We often view the water flowing from a tap as a symbol of cleanliness and safety.
But clear water is not necessarily chemically safe.
Unlike many waterborne microorganisms that may produce obvious symptoms of gastrointestinal illness, contaminants such as lead and mercury can cause harm without changing the colour, smell or taste of water.
Their effects can also develop over time.
This is particularly important because the nervous system is highly sensitive to toxic exposures, and developing brains are especially vulnerable to lead and certain forms of mercury.
Understanding how these environmental contaminants enter the body and how the body responds to them is an important part of environmental health.
ENVIRONMENTAL HEALTH PROBLEMS: WHERE CAN LEAD AND MERCURY COME FROM?
Heavy metals occur naturally in the environment, but human activities can increase their concentrations in air, soil and water.
The sources of exposure are not identical for lead and mercury, and understanding this distinction is important.
LEAD IN DRINKING WATER
Lead can enter drinking water when water comes into contact with plumbing components that contain lead.
Potential sources include:
Lead service lines
Older plumbing materials
Lead-containing solder
Certain brass fixtures and fittings
Corrosion of plumbing components
Lead cannot normally be seen, tasted or smelled in drinking water.
The amount entering water depends on factors including the plumbing materials, water chemistry, how long water remains in contact with the plumbing and other environmental conditions.
The U.S. Environmental Protection Agency identifies lead pipes, faucets and fixtures as important potential sources of lead in drinking water.
MERCURY IN THE ENVIRONMENT
Mercury has a different environmental pathway.
Mercury is naturally occurring, but human activities, including certain mining operations, coal combustion, industrial processes and waste incineration can release mercury into the environment.
Once released, mercury can move through air, soil and water.
Microorganisms can transform some forms of mercury into methylmercury, an organic form that can accumulate in aquatic food chains.
For most people, eating contaminated fish and shellfish is the major route of methylmercury exposure rather than ordinary drinking water.
WHO identifies mercury as one of the chemicals of major public-health concern because exposure can affect the nervous system, kidneys and other organs.
THE ANATOMICAL VULNERABILITY: THE BLOOD-BRAIN BARRIER
The brain is not directly exposed to everything circulating in the bloodstream.
One of its major protective structures is the blood-brain barrier (BBB).
The BBB is formed primarily by specialized endothelial cells lining the brain's small blood vessels, supported by surrounding cells and structures including pericytes and astrocytes.
Its function is highly selective.
It helps regulate which substances can move from the bloodstream into the brain's delicate extracellular environment.
However, the blood-brain barrier is not an impenetrable wall.
Certain chemicals can cross it because of their physical and chemical properties, their interaction with transport systems, or because they can alter the barrier itself.
HOW LEAD CAN REACH AND AFFECT THE NERVOUS SYSTEM
Lead does not simply "pick the lock" of the blood-brain barrier.
Its toxicity is more complex.
Lead can enter the bloodstream after absorption from the gastrointestinal tract or through other exposure routes. From the blood, it is distributed to several tissues, including the brain, kidneys, liver and bones.
WHO states that lead is stored particularly in bones and teeth and can remain in the body for long periods.
Lead can interfere with biological processes that normally depend on essential minerals such as calcium.
Because calcium plays critical roles in neuronal signalling, neurotransmitter release and development of the nervous system, interference with calcium-dependent processes can contribute to neurological toxicity.
This is particularly concerning during childhood, when the brain and nervous system are still developing.
HOW MERCURY CAN AFFECT THE BRAIN
Mercury exists in several chemical forms, and those forms behave differently in the human body.
Methylmercury, for example, can cross the blood-brain barrier.
WHO technical material explains that methylmercury can cross the blood-brain and placental barriers, with transport influenced by its interaction with molecules such as cysteine.
This is different from saying that mercury simply disguises itself as a nutrient.
The biological process is more complicated and depends on the chemical form of mercury and how it is absorbed, transported and metabolized.
Once in nervous tissue, mercury can interfere with cellular processes and contribute to neurological injury.
