Why this poison is interesting

Every other page in this library is organised around a dose or a concentration above which something happens. Lead is organised around the absence of one. Its UK toxicological overview states, of the endpoints it identifies as the most sensitive in adults, that there is no evidence for a threshold — for the anaemia, for the nephrotoxicity, and separately for the cardiovascular effect whose critical endpoint is a rise in systolic blood pressure.¹ The enzyme lead inhibits first is inhibited "at BLLs as low as 3 to 34 µg/dL with no threshold yet apparent", and the inhibition "inversely correlate[s] with BLLs over the whole dose range".¹ A poison with a dose–response that continues all the way down is a different kind of clinical object from one with a treatment threshold, and most of the confusion around lead comes from treating it as the second.

The second reason is that lead is not recognised as foreign. It is absorbed on machinery built for calcium, transported bound to erythrocyte proteins, and deposited in bone as an insoluble phosphate in exactly the places bone is growing fastest.¹ Nothing in the body is trying to keep it out or get it out; it is handled as a nutrient with the wrong atomic number, and the efficiency of that handling rises in precisely the groups least able to tolerate it — children, pregnant women, and anyone whose diet is short of calcium or iron.¹

The third is a measurement problem with the same shape as digoxin's, and it is more extreme here. About 90% of the adult body burden is in bone and roughly 5% is in blood.¹ The test in universal use samples the smallest compartment, and the compartment it samples is in slow exchange with a store whose half-life is measured in decades. This produces the page's strangest consequence: the blood lead concentration can rise years after the last exposure, when pregnancy, lactation, the menopause or osteoporosis resorbs the bone that has been holding it.¹

A poison is a drug whose kinetics have escaped its pharmacology.

The toxic principle

Lead's toxicity is the toxicity of a divalent cation occupying sites that belong to other divalent cations. The single best-characterised instance is in haem synthesis. Lead "inhibits δ-aminolevulinic acid dehydrogenase (ALAD), which leads to an increase in δ-aminolevulinic acid synthase",¹ and this is the lesion that produces the anaemia — through "reduced haemoglobin production and a shortened life-span of erythrocytes".¹ The enzyme is a zinc enzyme, and lead displaces the zinc. That is the whole mechanism in one sentence, and it generalises: what lead does elsewhere in the body is the same substitution in other places, less well characterised.

Why the absorption story is a calcium story

Lead competes with calcium for the binding proteins involved in gastrointestinal absorption, because the two ions are of similar size.¹ Three consequences follow directly, and all three are counter-intuitive in the same direction — the people who absorb most are the people who can least afford to. "Certain groups such as nursing mothers and children are more efficient at absorbing calcium than other populations. It has been hypothesised that this is why these groups also take up lead more efficiently (especially where there is a calcium deficiency)."¹ Adults absorb 5 to 15% of ingested lead, rising to as high as 45% under fasting conditions; in children and infants absorption "may be as high as 53%".¹ And low dietary iron, copper, zinc, selenium or phosphate all increase lead absorption¹ — a poisoning made worse by the deficiencies that accompany it.

The route of exposure turns out to matter much less than it does for the other metals in this band. The overview states plainly that "the toxicological effects of lead are the same regardless of the route of exposure",¹ which is why lead is described in terms of blood lead concentration rather than route — a convention that distinguishes it sharply from mercury, where the route determines which of three different diseases the patient gets. Inhalation still matters for how much gets in: 30 to 50% of inhaled lead is deposited in adults, particles under 1 µm reach the lower respiratory tract where "absorption appears to be complete", and particles over 5 µm are cleared by mucociliary transport and then swallowed, entering by the oral route after all.¹

A concentration–effect table in the source that this page does not reproduce

The UKHSA overview contains a table of acute effects against blood lead concentration. It is not reproduced here, and the reason is a rule this library has had to apply before. The table is attributed to seven references, one of which is TOXBASE — a login-gated NHS management database whose content cannot be verified from outside and must not be quoted. The individual figures cannot be traced to individual sources from the document, so the table cannot be separated into quotable and unquotable parts. The same judgement was made in Band D about the nitrobenzene overview's methaemoglobin table, for the same reason. Where this page does give a concentration attached to a decision, it comes from a UK marketing authorisation or from a named cohort instead, and the source is stated each time.

