Toxic Metals and Anions: Arsenic, Lead, Mercury, Thallium and Beyond

The forensic toxicologist cares about four arsenic species. Trivalent As(III), as arsenic trioxide As2O3 (classical white arsenic), is the most toxic: it binds enzyme thiols, locks up pyruvate dehydrogenase and generates mitochondrial ROS. Pentavalent As(V) is roughly 60 times less acutely toxic but the body reduces a fraction to As(III) in vivo, so chronic exposure still produces the full syndrome. MMA and DMA are urinary detoxification products. Arsenobetaine, the marine species in fish, is non-toxic and clears within 48 hours: a hair-arsenic finding from a victim who ate prawns needs speciation before any homicidal verdict.

Acute arsenic poisoning produces severe gastroenteritis within 30 minutes to 2 hours: violent vomiting, rice-water diarrhoea, faint garlic odour and hypovolaemic shock. Death follows within 24 to 48 hours above 100 to 200 mg of As2O3. Chronic exposure, what the Indian SFSL sees most often, presents differently: Mees lines, stocking-and-glove neuropathy, raindrop hyperpigmentation on the trunk, palmar and plantar keratosis, and on long exposure the malignancies (Bowen's disease, squamous skin cancer, bladder cancer, lung cancer).

The Indian groundwater crisis is one of the largest mass poisonings in history. The Bengal basin aquifer carries naturally occurring arsenic released from iron oxyhydroxide minerals into shallow tube-well water at concentrations routinely above the BIS IS 10500 limit of 50 micrograms per litre and the WHO guideline of 10. Worst-affected districts: Murshidabad, Nadia, North and South 24 Parganas and Maldah in West Bengal, where village surveys log wells at 500 to 3000 micrograms per litre. Bihar's Bhojpur, Buxar and Patna along the Ganga sit in the same hydrogeology. Assam, Jharkhand, eastern UP (Ballia, Ghazipur) and parts of Manipur add to the load. Estimates put the at-risk population at 50 million.

Detection begins with the Reinsch test, still run as a presumptive screen at smaller state FSLs. A copper strip in HCl-acidified sample at near-boiling for 30 to 60 minutes picks up arsenic, antimony, mercury and bismuth, but only arsenic gives the white octahedral As2O3 crystals on dry heating. The Marsh test (1836) is the sensitive successor: arsine from zinc and dilute H2SO4 decomposes in a heated tube to deposit a metallic mirror that dissolves in hypochlorite (antimony does not, the differentiating step). Modern HG-AAS runs the same chemistry at 193.7 nm with detection limits at 0.1 microgram per litre. ICP-MS reads arsenic at mass 75 with an oxygen reaction cell to eliminate ArCl interference. Hair segmental analysis (1 cm per month) reconstructs the past year. Treatment: intramuscular BAL for acute severe cases, oral DMSA for moderate, DMPS where available.

Lead enters Indian victims through routes the Western paediatric textbook does not list. The single largest contributor to paediatric blood lead is surma and kohl, applied to infants from the first weeks of life. Surma sold across north and eastern Indian bazaars routinely runs 60 to 90 percent lead sulphide (galena) by weight. The powder reaches the conjunctival sac, is partly ingested through the nasolacrimal duct, partly transferred to hands and mouth. A 2014 NIN Hyderabad survey found blood lead above 10 micrograms per decilitre in nearly half of surma-exposed urban infants. Cheap temple-market sindoor adds lead chromate and lead tetroxide in defiance of FSSAI rules.

Ayurvedic bhasma is the third route. Naga bhasma is calcined lead by classical formulation; patients with arthritis, infertility or chronic skin disease consume it for months, blood lead climbs to 40 to 80 micrograms per decilitre and they present with anaemia, abdominal colic and peripheral neuropathy. Occupational exposure rounds it out: battery recyclers, smelters in Loni and Sahibabad, e-waste clusters in Seelampur and Mustafabad. Lead-based paint was banned in 2016 with a 90 ppm BIS cap, but residual paint, toys and cosmetics still circulate. The pencil-tip "lead" is a myth: pencils have contained graphite for two centuries.

The mechanism is biochemical substitution. Lead's ionic radius is close to calcium and zinc and displaces both. The hallmark is inhibition of delta-aminolevulinic acid dehydratase (ALAD), the second enzyme in haem biosynthesis. ALA accumulates, porphobilinogen falls, haem production declines. The smear shows microcytic hypochromic anaemia with basophilic stippling. Lead also inhibits ferrochelatase, which raises zinc protoporphyrin (ZPP). The adult presents with abdominal lead colic, wrist drop from radial neuropathy, gum-line Burton's line (blue-black sulphide deposit from H2S generated by gum bacteria), hypertension and cognitive decline. The paediatric end is encephalopathy: irritability, ataxia, seizures and coma above 70 micrograms per decilitre.