ANNOTATED SCHEME: FROM ENVIRONMENTAL EXPOSURE TO NEUROLOGICAL EFFECTS
[ 1. ENVIRONMENTAL SOURCE ]
(Lead-containing plumbing, contaminated soil, industrial emissions, mining and other sources)
↓
[ 2. HUMAN EXPOSURE ]
(Drinking water, food, inhalation or other exposure pathways depending on the contaminant)
↓
[ 3. ABSORPTION ]
(Gastrointestinal tract, lungs or other routes)
↓
[ 4. BLOODSTREAM ]
(Contaminant is transported through the body)
↓
[ 5. BLOOD-BRAIN BARRIER ]
(Some forms of lead and mercury can reach nervous tissue through complex biological pathways)
↓
[ 6. NERVOUS SYSTEM ]
(Interference with cellular signalling, oxidative balance and other neurological processes)
↓
[ 7. HEALTH EFFECTS ]
(Effects may include impaired neurological development, cognitive difficulties, behavioural changes, sensory disturbances or other neurological effects depending on the contaminant and exposure level)
THE HEALTH CONSEQUENCES OF LEAD EXPOSURE
Lead affects multiple organ systems.
The developing nervous system is particularly vulnerable.
WHO reports that lead exposure can affect the neurological, cardiovascular, gastrointestinal and haematological systems. Children are especially susceptible because their brains and nervous systems are still developing.
CHILDHOOD NEURODEVELOPMENT
Even relatively low levels of lead exposure can affect children's learning and development.
Potential effects include:
Reduced intellectual ability
Problems with attention
Learning difficulties
Behavioural problems
Developmental delays
CDC states that no safe blood lead level in children has been identified. Even low blood lead levels can negatively affect learning, attention and development.
EFFECTS IN ADULTS
Lead exposure is not only a childhood concern.
WHO reports that lead exposure in adults can contribute to increased blood pressure and kidney damage, among other health effects.
Long-term exposure can therefore have consequences across the lifespan.
THE HEALTH CONSEQUENCES OF MERCURY EXPOSURE
Mercury toxicity depends strongly on the chemical form, dose and route of exposure.
WHO reports that mercury can affect the nervous, digestive and immune systems as well as the kidneys, lungs, skin and eyes.
Neurological effects may include:
Tremors
Memory problems
Headaches
Neuromuscular effects
Cognitive difficulties
Problems with coordination and motor function
The developing fetus is particularly vulnerable to methylmercury exposure because the nervous system is developing rapidly during pregnancy.
A NOTE ABOUT "DEMYELINATION"
The original version of this article described mercury as specifically attacking the myelin sheath and causing demyelination.
That description is too broad.
Mercury can cause neurological injury, but its effects depend on the chemical form and exposure pattern. It is more scientifically accurate to discuss mercury's neurotoxicity, oxidative stress, cellular dysfunction and effects on nervous-system signalling rather than presenting demyelination as the universal mechanism.
This distinction matters because Human & Habitat aims to separate established evidence from simplified explanations.
CAN YOU TRUST CLEAR WATER?
No.
Clear water can still contain dissolved contaminants.
Lead, for example, cannot normally be detected by looking at, smelling or tasting the water.
That is why visual inspection is not an adequate test for chemical safety.
If a household is concerned about possible contamination, laboratory testing is more useful than appearance alone.
This is particularly important for households using:
Older plumbing
Private wells or boreholes
Water sources close to industrial activity
Water from uncertain or poorly monitored sources
Plumbing systems containing older metal components
WHAT CAN YOU DO ABOUT POSSIBLE LEAD IN TAP WATER?
There is no single solution that works for every building.
The correct response depends on where the lead is coming from.
1. TEST THE WATER
If lead contamination is suspected, laboratory testing can determine whether lead is present.
Testing is particularly useful because lead cannot be detected reliably by taste, smell or appearance.
For private groundwater systems, periodic testing can also help identify changes in water quality.
2. FLUSH STAGNANT WATER WHEN APPROPRIATE
When water has remained in household plumbing for several hours, lead concentrations may increase because of contact with plumbing materials.