Toxicokinetics

The table below is unusual in this library because the interesting column is almost entirely about compartments rather than rates. Lead's clinical behaviour is set by the fact that it moves between three pools of wildly different size and turnover, and that the accessible one is the smallest.

Lead — three compartments, and the one that is measured holds the least
Parameter	Therapeutic	In overdose	Why it changes
Oral absorption	5 to 15% in adults¹	Up to 45% fasting; up to 53% in children and infants¹	Absorption rides on calcium's transporters, and calcium absorption is upregulated in growth, pregnancy and lactation. The physiological states that most need calcium are the ones that admit most lead
Modifiers of absorption	Diet-independent for most poisons	"Low levels of iron, copper, zinc, selenium, or phosphate in the diet can also increase lead absorption"¹	The deficiencies that cluster with lead exposure are the same ones that increase it — a poisoning that recruits its own risk factors
Distribution	Bound to erythrocyte proteins in blood; soft tissue and bone¹	~90% of adult body burden in bone, ~70% in children; blood ~5%¹	The measured compartment is not the storage compartment. A blood concentration is a window onto exchange, not onto burden
Half-life	Not a therapeutic concept	Blood and soft tissue 20–40 days; bone 10–30 years¹	Two half-lives three orders of magnitude apart in the same patient. A normal blood lead after a month of removal from exposure says nothing about the skeleton
Form in bone	—	Deposited as insoluble lead phosphate, in rapidly growing areas — radius, tibia and femur; "lead lines" visible radiographically, "their width related to the duration of exposure"¹	The store is chemically inert until the bone around it is remodelled — which makes the skeleton a record as well as a reservoir
Release from store	—	"Conditions causing bone resorption or increased calcium demand (particularly pregnancy, lactation, menopause, and osteoporosis) may cause increased mobilisation of lead from the bone... a rise in Blood Lead Levels after the original exposure has ceased"¹	The page's defining kinetic fact, and the library's longest latent interval. The exposure and the poisoning can be separated by decades
Placental transfer	—	Crosses from as early as 12 weeks; cord blood 85 to 90% of maternal concentration¹	A maternal skeleton laid down in childhood can poison a fetus. Two generations, one exposure
Metabolism	—	None as such: "binding to various proteins and reversible ligand reactions"; organic lead compounds are metabolised to inorganic lead¹	No activation and no detoxification. The only transformation in the system converts a lipophilic form into the toxic one
Excretion	—	"Primarily excreted in the urine, while approximately a third is excreted in the faeces"¹	Slow, unregulated and not saturable — but it is emptying the blood, not the bone

Metabolism and the metabolites

There is no metabolism in the usual sense and there is no toxic metabolite. Lead binds and unbinds; it is not chemically transformed. The one transformation the overview does describe runs in the direction that makes things worse: "organic lead compounds are metabolised to inorganic lead".¹ Tetraethyl lead, the anti-knock additive that was the largest source of UK emissions before 1999,¹ is lipophilic and crosses membranes easily — and then becomes the inorganic ion that this page is about.