Detection runs by GFAAS on whole blood (calibrated against NIST SRM 955c) or by ICP-MS on whole blood digested in nitric acid. Sample collection uses lead-free royal blue top tubes to avoid contamination from rubber stopper lead. Urine ALA, blood ZPP and blood ALAD activity are the biological-effect markers. Treatment escalates from source removal (the only intervention that works long-term) to oral DMSA, to intramuscular BAL plus IV Ca-EDTA in severe cases or encephalopathy, with D-penicillamine as a slower oral alternative.

Mercury's three forms are effectively three separate poisons. Elemental Hg0 is the silvery thermometer liquid: non-toxic if swallowed (GI absorption below 0.1 percent), but its room-temperature vapour produces inhaled exposure 50 times the WHO guideline in a closed room. Inhaled Hg0 crosses the alveolus, oxidises to Hg2+, accumulates in brain and kidney. The classical occupational picture from felt hat industry mercuric nitrate was "mad as a hatter": fine tremor, erethism (irritability, social withdrawal), gingivitis, salivation and peripheral neuropathy.

Inorganic mercury salts (HgCl2, "corrosive sublimate") are the homicidal form. Ingestion produces immediate corrosive gastroenteritis with grey mucosal burns, severe vomiting, bloody diarrhoea and renal tubular necrosis within 24 to 72 hours. Lethal dose is 1 to 4 grams. Organic methylmercury (CH3Hg+) is the bioaccumulating form: methylated by sulphate-reducing bacteria in anoxic sediments, biomagnified up the food web to predatory fish (tuna, swordfish, shark, large catfish). Methylmercury crosses the blood-brain barrier and the placenta; Minamata disease in 1950s Japan produced cerebral palsy, ataxia and constricted visual fields in the children of exposed mothers.

Indian relevance runs four ways. Sindoor: traditional cinnabar (HgS) is highly insoluble and minimally absorbed, but cheap modern sindoor substitutes red lead and other mercury salts. Dental amalgam contains roughly 50 percent elemental mercury; millions of older fillings still circulate. Artisanal gold mining in the Singhbhum belt of Jharkhand and parts of Karnataka and Andhra still uses elemental mercury for amalgamation, burning it off in the open. Ayurvedic rasashastra: parad bhasma is mercury by name, and makaradhwaja, rasasindur and many kushtha formulations contain calcined mercury sulphide.

Detection demands cold-vapour AAS (CV-AAS): nitric acid digest, stannous chloride reduction to Hg0 vapour, quartz cell at 253.7 nm with detection limits at 0.05 microgram per litre. ICP-MS at mass 202 is the alternative. Urine mercury is the biomarker for inorganic and elemental exposure (normal below 5 micrograms per litre); hair mercury, segmented, is the methylmercury biomarker. Treatment is BAL for inorganic (caution: not for organic, BAL can redistribute methylmercury into the brain), DMSA for moderate organic, DMPS as the preferred modern agent.

Thallium sulphate, acetate and chloride salts are colourless, odourless and tasteless. The salts (sulphate, acetate, chloride) are colourless, odourless, tasteless, water-soluble and absorbed completely from the gut. Lethal dose is 8 to 12 mg per kilogram, comparable to a teaspoon of salt in an adult. The clinical picture takes 1 to 3 weeks to develop, which removes the post-meal association that flags acute arsenic or cyanide. The Saddam Hussein regime used thallium against dissidents in the 1980s. The Australian poisoner Caroline Grills, "Aunt Thally", killed at least four relatives in 1940s Sydney with thallium-laced tea. Indian cases are rare but not absent: a 1970s Manipur case linked to political poisoning, occasional academic incidents, and rat-poison ingestions before Celio paste was phased out.

The clinical sequence is distinct. Within 24 to 72 hours: GI symptoms (nausea, vomiting, retrosternal pain). Days 3 to 14: painful ascending peripheral neuropathy starting at the feet, severe burning pain unresponsive to analgesics. Days 14 to 21: the most characteristic sign, alopecia. Hair falls out in handfuls, leaving a near-bald scalp by day 30. Mees lines appear on the nails. Blue gum line appears. Cranial nerve palsies, optic atrophy, encephalopathy and coma follow in severe cases. Untreated mortality runs above 50 percent at lethal-range doses.

Detection runs by ICP-MS at mass 205 (with 203 as the confirming isotope; natural 205Tl/203Tl ratio of 2.39 is the isotope-ratio check). GFAAS at 276.8 nm is the alternative. Urine thallium is the standard sample (normal below 0.5 microgram per litre, acute poisoning above 50). Hair segmental analysis is valuable when the exposure history is unclear because thallium incorporates into the hair shaft within days.

