Causes water contamination is the question millions are asking as polluted tap and groundwater threaten health, ecosystems, and economies. The short answer: agriculture, industry, sewage and faecal matter, and drinking‑water treatment byproducts dominate—now joined by emerging chemicals like PFAS. This guide ranks top sources by prevalence and severity, explains how contaminants move through the water cycle, quantifies risks (cancer, birth defects, diarrhoeal disease), and shows practical ways to cut exposure. You’ll see real‑world case studies like Iowa nitrates and global faecal contamination, understand regulatory gaps, and learn what you can do at home and in your community. The goal is simple: help you reduce risk today, and advocate for safer drinking water tomorrow.
What causes water contamination? Key sources at a glance
When people ask “how does water become polluted?” they’re often thinking of a single spill or a broken pipe. In reality, most contamination comes from multiple sources acting at once across a watershed and a water system. Knowing the major sources helps you spot risks and choose the right fixes.
Top four drivers (fast scan): agriculture, industry, sewage/faecal, treatment byproducts
Agriculture is the largest nonpoint source in many regions and one of the main causes water contamination that affects rural and urban watersheds alike.
Fertilizers, manure, and pesticide applications can wash off fields during rain, seep through soil, and contaminate water with nitrate, phosphorus, and chemical residues. This runoff fuels algal blooms and dead zones and raises the risk of infant “blue baby syndrome” from nitrate in well water.
Industry and mining release heavy metals like arsenic, chromium‑6, and lead, as well as synthetic chemicals such as PFAS, making them key contributors to what causes water contamination in industrial regions. Hotspots often occur near manufacturing corridors, smelters, and mine tailings. These pollutants can persist in groundwater and surface water for years and carry cancer and organ‑damage risks.
Sewage and faecal contamination occur when wastewater is untreated or partially treated, when combined sewer overflows discharge after storms, or when septic systems fail. The result is pathogen exposure that spreads waterborne diseases such as cholera, dysentery, and typhoid, especially where sanitation is limited.
Drinking‑water treatment byproducts, including trihalomethanes (THMs) and haloacetic acids (HAA5), form when chlorine reacts with natural organic matter. These chemicals are linked to long‑term cancer and reproductive risks, showing that even treatment processes can contribute to causes water contamination if not managed carefully. They remind us that even treatment can create new contaminants if source water and disinfection are not managed carefully.
To round out the top five sources many readers search for: natural geologic sources can also contaminate drinking water sources. In some aquifers, arsenic or fluoride occurs naturally and can enter well water without any human spill. Human activity often worsens the issue by lowering water tables or changing chemistry that releases these elements.
Fast facts: scope and risk
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More than 200 million Americans are exposed to cancer‑linked chemicals in tap water; nitrate alone affects an estimated 263 million across 49 states.
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Arsenic above strict health guidelines affects over 109 million Americans and occurs in all 50 states.
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The WHO estimates 1.7 billion people use water contaminated with faeces, and about 1 million diarrhoea deaths per year are preventable with safer water and sanitation.
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PFAS has been reported in a large share of U.S. drinking water systems, with recent analyses suggesting near‑universal detection in some datasets.
What is the main cause of water contamination?
It depends on where you live. In rural watersheds, agriculture tends to dominate because nutrient and pesticide runoff is widespread and hard to control. In low‑sanitation areas, sewage and faecal contamination drive most disease. In industrial corridors and mining regions, metals, solvents, and PFAS are common. Many cities face mixed risks: stormwater, aging pipes, and treatment byproducts.
To put it simply, the main cause shifts with land use. Look at your local water source, nearby farms or factories, and the condition of your water distribution system to see which risks apply.
Pathways: How contaminants travel through the water cycle
Understanding pathways shows how different pollutants move from land to tap. It also shows why fixing only one step—like adding a filter at the plant—won’t solve upstream sources of water pollution.