Lead — a pathway with no biotransformation, only relocation

Inorganic lead, ingested or inhaledThe poison as it arrives. No activation step; the container holds the active species
Absorption on calcium's binding proteins — "lead competes with calcium... as they have a similar ionic size"¹
Blood: bound to erythrocyte proteinsAbout 5% of the eventual body burden, and the only part a routine test can see¹
Soft tissue — marrow, kidney, nervous systemWhere the clinical effects are produced. Half-life 20 to 40 days¹
Bone — insoluble lead phosphate~90% of the adult burden. Half-life 10 to 30 years.¹ Inert while it stays there
Bone resorption — pregnancy, lactation, menopause, osteoporosis¹
Back into blood, decades laterThe reservoir becomes the exposure. A rising blood lead in a patient with no current source is this step, not a new ingestion

The lesion in haem synthesis, and why it is the most useful thing to know

Lead inhibits δ-aminolaevulinic acid dehydratase, and the substrate that accumulates behind the block drives an increase in δ-aminolaevulinic acid synthase.¹ Two things follow. First, the anaemia — "reduced haemoglobin production and a shortened life-span of erythrocytes"¹ — is a combined production and destruction problem, which is why it is not simply a microcytic picture. Second, and more useful: enzyme inhibition is detectable long before anaemia is. Reduced haemoglobin synthesis appears at blood lead concentrations of 50 µg/dL in adults and 40 µg/dL in children,¹ but ALAD activity is already inhibited "at BLLs as low as 3 to 34 µg/dL".¹ The biochemical lesion and the clinical syndrome are an order of magnitude apart, and that gap is the whole argument about whether lead has a threshold. A note on the enzyme's name: the overview writes "δ-aminolevulinic acid dehydrogenase", which is an error in the source — ALAD is a dehydratase. This page uses the correct name in its own prose and preserves the source's spelling inside quotation marks, which is why the two differ.

Elimination and accumulation

Lead is excreted, unlike iron — "primarily in the urine, while approximately a third is excreted in the faeces", with sweat, saliva, hair, nails and breast milk as minor routes.¹ The problem is not that there is no exit but that the exit drains the wrong tank. Clearance empties blood and soft tissue on a timescale of weeks; the skeleton, holding nine-tenths of the burden, empties on a timescale of decades, and only when it is being remodelled for reasons of its own.

Where this latent phase sits among the others

Lead — bone giving lead back — a store with a half-life of 10 to 30 years releasing its contents when pregnancy, lactation, menopause or osteoporosis resorbs it, so the blood concentration rises after the exposure has ended

The other 26 kinds of latent phase in this library

Amphetamines and MDMA — a hormone acting normally on a kidney behaving normally, while the patient supplies the water
Antipsychotics — a physical object in the stomach — extended-release quetiapine forming a pharmacobezoar
Arsenic — a tissue declaring on its own timetable rather than the poison's — the arsenic is excreted within days, but the nail that was growing while it circulated does not show its white transverse line for several weeks
Arsine and stibine — a red cell mass haemolysing faster than a kidney can cope with — the exposure is over, the haemolysis is silent until the urine changes colour, and the renal failure that follows is the cause of death
Beta-blockers — a repolarisation lesion waiting for an ectopic beat to fall inside it — sotalol prolongs the QT and then, for hours, nothing happens
Calcium-channel blockers — a tablet that has not yet dissolved
Carbon monoxide — an inflammatory process continuing after the poison itself has gone
Chlorine and ammonia — paraquat's cause at its shortest — the time an injured alveolar epithelium takes to leak
Cocaine — a package that has not yet failed — the calcium-channel blocker's cause moved outside the tablet, where wrapping integrity cannot be measured
Digoxin — distribution of the drug to its target — which that page argues is not a latent phase in the paracetamol sense at all
Ethylene glycol — paracetamol's cause with a different enzyme — glycolate accumulating behind glycolate oxidase while the patient looks merely drunk
GHB and GBL — not the poisoning but its withdrawal — a half-life under an hour means admission for any reason interrupts round-the-clock redosing
Hydrofluoric acid — an ion diffusing far enough to reach a nerve ending — and the thinner the solution, the further it travels before anybody feels it
Iron — a true remission — the corrosive injury settling while absorbed iron moves into the cells it will poison, so the patient improves on their way to the dangerous phase
Ketamine — nitrous oxide's cause on a longer clock — urothelial damage accumulating over months to years of repeated exposure
Lithium — transport across cell membranes
Mercury — distribution on two clocks — tissue concentrations peaking within 24 hours everywhere except the brain, which is not reached until 2 to 3 days, and which then cannot let the poison out again
Methaemoglobin inducers — paracetamol's cause at its shortest — one to four hours manufacturing the toxic species
Methanol — paracetamol's cause again — formate accumulating behind a folate-dependent disposal step that primates perform poorly
Nitrous oxide — damage accumulating to a threshold
Opioids — an antidote wearing off before the poison does — renarcotisation, the only gap in this set that treatment creates rather than reveals
Organophosphate insecticides — a fat store emptying — and, separately, a second and unexplained lesion declaring itself at a neuromuscular junction the first phase had already left
Paracetamol — time spent manufacturing a toxic metabolite
Paraquat — the body responding to an injury that is already complete
Sodium-channel blockade — a gap that cannot be shortened
Thallium — hair on its own clock rather than the poison's — sensory symptoms come first and the alopecia that makes the diagnosis obvious follows them, well after the interval in which treatment is recommended