Antimony shares chemistry with arsenic. Sb(III) (tartar emetic) and Sb(V) (sodium stibogluconate, used for visceral leishmaniasis in Bihar and West Bengal) are active. Acute antimony resembles arsenic. The Reinsch deposit on copper does not redissolve in hypochlorite. ICP-MS at mass 121.

Bismuth was a routine ingredient in colonial-era pharmacopoeia and reappears in ayurvedic preparations. Chronic bismuth produces encephalopathy with myoclonus and gum blackening.

Cadmium is the smoker's metal. One cigarette delivers 1 to 3 micrograms of inhaled cadmium. Chronic exposure produces renal tubular damage (proteinuria with beta-2-microglobulin), osteomalacia (Itai-itai disease in Japan from cadmium-laden rice) and lung cancer. Indian occupational clusters: battery manufacture, electroplating, e-waste recycling. ICP-MS at mass 111.

Selenium is the paradox metal: essential at microgram-per-day intake, toxic above 400 micrograms per day. Acute selenosis: garlic odour, hair loss, brittle nails with Mees-like lines, peripheral neuropathy. India has a selenium-rich groundwater belt in Hoshiarpur and Nawanshahr (Punjab) where seleniferous shales weather into the water table. ICP-MS at mass 78 with helium collision cell to remove Ar-Ar dimer interference.

Oxalate is the toxic metabolite of ethylene glycol, the antifreeze that is the second most common alcohol substitute poisoning in India after methanol. Alcohol dehydrogenase oxidises ethylene glycol through glycolaldehyde and glycolic acid to oxalic acid. Oxalate precipitates with serum calcium as needle-shaped birefringent crystals in renal tubules, urine sediment and brain perivascular spaces at autopsy. Severe anion-gap acidosis follows at 6 to 12 hours, oliguric renal failure at 24 to 72 hours. Detection runs by ion chromatography on urine and vitreous humour. Fomepizole is the modern antidote; ethanol infusion is the older but still standard Indian approach.

Fluoride enters Indian victims through groundwater and chronic chewing of khaini and kalsi (tobacco-lime mixture). Dental fluorosis and skeletal fluorosis (genu valgum, restricted spinal mobility, hyperostosis) are endemic across Rajasthan (Nagaur, Sirohi, Jodhpur), Andhra Pradesh (Nalgonda, Prakasam), Karnataka (Bellary, Tumkur) and Gujarat (Banaskantha, Mehsana). The BIS IS 10500 limit is 1 mg per litre with relaxation to 1.5; endemic villages run 3 to 12 mg per litre. Acute sodium fluoride ingestion produces gastritis, hypocalcaemia and cardiac arrhythmia. Detection: ion-selective electrode and ion chromatography.

Cyanide is treated in the volatile-poison chapter but the anion CN- is what the analyst measures. The Conway microdiffusion plate (outer well: sample plus dilute H2SO4 liberates HCN; inner well: NaOH traps as Na-CN) is the classical Indian SFSL screen. The trapped cyanide is measured by chloramine-T pyridine-barbituric acid spectrophotometry (König reaction) or by ion chromatography.

A viscera jar of 50 to 100 grams (liver, kidney, stomach with contents) arrives at the FSL with the chemical examiner's form, inquest copy and a request for the heavy-metal panel. A 5 gram aliquot goes to wet digestion in nitric acid with hydrogen peroxide, microwave-accelerated in a CEM MARS or Anton Paar Multiwave vessel at 200 C for 30 minutes. The clear digest is made up to 25 mL and aspirated into the ICP-MS for the multi-element panel (As, Pb, Hg, Tl, Sb, Cd, Se, Cr, Ni, Cu, Zn, Mn, Ba, Sr) in one injection, with internal standards (Sc, Ge, Rh, Re) added by online mixing.

A separate cold-vapour run on a Hg-specific digest (analysed within 24 hours because mercury is lost as vapour over time) gives the mercury number with the lowest blank. Hair and nail clippings are washed with acetone and deionised water, segmented to 1 cm pieces, digested in nitric acid in sealed PFA vessels and run on ICP-MS. Blood lead runs as a dedicated GFAAS method on whole blood digested in nitric acid plus Triton X-100, calibrated against NIST 955c, with bovine blood QC every 20 samples for NABL audit. CFSL Chandigarh, FSL Madhuban, FSL Kalina and FSL Hyderabad run this workflow as routine; state SFSLs in Lucknow, Bhopal, Bengaluru, Kolkata, Patna and Guwahati are at varying stages of the transition. Reinsch and Marsh are still tendered as confirmatory wet-chemistry evidence in lower courts when the instrumental tier is not available.