From fields and streets to rivers and aquifers
Rain and snowmelt wash fertilizers, animal waste, motor oil, and microplastics off land and streets into streams. On farms with tile drainage, nutrients move rapidly through buried pipes straight to ditches and creeks. In cities, stormwater runoff rushes across pavement, carrying metals, tire particles, and trash to water bodies. Some contaminants infiltrate the soil and reach an aquifer, where they can persist for decades and contaminate the water in wells far from the original spill.
Pipes, plants, and byproducts
Treatment plants are designed to remove microbes and many chemicals, but the process can create new byproducts when disinfectants react with natural organic matter. Inside the pipe network, low flow or dead‑ends allow biofilms and particles to build up, and intermittent pressure can draw in contaminants through leaks. Old service lines and premise plumbing—especially with lead—can leach metals into clean water just before it reaches your tap.
Point vs. nonpoint sources (why it matters for control)
Point sources are direct discharges from a known outlet, like a factory pipe or a wastewater outfall. These are easier to permit, monitor, and reduce. Nonpoint sources are diffuse: farm fields, lawns, roads, construction sites. They add up across a landscape, making them harder to regulate. This difference influences policy and explains why some problems improve with enforcement while others need incentives and community action.
Data‑backed deep dive into major sources
Before we dive into the specifics, it helps to get a big-picture view of where water contamination comes from. From farms to factories, urban systems to our very own taps, pollutants travel in ways that affect both human health and the environment. The sections below break down the major sources, the risks they pose, and the strategies experts recommend to keep our water safe.
Agricultural runoff (nitrates, phosphorus, pesticides, animal waste)
Agriculture is a major source of water contamination because nutrients and pesticides move easily with rain. Nitrate is highly soluble, so it travels with percolating water into ground water. High nitrate in drinking water can cause infant methemoglobinemia (“blue baby syndrome”), which reduces oxygen delivery in the body. Researchers have also found links between long‑term nitrate exposure and certain cancers. On the ecosystem side, excess phosphorus and nitrogen trigger algal blooms that deplete oxygen and stress aquatic life.
In the U.S., nitrate affects tens of millions, with the Midwest showing some of the highest levels due to intensive corn and livestock production. Rural families using private wells face the greatest risk because private wells are not regulated like public systems and may not be tested often.
Solutions are well known, but they require steady use: precision fertilization to match crop needs, cover crops to hold nutrients through winter, grass buffers and wetlands to trap runoff, and improved manure storage and timing. These practices reduce how much pollutants leave the field and help protect both surface water and aquifers.
Industrial and mining discharges (PFAS, arsenic, chromium‑6, lead)
Industrial processes release toxic substances. PFAS chemicals resist heat and oil, so they’re used in many products and often persist in the environment. Metal mining and smelting can release arsenic, chromium‑6, and lead. These harmful chemicals have well‑documented links to cancer, endocrine and immune disruption, and organ damage.
Communities near manufacturing plants or historic mines often become hotspots. Here, source control makes the biggest difference: strong pretreatment, closed‑loop or zero‑liquid discharge systems, and better monitoring. For legacy contamination, advanced treatment (ion exchange, granular activated carbon, and in some cases membrane systems) can reduce exposure at the utility scale. For PFAS, destruction technologies, like high‑temperature or advanced oxidation processes, aim to break the carbon‑fluorine bond after filters capture the chemicals.
Sewage, faecal contamination, and urban stormwater
Faecal contamination is a leading cause of waterborne disease because it carries viruses, bacteria, and parasites. More than a billion people still use water with faeces. Diarrhoeal diseases take a heavy toll on children, but outbreaks can hit any community if wastewater treatment fails or a storm causes a combined sewer overflow. In suburban and rural areas, septic tanks can leak when overloaded or poorly maintained.
Reducing this burden starts with sanitation upgrades: reliable wastewater treatment, separate storm and sanitary sewers, and decentralized systems where sewers don’t reach. Chlorination and other disinfection steps protect against microbes, but source control and infrastructure improvements reduce the need to rely only on chemical barriers.
Drinking‑water treatment byproducts (HAA5, THMs)
Disinfection saves lives. But when chlorine meets natural organic matter, it creates byproducts like HAA5 and THMs. Over many years, these compounds are associated with increased cancer risk and potential reproductive effects. Large systems often comply with legal limits, yet residents can still face higher exposure at the ends of long pipe networks or where water ages in storage.