Lead's entry is the longest in the set by three orders of magnitude, and it is the only one in which the gap is bridged by a physiological event in the patient rather than by the passage of time. Paracetamol's gap ends when enough metabolite has been made; lead's ends when a bone is resorbed, which may be thirty years later or never. It is also the only member of the set in which the poison re-enters the circulation without any new exposure.

Target organs — and why those

The organ list follows the substitution. Where a divalent cation is doing structural or catalytic work, lead can take its place; the organs that suffer are those whose function depends most on that work, or that are still being built.

Erythron

Targetδ-aminolaevulinic acid dehydratase, and the red cell itself

Why hereThe best-characterised lesion in the whole of lead toxicology, and the only one with an identified enzyme. Lead inhibits ALAD in haem synthesis, driving a compensatory increase in ALA synthase.¹ The anaemia is then produced twice over — "reduced haemoglobin production and a shortened life-span of erythrocytes"¹ — so it is a failure of manufacture and a shortening of survival at once. Established

At the bedsideAnaemia, with "basophilic stippling and reticulocytosis".¹ Reduced haemoglobin synthesis appears at 50 µg/dL in adults and 40 µg/dL in children,¹ but enzyme inhibition is measurable from 3 to 34 µg/dL with "no threshold yet apparent".¹ Basophilic stippling is a real finding and a poor screening test — it is neither sensitive nor specific enough to exclude the diagnosis.

Nervous system — developing

TargetThe growing brain

Why hereThe endpoint that has driven every regulatory decision about lead for fifty years, and the reason the threshold question matters. "The developing brain appears to be more vulnerable to the effects of lead. Features of neurotoxicity have been observed at lower BLLs in children than adults."¹ The mechanism is not resolved to a single enzyme the way the haem lesion is; what is established is the differential vulnerability and the direction of the dose–response. Inferred

At the bedside"Reduced nerve conduction velocity, postural sway, tremor, amyotrophic lateral sclerosis, and intelligence and neurobehavioral effects in children and adults".¹ Encephalopathy is the severe end, and children are more sensitive to these effects.¹ In the mild range the findings are developmental and behavioural rather than neurological, which is exactly why they are missed.

Nervous system — adult

TargetPeripheral nerve and cortex

Why hereA different clinical picture from the paediatric one, and separated from it in the source rather than merged.¹ The peripheral involvement is motor-predominant, which distinguishes it from the painful sensory neuropathies of arsenic and thallium — a useful discriminator when a patient presents with a neuropathy and a metal is suspected. Inferred

At the bedside"Dizziness, fatigue, sleep disturbance, headache, irritability, lethargy, malaise, slurred speech, and convulsions" at blood lead concentrations of 40 to 120 µg/dL, with "muscle weakness, paraesthesia, ataxia, tremors, and paralysis" also reported.¹ Every one of these is non-specific, which is the practical problem with the diagnosis.

