food and Drug Administration, which has its own standards and regulations.

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How can tap water become unsafe? 

Despite all these measures to keep public water sources safe, there are occasions where chemicals or other contaminants can still temporarily impact the safety of drinking water. 

This occurs when unsafe levels of heavy metals, harmful germs or chemicals get in the water at its source (ground water or water from lakes, reservoirs or rivers) or while water is traveling through the distribution system after the water treatment plant has already removed germs and chemicals, per the Center for Disease Control and Prevention. 

Among the ways contaminants can enter public water sources include "accidents that release industrial chemicals or fracking wastewaters that can also enter water supplies," says Richardson. 

Murray says that lead contamination can occur due to aging infrastructure or in older homes where lead pipes, service lines or plumbing fixtures still exist. "Microorganisms such as bacteria, viruses and parasites can also contaminate our drinking water sources, leading to waterborne diseases," she says. Such contamination may occur through sewage spills, agricultural runoff, animal waste or other sources of pollution that get into public water supplies. 

"While incidents of contamination or water quality issues do occur, we have systems in place to address problems quickly – including regulatory oversight, public notification and corrective actions by water utilities," says Noble. 

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How to know if your drinking water becomes unsafe?

In addition to paying attention to the news and related notifications, it's also possible to recognize contaminated water by its appearance (be wary of cloudy, foamy or off-colored water), by its smell (avoid water that has a sulfuric odor or smells like chlorine or swampland) and by its taste (steer clear of water that tastes salty, oily, fishy, gritty or metallic.) 

Suspicions of contaminated water can be reported to your utilities company, your local health department or directly to the EPA. "You can also learn the cleanliness of your tap water by using at-home water testing kits," says Murray.

It can also be helpful to protect against any potential contaminants from entering your home in the first place. This doesn't mean opting for bottled water, "because bottled water does not go through the same rigorous testing that tap water does and we contribute to the plastic problem when we use too much of it," says Richardson; but to instead consider installing a water filtration system, a reverse osmosis filter or an ultraviolet water purification system.

"It might not be economically viable for all Americans, but if people can afford it, I often recommend countertop filtration systems," says Noble.

She stresses again that "the number of outbreaks associated with tap water in the United States is relatively low per person, but by staying informed and by using commercially available filtration units, people can reduce their risk of tap-water exposure to chemicals and microbial contaminants."