Utilities can reduce byproducts by removing organic precursors before chlorination, optimizing contact time and pH, switching to alternative disinfectants where appropriate, and adding granular activated carbon filters. These changes protect public health while keeping microbial protection intact.
Emerging contaminants and accelerating trends
As new chemicals and materials enter our world, our water faces challenges we haven’t fully seen before. What makes water unsafe to use? Emerging contaminants—from “forever chemicals” to pharmaceuticals, microplastics, and nanomaterials—can linger in water and pose uncertain health risks. The sections below explore what these substances are, how they get into water, and what technologies and practices are helping to keep them under control.
PFAS (“forever chemicals”)—persistence, exposure, and removal
PFAS have become the poster child for emerging contaminants because they don’t break down easily and can travel long distances. National surveys show PFAS are widespread in public water supplies and private wells. Health concerns include certain cancers, impacts on the immune system, and developmental risks.
At the tap, removal options include reverse osmosis (RO), anion exchange resins, and specialized granular activated carbon. Each method has trade‑offs: RO is highly effective across many contaminants but wastes some water and requires maintenance. Ion exchange is targeted but needs resin regeneration and careful disposal. Carbon can be effective but must be sized and replaced based on PFAS type and levels. The hard part is what happens after capture: safe disposal or true destruction is needed to avoid re‑releasing PFAS.
Pharmaceuticals, hormones, and personal care products
These substances reach water when we excrete them or flush unused pills. Wastewater plants are not designed to remove all such compounds, so traces can remain in rivers and sometimes pass into groundwater. Most exposures are at very low doses, but the long‑term effects of mixtures are still being studied. Advanced oxidation, activated carbon, and membrane processes can reduce these micro‑pollutants at the plant. Source control—safe drug take‑back and not flushing medicines—helps too.
Microplastics and nanomaterials
Microplastics come from synthetic clothes, tire wear, and broken packaging. They can carry other pollutants on their surface and may move through the food chain. Data on direct health effects from drinking‑water exposure are still limited, but precautionary steps like better stormwater management, improved filtration, and reduced plastic use can lower inputs. For nanomaterials, research is ongoing; utilities may need upgraded filtration where industrial sources contribute.
Are microplastics in drinking water harmful?
Current evidence suggests most particles are excreted, and risk from drinking water alone is not yet proven. However, some very small particles might cross gut barriers. Because data are incomplete, simple steps—reducing plastic litter, installing better filters in treatment plants, washing synthetic clothing in ways that reduce fiber release, and managing road runoff—are reasonable ways to cut exposure while research catches up.
Comparison table: traditional vs. emerging contaminants
| Category | Persistence in environment | Main health concerns | Typical sources | Common removal options |
| Nitrate/phosphorus | Moderate; move quickly through water | Infant methemoglobinemia; ecosystem blooms | Fertilizers, manure | Ion exchange (nitrate), RO, source control |
| Metals (arsenic, lead) | High; can bind to sediments or leach from pipes | Cancer, neurological effects | Geology, mining, old plumbing | Adsorptive media, RO, corrosion control |
| Disinfection byproducts (HAA5, THMs) | Form during treatment; vary | Cancer, reproductive risks | Chlorination + organic matter | Precursor removal, carbon, optimize disinfection |
| PFAS | Very high; “forever chemicals” | Cancer, immune and developmental effects | Industry, consumer products | RO, ion exchange, specialized carbon + safe disposal |
| Pharmaceuticals/hormones | Variable; low concentrations | Chronic, mixture effects | Excretion, improper disposal | Advanced oxidation, carbon, membranes |
| Microplastics | High; fragment but persist | Uncertain; transport of other pollutants | Textiles, tires, packaging | Source control, advanced filtration, stormwater capture |
Health impacts and vulnerable populations
Water quality affects everyone, but not equally. While contaminants can pose long-term health risks, some groups—like infants, pregnant people, and those relying on private wells—are especially vulnerable. The sections below explore the health impacts of common pollutants, which populations are most at risk, and practical ways to reduce exposure while keeping water safe for daily use.