Kidney

TargetProximal tubule, then the glomerulus

Why hereNamed among the endpoints for which the overview states there is no evidence of a threshold — "nephrotoxicity, from depressed glomerular filtration rate to severe deficits in renal function".¹ It matters twice over, because renal function also determines how well the chelated complex can be excreted. Established

At the bedsideA tubulointerstitial and glomerular picture in chronic exposure — "glomerular sclerosis, interstitial fibrosis, and proximal tubular nephropathy"¹ — with progressive loss of filtration. The overview's description of an acute proximal tubular syndrome is attributed to its reference 22, which is TOXBASE, and is therefore not reproduced here. The kidney is also where succimer does most of its chelating,² which makes renal impairment a problem for both the poisoning and its treatment.

Cardiovascular system

TargetSystolic blood pressure

Why hereIncluded because the source identifies it as one of the most sensitive endpoints in adults, not because it is dramatic. "Lead causes cardiovascular toxicity with no apparent threshold, the critical endpoint being increased systolic blood pressure."¹ This is the endpoint that makes population-level lead exposure a public health problem rather than a clinical one. Inferred

At the bedsideA rise in systolic blood pressure, at exposures far below any that would produce symptoms. Nothing about this presents to an emergency department, and it is on this page to explain why the regulatory limits are where they are.

Bone

TargetMineralised matrix — as store, and as target

Why hereThe only organ in this library that is simultaneously the reservoir and a lesion. Lead is deposited as insoluble lead phosphate in the areas growing fastest,¹ which is why the radiographic "lead lines" appear at the metaphyses of growing children and why "their width related to the duration of exposure".¹ The bone is not merely storing the poison; it is recording the exposure history. Established

At the bedsideRadiographic lead lines in children, of historical and diagnostic interest. More importantly, the store determines the patient's future: any state that resorbs bone will re-expose them.¹

Timeline of effects

Lead — a timeline in which the exposure and the illness may be decades apart

Time

What you seeWhat is happening

Hours to days (high acute exposure only)Colic and gastrointestinal upset
What you see"Colic is a common early sign of acute lead poisoning, effects include abdominal pain, constipation, nausea, vomiting and anorexia."¹ Lead is described as a classic chronic toxin, and acute presentations of this kind require a high-level exposure.¹
What is happeningSufficient absorbed lead to produce systemic effects quickly. The constipation rather than diarrhoea is worth noting — it is shared with thallium and is unlike the profuse diarrhoea of acute arsenic.
WeeksAccumulation, without symptoms
What you seeNothing, in most patients. In the UK firing-range outbreak, only 9 of 63 tested individuals reported any symptoms at presentation, and the highest blood lead concentration at presentation was 11.7 µmol/L (242 µg/dL).³
What is happeningAbsorption exceeding excretion, with redistribution into bone. The enzyme lesion is already present and measurable at concentrations far below those producing symptoms¹ — the biochemistry is ahead of the clinical picture throughout.
Months to yearsThe chronic syndrome
What you seeAnaemia, the neurobehavioural and neurological features, renal impairment and the rise in systolic blood pressure.¹ In children, developmental and cognitive effects at concentrations below those causing anything an adult would report.¹
What is happeningCumulative substitution at divalent-cation sites across several tissues. For the most sensitive endpoints the overview states there is no threshold,¹ so this phase does not begin at a concentration — it begins at a level of exposure the patient's history determines.
Years to decadesRelease from the skeleton
What you seeA rising blood lead concentration in a patient with no current exposure, or the appearance of features during pregnancy, lactation or after the menopause.¹ In pregnancy, "BLLs gradually rise from the middle of the second trimester until delivery" in women with elevated body burdens.¹
What is happeningBone resorption returning stored lead to the circulation.¹ The half-life of lead in bone is 10 to 30 years,¹ so the store outlives most working lives. This is the only phase in the library in which the poison re-enters the blood without a new exposure.

