Water and Our Health

Water and the Human Body

Water constitutes approximately 60 percent of adult body weight and a higher proportion in infants and children. It functions as the medium for nearly all biochemical reactions and participates directly in temperature regulation, nutrient transport, waste elimination, and the maintenance of blood volume and cellular structure.

Total body water is distributed across two principal compartments. Intracellular fluid, contained within cells, accounts for approximately two-thirds of body water. Extracellular fluid, which includes plasma and the fluid surrounding cells, accounts for the remaining third. Movement between compartments is regulated by osmotic gradients and by the kidneys, which adjust water excretion to maintain plasma osmolality within a narrow range.

Water’s effect on health depends on two independent variables:

Quantity of intake: Whether daily consumption meets physiological requirements for the individual, accounting for age, body size, activity level, climate, and clinical conditions.
Quality of intake: What the water contains beyond H2O includes dissolved minerals, treatment chemicals, contaminants introduced during distribution, and substances leached from containers or plumbing.

Each variable is discussed separately in the sections that follow. Insufficient intake produces one category of health effects. Chronic exposure to contaminants produces another. The two are largely independent, and adequate hydration does not compensate for poor water quality.

Dehydration and Underhydration

Dehydration and underhydration describe different physiological states. Dehydration is an acute or subacute body water deficit sufficient to affect physiological function, typically corresponding to a loss of 1 percent or more of body weight as water. Underhydration is a chronic pattern of intake below recommended levels that may not produce clinical dehydration but is associated with adverse health outcomes over time.

Both are common in the general population, and both are underdiagnosed.

Prevalence in the United States

Clinical dehydration is particularly prevalent in older adults. Reported prevalence ranges from 17 to 28 percent among older adults in the United States, with higher rates observed in institutional settings such as nursing homes.

Chronic underhydration extends across a broader population. Analysis of National Health and Nutrition Examination Survey (NHANES) data found that:

72 percent of men aged 51 to 70 failed to meet hydration criteria based on serum osmolality.
66 percent of women aged 51 to 70 failed to meet the same criteria.
Similar patterns extend into younger and older age groups.

The NHANES analysis used serum osmolality above 300 mOsm/kg as the threshold for underhydration, a physiologically meaningful marker that reflects actual hydration status rather than self-reported intake.

Physiological Requirements

The Institute of Medicine established Adequate Intake values for total water at approximately 3.7 liters per day for adult men and 2.7 liters per day for adult women, from all sources, including food. Approximately 20 percent of daily water intake typically comes from food, with the remainder from beverages.

These values represent averages for temperate climates and sedentary to moderately active adults. Actual requirements vary substantially with:

Body size: Larger individuals require more water.
Physical activity: Sweat losses can exceed one liter per hour during vigorous exercise in heat.
Climate: Hot or humid conditions increase insensible water loss.
Clinical conditions: Fever, diarrhea, vomiting, uncontrolled diabetes, and certain medications increase requirements.
Life stage: Pregnancy, lactation, and advanced age modify normal requirements.

Documented Health Effects

Mild dehydration, defined as a body water deficit of 1 to 2 percent, produces measurable declines in cognitive performance, mood, and physical endurance. Effects documented in controlled studies include reduced short-term memory, impaired attention, slower reaction times, and reduced tolerance for physical exertion.

Sustained inadequate intake has been associated with:

Nephrolithiasis (kidney stones): Concentrated urine promotes crystal formation.
Urinary tract infection: Reduced urine flow decreases the mechanical clearance of pathogens.
Impaired thermoregulation: Reduced sweat capacity increases the risk of heat illness.
Constipation: Reduced colonic water content affects stool consistency.

The NHANES analysis identified statistical associations between underhydration and higher prevalence of obesity, insulin resistance, type 2 diabetes, hypertension, and metabolic syndrome. The same analysis observed an association with chronic disease mortality across a three- to six-year follow-up period.