Cancer and chronic disease links (arsenic, chromium‑6, nitrate, HAA5)
Exposure risk depends on dose and duration. Arsenic in groundwater is linked to skin, lung, and bladder cancers and other chronic diseases. Chromium‑6 is a carcinogen at certain doses. Nitrate may be associated with some cancers when combined with other conditions in long‑term exposure. HAA5 and THMs are linked to increased cancer risk over decades of use. Because many people face mixtures rather than single contaminants, it’s smart to minimize exposure across the board.
Infants, children, and pregnancy risks
Infants are especially vulnerable to nitrate in well water, which can lower blood oxygen and cause blue baby syndrome. Lead from plumbing harms brain development and behavior in children, even at very low levels. Some disinfection byproducts and PFAS are associated with pregnancy complications. Schools and daycares located in buildings with older pipes or fountains need regular testing and maintenance to ensure safe drinking water quality.
Infectious disease burden from faecal contamination
Faecal contamination carries germs that cause diarrhea, cholera, and other infections. Outbreaks rise after floods or sewer overflows and during warm seasons that favor pathogen growth. WASH interventions—safe water, sanitation, and hygiene—reduce disease. In homes, simple behaviors like handwashing and safe storage of treated water can lower risk dramatically when sanitation is lacking.
Is tap water safe if it meets legal limits?
Legal limits are important, but they don’t always match the newest health research or reflect mixture effects. Limits also do not cover every contaminant. If your water meets all regulations, it is generally safe to use; however, some families still choose extra protection like water filtration to reduce specific risks (for example, PFAS or lead). This is a personal risk decision based on local reports, household members (like infants), and budget.
Real‑world case studies and lessons learned
Looking at real-world examples makes the challenges of water contamination more tangible. From nitrate spikes in Iowa to PFAS spread across the U.S., and global faecal contamination issues, these case studies reveal patterns, successes, and ongoing gaps. The stories below highlight lessons learned and practical strategies that can guide better water management everywhere.
Iowa’s nitrate crisis—agriculture and rising cancer rates
Heavy fertilizer use, tile drainage, and livestock manure have pushed nitrate levels higher in rivers and some wells. In summer 2025, some utilities faced treatment challenges and restrictions as nitrate spiked after spring rains. Communities debated how to balance crop yields with water quality, and pilot projects—cover crops, saturated buffers, and targeted wetland restoration—showed promise. The lesson: voluntary practices help, but scaling them takes funding, technical support, and clear goals.
PFAS spread across the U.S. (2025)
Community testing continues to find PFAS in large portions of the country. Many utilities are installing new filters, while households near hotspots are using RO systems or point‑of‑use carbon filters while long‑term fixes are built. Funding gaps and disposal challenges remain, and public pressure is pushing for phase‑outs and safer chemicals.
Global faecal contamination burden
Roughly 1.7 billion people use water with faeces, and outbreaks are common where sanitation is weak. Countries that invested in sewers, reliable disinfection, and household hygiene saw sharp drops in diarrhoeal disease. The lesson is clear: clean water alone is not enough without safe sanitation and hygiene.
Regulations, standards, and policy gaps
Regulations and standards shape how we protect water, but rules alone don’t guarantee safety. From WHO guidelines to EPA limits, each system has strengths—and gaps. The sections below explore who sets the rules, where legal limits may fall short of health science, and how monitoring, enforcement, and shared responsibility play a role in keeping water clean.
Who sets the rules? WHO, EPA, and national regulators
The World Health Organization (WHO) issues health‑based guidelines. Countries adopt their own standards. In the U.S., the Environmental Protection Agency enforces the Safe Drinking Water Act and sets Maximum Contaminant Levels (MCLs) for many chemicals and microbes. States can set stricter standards. For private wells, owners are responsible for testing and maintenance.