What the mechanism predicts at the bedside

A normal blood lead concentration does not mean a normal body burden. Blood holds about 5% of it and bone about 90%.¹ The test is a measure of recent exchange, and it is the right test to order — but it answers a narrower question than it appears to.
A rising concentration in someone with no current source is a bone story, not a new exposure. Ask about pregnancy, lactation, the menopause, immobilisation and anything else that resorbs bone.¹
Ask about the household's calcium and iron status, not only its lead sources. Deficiency of iron, copper, zinc, selenium or phosphate increases absorption,¹ so nutrition is part of the exposure assessment rather than an afterthought.
Fasting increases absorption from 5–15% to as much as 45%.¹ The same environmental exposure produces a different dose in a child who has not eaten.
Expect the presentation to be non-specific and the history to do the work. In a UK firing-range outbreak, 87 people were identified as possibly exposed, 63 were tested, and only nine reported any symptoms at all.³ The authors' conclusion is about "the vague symptoms of lead poisoning" and the need for vigilance about it as an occupational hazard.³
A motor-predominant peripheral neuropathy points to lead rather than to the other metals in this band. Arsenic and thallium both produce a painful, sensory-predominant, ascending neuropathy; lead does not.
Do not read the occupational action levels as clinical thresholds. The UK enforces an occupational exposure limit of 0.15 mg/m³ in air and action levels of 25 and 50 µg/dL in blood for women of childbearing capacity and for general employees respectively.¹ These are regulatory biological-monitoring triggers designed to keep a workforce below a level of exposure. They are not treatment thresholds and they are not a statement that a lower concentration is safe — the same document says the most sensitive effects have no demonstrated threshold.¹
Check the availability of the antidote early rather than at the point of use. In the firing-range outbreak, the choice between oral succimer and intravenous sodium calcium edetate was made "dependent on stock availability",³ and the authors flag "potential issues surrounding stock availability of rarely used antidotes" explicitly.³ This is a real constraint on a real UK incident, not a theoretical one.

The antidote, from the poison's side

Lead is the only poison in this band with four plausible chelators and a genuine, published comparison between the two principal ones. A systematic review of the experimental and clinical literature to 2009 found that "oral DMSA and parenteral sodium calcium edetate are both effective chelators of lead" but that "there are currently insufficient data... to conclude that either antidote is superior in enhancing lead excretion".² What the review does establish is that they work in different places, and that difference is the most useful thing on this section of the page.