These findings describe statistical associations rather than established causation. Confounding variables, including diet composition, socioeconomic factors, and health behaviors, may contribute to observed relationships. The direction of the evidence is nonetheless consistent across multiple studies and populations, and the mechanisms proposed to link chronic underhydration to metabolic disease are physiologically plausible.

Tap Water

Municipal water treatment in the United States operates under the Safe Drinking Water Act, enacted in 1974 and administered by the Environmental Protection Agency. Treated water reaching community water systems is generally free of pathogens that can cause acute illness. This public health achievement has led to the near-elimination of waterborne epidemic disease.

Chemical composition and contaminant profiles vary substantially by source water, treatment approach, and distribution infrastructure.

Chemicals Added During Treatment

Standard municipal treatment introduces multiple chemical agents at defined concentrations. Each serves a specific engineering purpose:

Aluminum sulfate: A coagulant that binds suspended particles into larger aggregates for removal.
Ferric chloride: Improves settling of particulates. At some source reservoirs, ferric chloride is added upstream of the treatment plant to control phosphorus and limit algal growth.
Calcium hydroxide (lime): Raises pH to reduce pipe corrosion and improve coagulation efficiency.
Chlorine: Primary disinfectant used by the majority of United States utilities.
Chloramine: A more stable secondary disinfectant used by approximately one-third of United States utilities, formed by combining chlorine with ammonia.
Ammonia: Precursor to chloramine formation.
Stannous chloride, sodium silicate, zinc orthophosphate, sodium hydroxide: Corrosion inhibitors that reduce metal leaching from distribution infrastructure.
Fluoride: Added to many United States water supplies at approximately 0.7 mg/L to prevent dental caries.

Concentrations are regulated and monitored. The chemicals themselves are not removed before consumption; they remain in the finished water at their treatment concentrations, with some undergoing chemical reactions that produce additional compounds discussed below.

Disinfection Byproducts

Chlorine and chloramine react with naturally occurring organic matter present in source water. These reactions produce chemical compounds collectively termed disinfection byproducts (DBPs). The two federally regulated groups are:

Trihalomethanes (THMs): Include chloroform, bromodichloromethane, dibromochloromethane, and bromoform. Regulated at a maximum of 80 micrograms per liter as a running annual average.
Haloacetic acids (HAAs): A group of five acids regulated together at 60 micrograms per liter.

The National Cancer Institute reports that long-term exposure to disinfection byproducts has been associated with bladder and rectal cancers in multiple epidemiological studies. A cumulative risk analysis estimated that the lifetime cancer risk from disinfection byproducts, based on epidemiological data, approaches 3 cases per 1,000 people served by community water systems.

Exposure to DBPs occurs through three routes:

Ingestion: Drinking treated water.
Dermal absorption: DBPs cross the skin during bathing and showering.
Inhalation: Volatile DBPs vaporize into the shower and bath air, where they are inhaled.

The dermal and inhalation routes contribute meaningfully to total DBP exposure. EPA-funded research predicted that dermal absorption during a 10-minute shower can exceed 30 percent of the equivalent ingestion dose for trihalomethanes.

Disinfection is necessary to prevent waterborne infectious diseases, and the benefits substantially outweigh the risks of DBP exposure at regulated concentrations. Point-of-use treatment can reduce DBP exposure without compromising the microbial safety of the distributed water supply.

Per- and Polyfluoroalkyl Substances (PFAS)

Per- and polyfluoroalkyl substances are a class of synthetic compounds characterized by carbon-fluorine bonds resistant to environmental degradation. The class includes thousands of individual chemicals, of which perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) are the most extensively studied.

PFAS have been used since the 1940s in stain-resistant coatings, nonstick cookware, water-repellent fabrics, food packaging, and firefighting foams. Their environmental persistence has led to the informal designation “forever chemicals.”

Prevalence in Drinking Water

A 2023 United States Geological Survey study using a validated analytical method estimated that at least 45 percent of the nation’s tap water contains one or more PFAS compounds. Detections were higher in urban areas and in regions with historical industrial PFAS use, but rural and remote supplies also showed contamination.

