Squeamish? Proceed with caution… 

From rocket fuel to flesh-eating bacteria, what’s hiding in your tap could be straight out of a horror movie. Some dangers are rare. Others are shockingly common. All of them raise questions about what we’re really drinking.

Of the 324 different contaminants detected in U.S. tap water supplies, the following are the 11 nastiest—in no particular order.

And just to be clear, nastiest doesn’t always mean most dangerous. While many come with serious health concerns, this list focuses on the grossest, stomach-turning stuff you’d never actually expect to find in your water.

Our apologies in advance.

Imagine drinking someone else’s prescription meds…

As crazy as it sounds, traces of hundreds of medications from birth control hormones and antibiotics to antidepressants and painkillers have been legally found in U.S. tap water. 

How does that happen?

Most medications aren’t fully broken down by the body, so they’re released in urine or flushed. Others are tossed down the drain or toilet when people dispose of unused prescriptions. Since most water treatment plants can’t filter them out, the leftovers can end up in your glass. 

Worse, this is more common than you might think. 

Pharmaceuticals have been found in water served to at least 46 million Americans and in at least 24 major metro areas from Philadelphia to Los Angeles. Since these drugs can seep into underground aquifers (which supply up to 40% of the nation’s tap water), actual exposure may be even more widespread. 

At the time of publication, the EPA has no standards or regulations in place to protect our tap water from pharmaceuticals. That means even though they’ve been detected in our tap water, contamination remains completely legal.

Yes, fecal matter is exactly what you think it is. Poop. In your water. 

Heavy rains, flooding, and sewer overflows can send human waste and animal feces straight into rivers, reservoirs, and aquifers. Failing septic tanks and crumbling pipes only add to the contamination.

The result is E. coli and coliform bacteria in your tap water. 

While exact figures in the U.S. are harder to track, in 2015 alone, more than 5,700 American water systems failed to meet federal coliform standards. Plus, mere months before this article was first published, boil water advisories were issued in both Massachusetts and California due to related outbreaks. 

While the EPA does regulate coliform bacteria, violations and outbreaks still happen. 

In other words, legal limits aren’t always enough to fully protect you from fecal matter or other nasty contaminants that can sneak into public water supplies.

Originally developed by the U.S. military in the 1960s, AFFF (aqueous film forming foam) was designed to quickly smother jet fuel and chemical fires. It became standard at military bases, airports, and industrial sites.

The problem?

AFFF contains PFAS (per- and polyfluoroalkyl substances) linked to cancer, hormone disruption, liver damage, developmental issues, and more.

For decades, PFAS-laced foam was sprayed during emergency responses, routine training, and equipment testing, allowing it to soak into soil and groundwater, especially around military bases and airports.

Now, we’re paying the price… 

These “forever chemicals” don’t break down in the environment and can persist in water, soil, and the human body for decades.

The Department of Defense (DoD) planned to transition to PFAS-free foams by October 2024, but the deadline has been extended. Meanwhile, many airports and private facilities have been slow to adopt safer alternatives due to logistical challenges, costs, and more. 

AFFF may have saved lives in fires, but it's left a toxic legacy in our water. And the cleanup has barely begun.

It sounds like science fiction, but flesh-eating bacteria in water is very real. 

Aeromonas hydrophila is a naturally occurring bacteria that thrives in warm, stagnant water. It has been detected in more than 14% of public water systems tested across 35 U.S. states.

Although no widespread outbreaks have been linked directly to municipal tap water, serious infections have been documented when open wounds have been exposed to contaminated freshwater (such as lakes, rivers, or even neglected pipes and plumbing systems). 

The EPA has placed Aeromonas on its Contaminant Candidate List (CCL)—a federal watchlist of unregulated substances that may pose health risks. But there are currently no safety limits or federal regulations in place.

In short, it’s nothing to panic about, but it is something to know about.

Naegleria fowleri is a single-celled amoeba that lives in warm, fresh water and has been detected in tap water in states like Louisiana and Florida.