EPA guidance recommends flushing the cold-water tap before drinking or cooking after water has been sitting for several hours. Depending on the plumbing system, this can take approximately 30 seconds to 2 minutes, although systems with lead service lines may require different procedures.
This should not be presented as a universal guarantee of safety.
If a home has a significant lead source, flushing may reduce exposure but does not necessarily eliminate it.
3. USE COLD WATER FOR DRINKING AND COOKING
Use cold tap water for drinking, cooking and preparing infant formula.
If hot water is required, draw cold water first and heat it separately.
EPA notes that hot water can dissolve lead more readily from plumbing materials and that boiling does not remove lead from water.
4. USE A CERTIFIED FILTER WHEN APPROPRIATE
A water filter should not be selected simply because the packaging says "purifies water."
If lead is the concern, choose a treatment device that has been independently certified for lead reduction.
EPA recommends looking for products evaluated by accredited third-party certification bodies for lead reduction. NSF/ANSI Standard 53 is one important certification standard for lead reduction claims.
Different filters remove different contaminants.
A filter that improves taste or removes chlorine is not automatically a lead-removal filter.
WHAT ABOUT REVERSE OSMOSIS?
Reverse osmosis (RO) can reduce a wide range of dissolved contaminants, including certain heavy metals.
However, RO should not be presented as the only appropriate solution.
A properly certified point-of-use filter designed for lead reduction may be sufficient for a household's specific needs.
The important principle is:
Choose the treatment technology according to the contaminant that has actually been identified or reasonably suspected.
Do not buy an expensive treatment system simply because it sounds more advanced.
WHAT ABOUT BOREHOLE WATER?
Private groundwater can be extremely useful, but groundwater quality varies from location to location.
A borehole does not automatically mean that water is chemically safe.
Potential contaminants depend on local geology, land use, sanitation, industrial activities and the construction of the borehole.
Therefore, households relying on private groundwater should consider laboratory testing appropriate to their local risk profile.
Testing only for bacteria does not tell you whether heavy metals or other chemical contaminants are present.
A comprehensive water-quality assessment may need to include microbiological and chemical parameters.
WHAT ABOUT MERCURY IN DRINKING WATER?
This is where environmental-health communication needs to be precise.
Mercury contamination can affect water environments, but for many populations, dietary exposure to methylmercury through contaminated fish and seafood is a more important exposure pathway than ordinary drinking water.
WHO identifies fish and shellfish containing methylmercury as the main source of human exposure to this form of mercury.
Therefore, someone concerned about mercury exposure should consider the entire exposure pathway rather than automatically assuming that tap water is the main source.
In areas affected by mining or industrial contamination, however, local water and food sources may require specific investigation.
THE PUBLIC-HEALTH RESPONSE
Individual households can reduce exposure, but heavy-metal contamination is ultimately an environmental-health and infrastructure problem.
Effective prevention requires cooperation among:
Water utilities
Environmental regulators
Public-health authorities
Industrial operators
Local governments
Laboratories
Communities
Households
INFRASTRUCTURE
Where lead-containing service lines or plumbing components exist, identifying and replacing the source is a long-term solution.
Flushing and filtration can reduce exposure, but they do not remove the underlying contaminated infrastructure.
INDUSTRIAL CONTROL
Industrial facilities that handle metals must properly manage waste and wastewater.
Environmental monitoring and enforcement are essential where mining, manufacturing or other activities create risks of metal contamination.
WATER-QUALITY MONITORING
Routine water-quality monitoring can help identify contamination before it becomes a widespread public-health problem.
Where communities rely heavily on groundwater, monitoring should reflect local environmental risks rather than assuming that groundwater is automatically clean.
THE DEVELOPING-BRAIN CONNECTION
One of the most important lessons from lead exposure is that environmental health is inseparable from human development.
A child's brain is not simply a smaller adult brain.
It is undergoing rapid growth, formation of neural connections and functional development.
Environmental toxicants that interfere with these processes can have consequences that persist long after the original exposure has occurred.
That is why preventing exposure is more important than waiting for symptoms.