Federal Regulation

PFAS drinking water regulation has undergone rapid change:

April 2024 – EPA established the first legally enforceable federal PFAS drinking water limits, setting maximum contaminant levels of 4 parts per trillion for PFOA and PFOS. Additional MCLs of 10 parts per trillion were set for PFHxS, PFNA, and HFPO-DA (GenX chemicals). A hazard index applied to mixtures of four PFAS was also established. Water systems were required to complete initial monitoring by 2027 and to comply with MCLs by 2029.
May 2026 – EPA proposed two rules significantly narrowing this framework:
- A proposed rescission rule would eliminate federal limits on PFHxS, PFNA, and GenX chemicals, as well as the hazard index for mixtures. Federal regulation would remain in place only for PFOA and PFOS.
- A proposed compliance extension rule would delay the PFOA and PFOS compliance deadline from 2029 to 2031.

Health Concerns

Peer-reviewed epidemiological and toxicological research has linked PFOA and PFOS exposure to elevated cholesterol, reduced vaccine response in children, increased risk of certain cancers (including kidney and testicular cancer), thyroid disease, low birth weight, and pregnancy-induced hypertension. Half-lives of PFOA and PFOS in the human body are measured in years, meaning that ongoing exposure leads to bioaccumulation.

Lead

Lead enters drinking water through corrosion of plumbing materials containing lead, including service lines connecting water mains to individual buildings, older solder in copper piping, and brass fixtures. Water with low pH or low mineral content accelerates corrosion.

Scale of the Problem

The EPA estimates that approximately 9.2 million lead service lines remain in use across the United States. Concentrations of lead service lines are highest in older cities and in the Midwest and Northeast, though lines exist across every state.

Health Effects

No safe threshold for lead exposure has been established. Children are particularly vulnerable, as lead readily crosses the developing blood-brain barrier and interferes with neurodevelopment. Documented effects of childhood lead exposure include reduced IQ, attention, and behavioral problems, and reduced academic achievement.

In adults, chronic lead exposure has been associated with hypertension, cardiovascular disease, kidney dysfunction, and reproductive effects. Bone stores of lead can be mobilized during pregnancy and menopause, producing exposure to fetuses or resuming systemic exposure decades after the initial contamination.

Regulatory Changes

The Lead and Copper Rule Improvements (LCRI), finalized in October 2024, established the current regulatory framework:

Service line replacement: All lead service lines must be replaced by 2037, with replacement required regardless of measured water lead concentrations.
Action level: The lead action level decreases from 15 to 10 parts per billion beginning in 2027. Exceedances trigger public notification and treatment adjustments.
Sampling: Revised sampling protocols aim to capture worst-case exposures more accurately.
Public notification: Enhanced disclosure requirements to affected residents.

Until service line replacement is complete, households served by lead lines remain at risk of lead contamination, even when the water leaving the treatment plant contains no lead.

Fluoride

Fluoride is added to public water supplies in most communities in the United States for dental caries prevention. The current U.S. Public Health Service recommendation is 0.7 milligrams per liter.

The health effects of fluoride at various exposure levels have been studied since community water fluoridation began in the 1940s. The topic remains scientifically and legally contested, and both the evidence and the regulatory status have changed within the past two years.

The 2024 NTP Monograph

In August 2024, the National Toxicology Program (NTP), part of the National Institute of Environmental Health Sciences, published a monograph on fluoride and neurodevelopment. Key findings:

The NTP concluded with moderate confidence that fluoride exposure above 1.5 milligrams per liter is associated with lower IQ in children.
The threshold identified is more than double the current United States water fluoridation target of 0.7 mg/L.
The NTP stated that the available data were insufficient to determine whether fluoride at lower levels affects children’s IQ.
Higher exposure levels are more common outside the United States, particularly in regions with naturally elevated fluoride levels in groundwater.

