Hydrogen sulfide toxicity

Background

  • Hydrogen sulfide (H₂S) is an extremely toxic, colorless, flammable gas that inhibits mitochondrial cytochrome C oxidase — a mechanism nearly identical to cyanide.
  • It is the most common cause of fatal toxic gas exposure in the workplace.[1]
  • The classic toxidrome is knockdown (rapid loss of consciousness), pulmonary edema, conjunctivitis, and olfactory paralysis.[2]


General information

  • Colorless, highly flammable, heavier-than-air gas
  • "Rotten egg" odor at low concentrations (detectable at ~0.5 ppb)[3]
  • WARNING: Odor is NOT a reliable safety indicator — olfactory nerve fatigue occurs at ≥100 ppm (within 2-15 minutes), causing complete loss of smell at the very concentrations that are dangerous[2]
  • Heavier than air → accumulates in enclosed, low-lying, and poorly ventilated spaces (sewers, manure pits, tanks)
  • Encountered in:
    • Oil and gas industry (primary chemical hazard of natural gas production)
    • Sewers, wastewater treatment
    • Manure pits, animal confinement facilities
    • Paper/pulp mills, tanneries
    • Roofing/asphalt operations
    • Mining, smelting
    • Hot springs, volcanic areas
  • Suicide method ("detergent suicide"): mixing acidic household cleaners (toilet bowl cleaner) with sulfur-containing products (bath salts, lime sulfur pesticide) to generate H₂S in an enclosed space — increasing in prevalence[1]
  • Most fatalities occur in confined spaces; rescuers entering without SCBA frequently become secondary victims[3]
  • Children are at higher risk at the same ambient concentration due to higher minute volume:weight ratios and shorter stature (gas settles near the ground)[3]

Concentration-dependent effects

Concentration (ppm) Effects
0.01-0.03 Rotten egg odor detectable
2-5 Nausea, headache, tearing with prolonged exposure
10-20 Eye irritation ("gas eye"), mild respiratory irritation
50-100 Conjunctivitis, respiratory tract irritation; olfactory fatigue begins at 100 ppm
100-200 Olfactory paralysis (loss of smell); cough, eye irritation, drowsiness; potentially fatal after 4-48 hours
200-300 Marked conjunctivitis, respiratory irritation after 1 hour; pulmonary edema risk
500-700 Staggering, collapse within 5 minutes; serious eye damage; fatal in 30-60 minutes
700-1000 Rapid "knockdown" — LOC within 1-2 breaths; respiratory paralysis; death within minutes
>1000 Immediate collapse, cardiac arrest, death after single breath

Mechanism of toxicity

  • Rapidly absorbed through the lungs
  • Inhibits mitochondrial cytochrome C oxidase (complex IV) by binding to the ferric (Fe³⁺) moiety → blocks aerobic metabolism → cellular asphyxia[2]
    • Identical mechanism to cyanide
    • Tissues with high oxygen demand (brain, heart) are most sensitive
  • Causes lactate accumulation and metabolic acidosis from impaired oxidative phosphorylation
  • Causes hyperpolarization of potassium-mediated neuronal channels → CNS depression[1]
  • Potentiates neuronal inhibitory mechanisms; alters brain neurotransmitter content
  • Direct mucosal irritant → conjunctivitis, pulmonary edema
  • At very high concentrations, stimulates carotid/aortic chemosensors → initial hyperpnea, then central respiratory arrest

Healthcare worker safety

  • Rescuers entering confined spaces without SCBA frequently become fatalities themselves[3]
  • SCBA is mandatory for any responder entering the hot zone
  • Chemical-protective clothing is generally NOT needed (H₂S is not significantly absorbed through skin)
  • In detergent suicide scenarios:
    • Approach with extreme caution; do not enter enclosed space (car, bathroom) without respiratory protection
    • Look for warning signs posted by the patient
    • Ventilate the space before entry
  • Patients exposed only to H₂S gas do not generally require decontamination
  • H₂S burns to produce sulfur dioxide (SO₂) — a secondary hazard if ignited