Sodium calcium edetate — a bone chelator: It "chelates lead by displacement of the central Ca2+ ion with Pb2+",² which is chemically the mirror of how lead got into the patient in the first place. "The primary source of lead mobilized by sodium calcium edetate is bone with an additional contribution from kidney and liver."² It must be given parenterally, is not metabolised, and causes dose-related nephrotoxicity.² It also depletes zinc and copper, the zinc effect "being significantly greater" than with succimer.²
Succimer (DMSA) — a kidney chelator: Absorption of oral DMSA "is more complete" than sodium calcium edetate, which has to be given parenterally.² It is "extensively metabolized to mixed disulfides of cysteine", and there is evidence those mixed disulfides are the active chelating moiety in humans — which, as the review notes, "suggests that chelation occurs principally, if not exclusively, in the kidney".² Correspondingly, in the experimental studies reviewed, DMSA was more effective than edetate at reducing kidney lead concentrations and edetate more effective at reducing bone lead concentrations.² Those are animal data; the statement that edetate mobilises lead principally from bone is made unqualified in the review's pharmacokinetic section and does not depend on them.² Its characteristic adverse effects are a transient transaminase rise and, occasionally, "a severe mucocutaneous reaction necessitating discontinuation".²
Penicillamine — the one with a UK licence: Penicillamine is the only chelator in this band whose UK marketing authorisation names lead poisoning, listing "Lead poisoning in adults and children (0 to 18 years)" among its indications.⁴ Its dosing is instructive about how the poisoning is thought about: adults 1000 to 1500 mg daily in divided doses "until urinary lead is stabilised at less than 0.5 mg per day", and in children 15 to 20 mg/kg/day in 2 to 3 doses — but "only... in cases where blood lead levels <45 mcg/dL".⁴ The paediatric licence is restricted to the less poisoned child, which is the opposite of the intuition that worse poisoning earns oral chelation, and it is worth knowing before reaching for it in a severe case.
Dimercaprol — the oldest, and the one iron forbids: The original dithiol, developed in wartime Britain as an antidote to an arsenical warfare agent.⁶ It remains in use for severe lead poisoning, historically alongside edetate. It is also the chelator explicitly contraindicated on the iron page, where the label states it "should not be used since it forms a toxic complex with iron" — a reminder that the chelators in this band are not interchangeable across metals.
What the UK actually used, in a real outbreak: In the 2024 firing-range series, 15 patients were chelated with oral succimer 30 mg/kg/day or intravenous sodium calcium edetate 75 mg/kg/day, "dependent on stock availability".³ The choice was made on supply rather than on pharmacology, which is a fair description of how this decision is often made.

The antidote does not reach the organ that matters most

The claim that chelation treats lead encephalopathy by removing lead from the brain is not supported by the evidence assembled in its own systematic review. "There is no evidence that either antidote crosses the blood-brain barrier to any major extent", both being "distributed predominantly extracellularly", and in the experimental studies reviewed there was "no consistently observed effect of chelation therapy on brain lead concentrations".² Traditional teaching This is not an argument against chelating an encephalopathic patient — the same review records that both antidotes "resolve the symptoms of moderate and severe lead toxicity rapidly", and that there is "greater clinical experience with sodium calcium edetate, particularly in the treatment of lead encephalopathy".² It is an argument against the mechanism usually given for why it works. Lowering the blood and soft-tissue burden may well help a brain by changing the gradient it sits in; that is a different claim from extracting lead out of neurons, and only the second is refuted.

Critical appraisal

One evidence-tier downgrade appears on this page, on the claim that chelation works by removing lead from the brain. The citation for the doubt is direct and is not an absence: the systematic review states there is "no evidence that either antidote crosses the blood-brain barrier to any major extent" and reports "no consistently observed effect of chelation therapy on brain lead concentrations" in the experimental studies it reviewed.² The clinical practice is not downgraded — only the mechanism offered for it.
Two claims that might have earned a badge and deliberately did not. First, that chelation improves outcome: the review's finding of "insufficient data" to rank the two antidotes² is a statement about comparison, not about efficacy, and the same review records rapid symptom resolution with both. Second, the ALAD mechanism itself, which is standard, uncontested biochemistry and is badged established under the house rule that a textbook mechanism not actively contested defaults to that tier.
The blood lead concentration table in the source is deliberately not reproduced, because TOXBASE is among the seven references it carries and the individual figures cannot be traced to individual sources. This is the second time in two bands that a free government document has been found carrying a login-gated table. The rule generalises and should be applied on sight.
The occupational action levels of 25 and 50 µg/dL are presented as regulatory limits and not as clinical thresholds, and this distinction is load-bearing on a page whose central claim is that the sensitive endpoints have no threshold.¹ Presenting a biological-monitoring trigger as a treatment threshold would contradict the page's own argument.
No lethal dose or comparative-lethality figure appears anywhere on this page, consistent with the library's editorial boundary. The concentrations given are those a cited source attaches to a decision — an anaemia endpoint, a licensed paediatric restriction, an observed peak in a named cohort.
EXTRIP has not addressed lead.⁵ The absence is uninformative rather than a gap, and for a clearer reason than on most pages: extracorporeal removal treats what is in the plasma, and on this page the plasma holds about 5% of the poison¹ in slow exchange with a skeletal store. The kinetics answer the question before a guideline is needed.
The firing-range outbreak is a single UK case series and is cited as one.³ Its value here is that it is recent, British, and describes the diagnostic problem — 9 symptomatic of 63 tested — rather than that it establishes anything about mechanism.