Legal Status

Federal legal status has shifted:

2024. A federal district court in California ruled that fluoridation posed an unreasonable risk under the Toxic Substances Control Act and ordered the EPA to take regulatory action.
May 2026. The Ninth Circuit Court of Appeals reversed that ruling on procedural grounds, concluding the district court applied an incorrect legal standard.
Ongoing. The EPA is conducting a new human health toxicity assessment of fluoride.

The scientific consensus remains that fluoride at approximately 0.7 mg/L provides dental caries prevention. Ongoing research examines potential neurodevelopmental effects across various exposure levels.

Bottled Water

The Food and Drug Administration regulates bottled water in the United States under standards that differ from the EPA regulations governing municipal water. FDA standards for many contaminants are less stringent than EPA standards, monitoring frequency is lower, and public disclosure requirements are minimal.

Bottled water is not, as a general category, purer than tap water.

Sources and Categories

FDA regulations recognize several categories:

Spring water: Water derived from a spring or borehole tapping an underground formation.
Purified water: Water processed through distillation, deionization, or reverse osmosis.
Mineral water: Groundwater containing at least 250 mg/L of total dissolved solids from the source.
Artesian water: Water from a confined aquifer under pressure.

A substantial portion of bottled water sold in the United States originates from municipal supplies and is further treated before bottling. Source disclosure requirements are limited.

Historical Contamination Data

A Natural Resources Defense Council study conducted over four years found that approximately one-third of tested bottled waters contained contamination exceeding allowable limits. Contaminants detected included bacteria, arsenic, and synthetic organic chemicals. The study is now historical, and comparable large-scale independent testing has not been repeated.

The finding that bottled water is not automatically safer than tap water has not been contradicted by subsequent research.

Microplastics and Nanoplastics

The presence of plastic particles in bottled water has been documented in multiple studies. The most significant methodological advance came in January 2024, when researchers at Columbia University and Rutgers University published findings in the Proceedings of the National Academy of Sciences using a new laser imaging technique.

Key findings from the Columbia-Rutgers study:

An average of approximately 240,000 detectable plastic fragments per liter across the bottled waters tested.
Approximately 90 percent of the fragments were nanoplastics, defined as particles smaller than one micrometer.
The total exceeded prior microplastic estimates by 10 to 100 times.

Nanoplastics differ from larger microplastics in ways that carry biological significance. Particles below one micrometer can cross biological barriers, including the intestinal epithelium, the blood-brain barrier, and the placental barrier, whereas larger particles cannot traverse.

Emerging Cardiovascular Data

A 2024 study published in the New England Journal of Medicine represents the most consequential published research to date on the health effects of microplastics and nanoplastics.

Study design:

257 patients undergoing carotid endarterectomy for symptomatic carotid stenosis.
Arterial plaque samples were analyzed for microplastic and nanoplastic content using pyrolysis-gas chromatography mass spectrometry.
34 months of follow-up for cardiovascular events.

Findings:

Microplastics and nanoplastics were detected in the plaque of 58 percent of patients.
Patients with detectable plastic experienced myocardial infarction, stroke, or death at approximately 4.5 times the rate of patients without detectable plastic.

Interpretation:

The authors stated that the study does not establish causation. The association does not demonstrate that plastic caused the observed events.
Independent researchers have raised methodological concerns, including the possibility of sample contamination during collection and analysis.
The findings require replication with attention to contamination controls.

Regardless of the final interpretation, the study represents the first published evidence connecting plastic particle exposure to hard cardiovascular endpoints in humans.

Chemistry of Plastic Bottles

Chemicals used in the manufacture of plastic bottles, primarily polyethylene terephthalate (PET), migrate into bottled water over time. Migration is affected by:

Storage duration – Longer storage produces greater migration.
Temperature – Elevated temperatures substantially accelerate migration.
Ultraviolet exposure – UV light degrades plastic and increases migration.