Gaps between legal and health‑based limits
Some MCLs are older and may not reflect new science or mixture effects. For example, legal limits exist for arsenic and nitrate, but studies suggest risk may start below those levels for some groups. Disinfection byproducts are regulated, yet sensitive populations might want further reductions. And many emerging contaminants—like PFAS chemicals in the past—may lack enforceable national limits until risk assessments and rules are finalized.
Monitoring, reporting, and enforcement challenges
Small water systems can struggle with testing costs, staffing, and aging infrastructure. Underfunded labs slow sample analysis. Some contaminants, like PFAS, require advanced methods and specialized equipment. Citizen science and transparent Consumer Confidence Reports help, but they don’t replace strong oversight and funding to fix the root causes.
Who is responsible for preventing water contamination?
Everyone shares responsibility. Utilities treat and deliver safe water. Industry must prevent releases and treat waste. Farmers and land managers reduce runoff. Households maintain septic systems and choose safer products. Regulators set rules and enforce them. Communities advocate for investments and hold polluters accountable.
How to stop water pollution: prevention and solutions—from watershed to tap
Stopping water pollution requires action at every level—from entire watersheds down to the tap in your home. Utilities, farmers, industries, and households all play a role. The sections below cover practical solutions, from large-scale treatment and infrastructure upgrades to agricultural best practices, industrial controls, and household testing and filtration, helping ensure safe water for people and the environment.
Utility‑scale treatment and infrastructure upgrades
Utilities can add treatment layers targeted to local risks. Ion exchange can cut nitrate. Adsorptive media can remove arsenic. RO and advanced carbon can capture PFAS and many other chemicals. Corrosion control reduces lead leaching, and replacing lead service lines removes the source. Smart monitoring—continuous sensors and targeted sampling—catches problems early. Upgrading storage tanks and boosting flow in dead‑ends can lower byproducts and biofilm growth.
Agricultural best practices and incentives
Nutrient management plans set the right rate, source, and timing of fertilizer. Cover crops reduce erosion and hold nutrients through winter. Riparian buffers and restored wetlands trap sediment and phosphorus before it reaches streams. Livestock operations can improve manure storage, avoid spreading before storms, and expand treatment. Incentives, cost‑share programs, and market rewards (like water‑friendly certification) help bring practices to scale.
Industrial wastewater controls and safer chemistry
Pretreatment and closed‑loop water reuse cut discharges. Phase‑outs of high‑risk substances, safer chemical substitutions, and process changes reduce the chance to contaminate water at the outset. Environmental audits and third‑party certifications keep improvements on track. Clear permits and strict enforcement create a level playing field for firms that invest in better controls.
Household actions: testing and filtration
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If you face bacteria or protozoa: use a certified ultraviolet (UV) disinfection unit or boil during advisories. Filters alone don’t kill viruses without a disinfection step.
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If nitrate is elevated: consider ion exchange or RO rated for nitrate.
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For arsenic: use adsorptive media rated for arsenic or RO; test periodically to confirm performance.
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For lead: install certified point‑of‑use filters at drinking taps and replace cartridges on time; also address corrosion control and consider replacing lead service lines.
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For PFAS: choose RO, anion exchange, or specialized carbon filters rated for PFAS; replace media on schedule.
Boil advisory basics (step‑by‑step)
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Use bottled or boiled water for drinking, cooking, brushing teeth, making ice, and preparing baby formula.
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Bring water to a rolling boil for at least one minute (three minutes at high elevation), then cool.
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Do not rely on filters to remove microbes unless they include a certified microbiological purifier or UV.
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Flush plumbing and discard ice after the advisory ends, following local guidance.
Conclusion
Water contamination comes from multiple sources—agriculture, industry, sewage, treatment byproducts, and natural factors. Understanding the reasons for water pollution helps you take action, from supporting safer practices and regulations to testing and filtering water at home. With awareness and proactive steps, we can protect both health and the environment and ensure safer water for everyone.