How this page connects to the rest of the library

Lead is the band's argument for why the band exists. Its mechanism is molecular mistaken identity — a divalent cation occupying calcium's and zinc's places — and the same shape recurs on thallium, which is mistaken for potassium, and on arsenic, where arsenate is mistaken for phosphate. Its measurement problem is digoxin's, taken further: a large distributed burden and a small sampled compartment. Its store-and-release kinetics have no parallel anywhere else in the library. And it is the one page here where the antidote's target organ and the poison's target organ are different² — on experimental evidence, which the page flags where it is used — a distinction the thallium page pushes further still, since there the standard metal chelators are not merely ineffective but positively contraindicated.

Evidence tier of the mechanisms on this pageEstablishedDemonstrated in humans, or in a model that reproduces the human syndromeInferredConsistent with the biochemistry and widely accepted, but the causal step has not been shown in humansTraditional teachingTaught and repeated but not demonstrated — the source questioning it is cited

References

1
UK Health Security Agency. Lead: toxicological overview. Compendium of Chemical Hazards. gov.uk. Supplies the kinetics, the compartment fractions, the two half-lives, the ALAD lesion, the no-threshold statements and the UK occupational limits. Its reference 22 is NPIS TOXBASE; claims resting on that reference alone are not used here, and its table of effects against blood lead concentration is deliberately not reproduced.
2
Bradberry S, Vale A. A comparison of sodium calcium edetate (edetate calcium disodium) and succimer (DMSA) in the treatment of inorganic lead poisoning. Clinical Toxicology 2009;47(9):841–58. PMID 19852620. Systematic review of Medline, Toxline and Embase to June 2009. Source for the blood–brain barrier statement that carries this page's evidence-tier downgrade, for the bone-versus-kidney difference between the two chelators, and for their adverse-effect profiles.
3
Warsi A, Pucci MR, Bradberry SM, et al. Outbreak of lead poisoning from a civilian indoor firing range in the UK. Occupational and Environmental Medicine 2024;81(3):159–62. PMID 38302418. 87 identified as possibly exposed, 63 tested, ages 6 months to 78 years, highest presenting blood lead 11.7 µmol/L (242 µg/dL), 9 symptomatic, 15 chelated with succimer 30 mg/kg/day or sodium calcium edetate 75 mg/kg/day depending on stock availability.
4
Penicillamine 125 mg film-coated tablets — Summary of Product Characteristics, §4.1 and §4.2(d). electronic Medicines Compendium, product 2712. The only UK marketing authorisation among this page's chelators to name lead poisoning. Supplies the adult, elderly and paediatric dosing and the paediatric restriction to blood lead below 45 µg/dL.
5
EXTRIP Workgroup. Recommendations index. extrip-workgroup.org. Cited for an absence: the published set covers 22 substances and lead is not among them. Verified against the site's own menu, 7 September 2026.
6
Kosnett MJ. The role of chelation in the treatment of arsenic and mercury poisoning. Journal of Medical Toxicology 2013;9(4):347–54. PMID 24178900. Cited here only for the origin of dimercaprol as a wartime antidote to an arsenical agent; its substantive use is on the arsenic and mercury pages.
7
National Poisons Information Service. TOXBASE — NPIS 0344 892 0111. The authoritative UK source for lead risk assessment and chelation indications. Login-gated, and deliberately not quoted anywhere on this page.

Last reviewed 2026-09-08 · Author: Dr Nirmalya Hore

Lead

At a glance