Thankfully, you can’t get infected by drinking Naegleria. But if contaminated water enters through your nose—typically during sinus rinsing or swimming—it can travel to the brain and cause primary amebic meningoencephalitis (PAM), a rare but almost-always fatal brain infection.

That’s why the CDC recommends using distilled, sterile, or boiled-and-cooled tap water for any kind of nasal irrigation, like with a neti pot.

It’s like watering your body with plant poison…

After rain or irrigation, both chemicals wash off fields and seep into rivers, reservoirs, and underground aquifers, where they’re incredibly difficult to remove.

Nitrate is legally allowed in tap water up to 10 mg/L, a limit set back in 1962 to protect infants from “blue baby syndrome,” a potentially fatal condition that interferes with oxygen in the blood. However, more recent research suggests legal levels of nitrate could be far too high. In fact, researchers in Europe have found increased risks of colorectal cancer at levels 10 times lower than the current U.S. legal limit. Plus, other studies have linked long-term exposure to thyroid cancer, ovarian cancer, and other serious diseases.  

Glyphosate, meanwhile, has an MCL of 700 ppb. This limit is much higher than advisory levels from groups like the EWG, which recommends a much stricter guideline of 5 ppb to help minimize cancer risk.

Think: diarrhea-inducing bugs that chlorine might not kill.

They’re tiny, nasty, and yes… they’ve been found in U.S. tap water. 

In fact, the largest waterborne disease outbreak in U.S. history was caused by Cryptosporidium. In 1993, over 400,000 people in Milwaukee WI suffered from diarrhea, after the city’s water supply was contaminated with untreated runoff even though it had been treated with chlorine. 

Giardia, while slightly more susceptible to standard water treatment, is also common in contaminated water and can cause nausea, diarrhea, stomach cramps, dehydration, and fatigue.

The EPA regulates both under the Surface Water Treatment Rule, which requires public water systems to use filtration or proven disinfection methods. Still, outbreaks can occur—especially after heavy rain, flooding, or system failures. 

For example, among 29 waterborne Giardia outbreaks reported from 2012-2017, nine were linked to tap water systems (municipal systems or private wells). From 2015–2020, five tap water outbreaks were tied to Cryptosporidium or Giardia. 

It’s all proof that even treated tap water isn’t immune to contamination.

Radioactivity and drinking water should never mix. But in much of the U.S., they already do.

Naturally occurring radioactive elements in soil and rock can leach into groundwater and surface water, which many communities rely on for drinking water. The risk grows dramatically when human activity accelerates the process. Fracking, mining, and drilling can disturb underground rock layers, releasing radioactive particles into nearby water sources.

And it’s more widespread than you might think…

According to an EWG analysis of water utility tests from 2010-2015, more than 170 million Americans were served drinking water that contained radioactive elements at levels that may increase cancer risk. 

Radium is linked to bone cancer. Uranium is associated with kidney damage. And Strontium-90 has been tied to bone cancer and leukemia. 

The EPA does set maximum contaminant levels (MCLs) for these substances, but there’s a catch: The current legal limits were established in 1976 and haven’t been updated since. That means today’s regulations may be dangerously outdated.

During the Cold War, perchlorate was regularly used in missile manufacturing, rocket testing, and explosives—especially near military and defense facilities. Waste from these operations was often dumped or poorly stored, allowing perchlorate to seep into nearby soil and groundwater. 

Many of those contaminated sites were never fully cleaned up. So even decades later, perchlorate continues to leach into drinking water supplies today. 

Meanwhile, modern use only makes things worse: Today, perchlorate is still used in fireworks, flares, airbags, and more. 

Without effective water filtration, the colorless, tasteless contaminant can travel miles underground and persist for years without breaking down. That’s why more than 20 million Americans may be exposed through their tap water. 

Perchlorate is known to disrupt thyroid function, which is especially concerning for pregnant women, infants, and anyone with thyroid conditions.