FAQs
1. What are the causes of water contamination?
Water contamination happens when harmful substances or microbes get into our drinking water or natural water bodies. The causes are actually a mix of human activity and natural factors. On the human side, agriculture is a big player: fertilizers, manure, and pesticides can wash into rivers, lakes, and groundwater. Industry and mining release chemicals like PFAS, heavy metals like arsenic and lead, and solvents that can persist in water for years. Then there’s sewage and faecal contamination—think untreated wastewater or overflowing sewers—which spreads bacteria and viruses. Even our attempts to treat water can backfire a bit, because disinfection can create byproducts like THMs and HAA5. And don’t forget natural sources: some aquifers naturally contain arsenic or fluoride, which can affect wells without any human spill. So really, contamination is rarely caused by just one thing—it’s usually a combination of multiple sources interacting across a watershed.
2. What are the 5 sources of water contamination?
The five main sources you should know about are:
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Agriculture – runoff from fertilizers, manure, and pesticides. These nutrients can enter water and cause problems like algae blooms and health risks such as blue baby syndrome.
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Industry and mining – factories and mines release heavy metals (arsenic, lead, chromium‑6) and persistent chemicals like PFAS, which can stick around in the environment for decades.
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Sewage and faecal contamination – untreated or partially treated wastewater, septic tank leaks, and combined sewer overflows. These bring pathogens into the water.
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Drinking‑water treatment byproducts – chemicals like THMs and HAA5 that form when disinfectants react with natural organic matter during water treatment.
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Natural geologic sources – things like arsenic and fluoride can naturally occur in groundwater, sometimes exacerbated by human activity like over-pumping or altering water chemistry.
These five sources cover most of the contamination people face, whether in urban or rural settings.
3. How to tell if tap water is contaminated?
Honestly, just looking at or tasting your water won’t tell you the full story. Some contaminants are invisible and tasteless. But there are a few ways to check: first, if you’re on public water, read your annual Consumer Confidence Report—it shows tested levels of common contaminants. For private wells or deeper insight, use a certified lab to test for bacteria, nitrate, arsenic, and lead at least once a year. At-home screening kits can give a rough idea, but they aren’t as reliable as lab tests. You might notice odors like chlorine or “rotten egg” smells, which can hint at issues, but these are not foolproof. So, the safest bet is testing.
4. What makes water unsafe for use?
Water becomes unsafe when it contains harmful microbes, chemicals, or byproducts above health-based limits. Microbes include bacteria, viruses, and parasites that can cause diarrheal disease or more serious infections. Chemicals might be heavy metals like lead or arsenic, synthetic pollutants like PFAS, or industrial solvents. Even water that started clean can pick up contaminants if it flows through old pipes or cross-connections. So unsafe water isn’t always obvious—it might look fine but still pose a health risk, especially for infants, children, and pregnant women.
5. What is the meaning of contaminate water?
To contaminate water means to introduce something harmful into it—this could be a chemical, microbe, or any substance that lowers its quality and poses a risk to health or the environment. It’s not just “dirty water”; even small amounts of certain chemicals like PFAS or nitrates can contaminate water without changing how it looks or tastes. Essentially, contamination turns safe water into something that could make people or ecosystems sick.
6. What happens if your water is contaminated?
The effects depend on the type and level of contamination. Microbes can cause stomach upset, diarrhea, or more severe waterborne diseases like cholera or typhoid. Long-term chemical exposure—like arsenic, nitrates, or THMs—can increase cancer risk or affect organ function. Children and infants are particularly vulnerable: nitrate in well water can cause blue baby syndrome, and lead can harm brain development. Ecosystems can also be affected—nutrient runoff triggers algae blooms and dead zones that kill fish. How you respond depends on the contaminant: boil water for microbes, avoid tap water for chemicals, or use bottled/filtered water for sensitive groups.
7. Can RO water be contaminated?
RO (reverse osmosis) systems are excellent at removing many pollutants—nitrates, PFAS, heavy metals, and more—but they aren’t perfect. If filters or membranes aren’t properly maintained, microbes can grow on cartridges, leading to recontamination. Storage tanks or faucets can also get dirty. RO doesn’t remove all gases or volatile chemicals, so additional treatment might be needed for substances like radon or solvents. Basically, RO water is very safe if the system is maintained correctly, but neglecting it can compromise its effectiveness.
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