THE LEAD BLOOD-LEVEL QUESTION
You may still see older articles referring to 5 µg/dL as a blood-lead "level of concern."
This terminology needs updating.
CDC currently uses a blood lead reference value of 3.5 µg/dL for children. This value is a population-based reference used to identify children whose blood lead levels are higher than those of most U.S. children.
It is not a safety threshold.
CDC emphasizes that no safe blood lead level in children has been identified.
WHO similarly states that there is no known safe blood lead concentration and notes that even low concentrations can be associated with reduced intelligence and behavioural or learning problems in children.
The practical message is therefore simple:
Lower exposure is better, and preventing exposure is the goal.
A SIMPLE HOUSEHOLD CHECKLIST
If you are concerned about heavy metals in your water, ask:
☐ Is the plumbing old or of uncertain composition?
☐ Does water remain unused in the pipes for several hours?
☐ Is the water supplied from a private borehole?
☐ Has the water ever been laboratory tested for chemical contaminants?
☐ Is there industrial or mining activity nearby?
☐ Is the household using a filter specifically certified for the contaminant of concern?
☐ Are filter cartridges replaced according to the manufacturer's instructions?
☐ Are children or pregnant people potentially exposed?
☐ Could contaminated food be a more important source of mercury exposure?
These questions can help turn a vague concern about "dirty water" into a specific environmental-health investigation.
THE HUMAN & HABITAT TAKEAWAY
Water can look perfectly clean and still contain substances that deserve attention.
Lead and mercury are not interchangeable contaminants, and they do not follow identical pathways through the environment or human body.
Lead exposure can affect multiple organ systems, with particularly serious consequences for the developing nervous system.
Certain forms of mercury especially methylmercury can cross the blood-brain barrier and cause neurological toxicity.
The most effective response is not fear.
It is identification, testing, source control and evidence-based prevention.
If lead is suspected in drinking water, investigate the plumbing and test the water.
If mercury contamination is suspected, investigate the local environmental and dietary exposure pathways.
If a contaminant is identified, select a treatment method that is specifically designed and certified to reduce that contaminant.
Your water may look clean.
But environmental health begins with understanding what cannot be seen.
REFERENCES AND FURTHER READING
World Health Organization (WHO). Lead poisoning.
WHO — Lead poisoningWorld Health Organization (WHO). Lead in drinking-water: Health risks, monitoring and corrective actions.
WHO — Lead in drinking-waterWorld Health Organization (WHO). Mercury and health.
WHO — Mercury and healthU.S. Environmental Protection Agency (EPA). Basic Information about Lead in Drinking Water.
EPA — Lead in drinking waterU.S. Environmental Protection Agency (EPA). Consumer Tool for Identifying Point-of-Use and Pitcher Filters Certified to Reduce Lead in Drinking Water.
EPA — Certified lead-reduction filtersU.S. Environmental Protection Agency (EPA). Will my filter remove lead?
EPA — Will my filter remove lead?Centers for Disease Control and Prevention (CDC). CDC Updates Blood Lead Reference Value.
CDC — Blood Lead Reference ValueCenters for Disease Control and Prevention (CDC). Why CDC Avoids the Term "Elevated" for Blood Lead Levels.
CDC — Blood Lead Levels
AUTHOR'S NOTE
This article provides general educational information about environmental health and toxic exposures. It is not a substitute for medical diagnosis, treatment or professional water-quality assessment.
The health effects of lead and mercury depend on the chemical form, dose, route and duration of exposure, as well as age and other individual factors.
If you believe you or a child has been exposed to lead or mercury, seek advice from an appropriate healthcare professional and consider environmental testing to identify and control the source of exposure.
Human & Habitat aims to present environmental-health information in a scientifically grounded and accessible way while distinguishing established evidence from simplified explanations and areas where research continues to develop.
ABOUT THE AUTHOR
Ugochukwu is the creator and primary writer of Human & Habitat. He holds a Bachelor of Science (Hons) in Anatomy from the University of Calabar and has an academic interest in Environmental Health Science.
For more information, visit the About the Author page.
Last reviewed: September 28, 2026
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