And yet, there’s still no enforceable federal limit for perchlorate in U.S. tap water. However, that should change soon: in 2023, a federal court ordered the EPA to propose a regulation by November 2025, with a final rule due by May 2027.

It's like drinking fabric cleaner or engine wash. Not exactly what you want in your water…

PCE (tetrachloroethylene) was once the go-to solvent for dry cleaning thanks to its ability to remove grease and stains from fabric. Classified as a likely human carcinogen, PCE has been linked to several cancers and has caused widespread, long-term contamination—particularly near dry cleaners. Spills, leaks, and improper disposal have allowed it to seep into soil and groundwater, where it can persist for decades.

TCE (trichloroethylene), a metal degreaser once commonly used on military bases and in factories and aircraft hangars, is just as alarming. Considered a known human carcinogen, TCE contaminates water through industrial runoff and underground chemical plumes (slow-moving clouds of contamination that spread through spill and groundwater over time). 

Both chemicals are invisible, persistent, and dangerous in even small amounts. 

On December 17, 2024, the EPA banned all uses of TCE and many uses of PCE (including dry cleaning) to help protect public health. The TCE ban is already in effect with specific exceptions, while PCE use is being phased out over 10 years. 

The EPA’s Maximum Contaminant Level Goal (MCLG) for both is zero—meaning no amount is considered truly safe. However, the enforceable legal limit (MCL) is currently set at 5 parts per billion (ppb) each, due to limitations in detection and treatment technology. 

At this time, the EPA bans are at ongoing risk of delays and revisions. Until both chemicals are completely phased out, you could be sipping water with trace amounts of what used to clean jet engines and tuxedos.

Imagine turning on your tap and smelling gasoline… 

That’s the reality for some communities exposed to MTBE (methyl tertiary-butyl ether)—a chemical once added to gasoline nationwide to help make fuel burn cleaner and reduce air pollution. But what it did for the air, it nearly undid for the water.

MTBE is highly soluble, meaning it spreads quickly through groundwater. Even a tiny gasoline leak—from an underground storage tank or pipeline—can contaminate large quantities of drinking water. And with tens of thousands of gas stations and pipelines scattered across the country, these leaks are far from rare.

According to the EWG, MTBE has been found in public water supplies in 26 states. Though it’s now banned or restricted in more than 25 states, the EPA has never issued a national drinking water standard, and legacy contamination remains a major concern.

It only takes a few drops to ruin an entire water supply. And decades to clean it up.

Here are just a few more disturbing things that have been found in U.S. tap water supplies: 

• Vinyl chloride: This colorless gas used to make PVC pipes—the stuff that lines sewer systems—is a known human carcinogen and was the chemical at the center of the 2023 East Palestine, OH train disaster. 

• Microplastics: These teeny fragments of plastic—shredded from bags, bottles, and synthetic clothes—have been found in 94% of tested U.S. tap water. While the science is still emerging, studies suggest microplastics can lodge in everything from brain tissue to reproductive organs.

• Atrazine (Herbicide): Some studies show this potent weed-killer turned male frogs into egg-laying females. Though it’s been banned in more than 60 countries, atrazine is still sprayed on U.S. crops today. And that means it can still wash into our water supplies. 

With new threats and deeper research emerging all the time, the scary truth is this list could soon be much longer.

Want to know exactly what’s in your tap water? 

Simply enter your zipcode here to get a free report that reveals the specific contaminants in your local tap water supply, risks associated with each, and personalized recommendations to protect yourself and your loved ones from the dangers flowing from your tap. 

And if you’re ready to upgrade your water, tap here to shop our entire collection of best-selling, advanced filtration systems dedicated to your health and safety. 

While ordinary water filters protect you from as few as 5 contaminants, ours target up to 365+ dangers—including the nastiest stuff in your tap. 

Note: For microbiological threats like Giardia or Cryptosporidium, disinfection (like boiling or UV light) is recommended in addition to filtration.