Substances documented to migrate from PET bottles include antimony (used as a catalyst in PET production) and a range of low-molecular-weight organic compounds. Migration levels typically fall below current regulatory limits when bottles are stored under normal conditions, but exceed them under prolonged storage at elevated temperatures.

Environmental Impact

Plastic bottles contribute substantially to global plastic waste. Recycling rates for PET bottles in the United States remain below 30 percent. A significant portion of the remainder enters landfills or incinerators, or is released into environmental pathways that reach oceans, rivers, and lakes.

Regulatory and Distribution Context

Compliance with federal drinking water standards at the treatment plant does not fully describe the water reaching consumer taps. Three structural factors contribute to the gap between treated water and consumed water.

Regulatory Scope

The Safe Drinking Water Act regulates more than 90 drinking water contaminants under the National Primary Drinking Water Regulations. Additional contaminants are monitored under the Unregulated Contaminant Monitoring Rule (UCMR), which requires periodic testing without setting enforceable limits.

Substances detected in United States tap water at measurable concentrations exceed the number regulated. Regulatory action requires scientific evidence of health effects, technical feasibility of removal, and economic analysis, all of which take years to complete for individual contaminants.

Treatment Technology Adoption

Conventional water treatment, designed for pathogen and particulate removal, does not effectively address many synthetic organic chemicals, including PFAS and pharmaceutical residues.

Advanced treatment technologies capable of removing these contaminants include granular activated carbon, ozone, and membrane filtration (reverse osmosis or nanofiltration). These technologies are effective when implemented, but require capital investment and ongoing operating costs.

Fewer than 10 percent of drinking water treatment facilities in the United States employ advanced treatment technologies capable of removing PFAS and similar emerging contaminants at scale. Adoption is concentrated in larger urban systems with greater financial capacity.

Distribution Infrastructure

Water leaves the treatment plant, meeting applicable standards, and travels through distribution infrastructure before reaching consumer taps. Contaminants introduced during distribution do not appear in treatment plant compliance monitoring:

Lead service lines: Approximately 9.2 million remain in service.
Corroding fixtures and internal plumbing: Lead, copper, and other metals leach from aging materials.
Main breaks and pressure loss events: They can introduce contamination from the surrounding soil and groundwater.
Cross-connections and backflow: Can allow non-potable water to enter the distribution system.

Treatment plant compliance monitoring describes water leaving the plant. It does not describe water arriving at the tap.

Summary

Water affects human health through two independent variables: the quantity consumed relative to physiological requirements and the quality of what is consumed beyond H2O. Both variables show substantial gaps in the United States population, with chronic underhydration affecting more than 60 percent of middle-aged adults and drinking water contamination affecting nearly every water source at some level.

Chronic underhydration has been associated with increased risk of kidney stones, urinary tract infections, and cardiovascular disease. Even mild dehydration produces measurable declines in cognitive performance, mood, and physical endurance. Meeting adequate intake requires attention to individual factors including body size, activity level, climate, and clinical conditions.

Municipal water quality varies substantially by source, treatment approach, and distribution infrastructure. Treatment introduces regulated chemicals including chlorine and chloramine, which react with organic matter to produce disinfection byproducts associated with cancer risk. PFAS contamination affects at least 45 percent of tap water, and lead exposure risk persists in the estimated 9.2 million lead service lines still in use.

Bottled water is not, as a general category, safer than tap water. Recent research documented approximately 240,000 plastic fragments per liter in bottled water, with 90 percent classified as nanoplastics capable of crossing biological barriers. A 2024 New England Journal of Medicine study found microplastics in arterial plaque of 58 percent of cardiovascular patients, with associated cardiovascular event rates 4.5 times higher than patients without detectable plastic.

Regulatory compliance at treatment plants does not describe water at the tap. Fewer than 10 percent of treatment facilities employ advanced treatment capable of removing emerging contaminants, and distribution infrastructure can introduce contamination between plant and consumer. Point-of-use treatment provides one approach to addressing gaps between regulatory compliance and actual water quality reaching the tap.