Clinical features

  • The classic toxidrome: "knockdown + pulmonary edema + conjunctivitis + olfactory paralysis"[2]

Respiratory

  • Dyspnea, cough, chest tightness
  • Hemoptysis
  • Bronchospasm, wheezing
  • Rales, rhonchi
  • Pulmonary edema — may be immediate or delayed up to 72 hours[3]
  • ARDS
  • Central respiratory arrest at high concentrations

Ophthalmic ("gas eye")

  • Conjunctivitis (often the earliest sign at low concentrations)
  • Corneal ulceration, corneal scarring
  • Photophobia, tearing
  • Blepharospasm

Central nervous system

  • Headache, dizziness, weakness (early)
  • "Knockdown" — sudden loss of consciousness within 1-2 breaths at ≥500 ppm; may be followed by apparent rapid recovery, but can leave lasting neurologic injury[2]
  • Disequilibrium, ataxia
  • Intention tremor, muscle rigidity
  • Seizures
  • Coma

Neuropsychiatric (may be delayed/persistent)

  • Amnesia (particularly surrounding the event)
  • Lack of insight, disorientation
  • Delirium
  • Cognitive impairment, dementia
  • Depression, personality changes
  • Peripheral neuropathy
  • Persistent neurologic deficits may follow "knockdown" events even with apparent recovery[2]

Cardiovascular

  • Chest pain
  • Bradycardia
  • Tachycardia (early sympathetic stimulation)
  • Hypotension
  • Myocardial injury (elevated troponin/CK reported)[4]
  • ECG: T-wave changes, P-wave abnormalities, QTc prolongation
  • Cardiac arrest (PEA, asystole)

Gastrointestinal

  • Green-gray line on gingiva (characteristic but not always present)
  • Nausea, vomiting, diarrhea

Other

  • Cyanosis (from respiratory failure, not methemoglobinemia in most cases)
  • Metabolic acidosis with elevated lactate
  • Gray-green discoloration of skin/blood on autopsy (sulfhemoglobin formation — a postmortem finding)

Differential diagnosis

Toxic gas exposures

Toxic gas exposure

Evaluation

  • No single rapid bedside test exists to confirm H₂S exposure[2]
  • Diagnosis is clinical: exposure history (confined space, rotten egg odor, industrial setting, detergent suicide setup) + compatible symptoms

Labs

  • ABG/VBG: metabolic acidosis with elevated lactate + normal or elevated PaO₂/SaO₂ (cellular asphyxiant — oxygen delivery is adequate but cells cannot use it)
    • Classic finding: lactic acidosis with normal oxygen saturation (unless concurrent pulmonary edema reduces oxygenation)
  • Serum lactate: elevated (reflects impaired oxidative phosphorylation)
  • BMP: electrolytes, renal function
  • CBC: leukopenia, neutropenia reported
  • Hepatic function panel (hepatic injury reported in severe cases)
  • Troponin, CK — myocardial and skeletal muscle injury
  • Methemoglobin level — obtained if nitrite therapy administered (target <30%)
  • Whole blood sulfide: >0.05 mg/L is abnormal; must be obtained within 2 hours of exposure and analyzed immediately; not available in most hospitals and will not affect acute management[2]
  • Urine thiosulfate: reflects H₂S exposure in acute setting; useful for workplace monitoring but rarely available acutely

Imaging

  • Chest radiograph: pulmonary edema (may be delayed 24-72 hours)
  • CT head: consider if persistent altered mental status; basal ganglia necrosis has been reported after severe exposure

Other

  • ECG: ST-T changes, QTc prolongation, dysrhythmias
  • Discolored copper coins found on the patient may turn dark/black (copper sulfide formation) — can serve as a supportive diagnostic clue[1]

Management

Immediate

  • Remove from exposure — ensure scene safety; do not enter confined space without SCBA
  • 100% high-flow oxygen via NRB mask — cornerstone of treatment
    • Competes with H₂S for cytochrome oxidase binding
    • Administer to all patients regardless of SpO₂
  • Airway management: intubate early for altered mental status, respiratory failure, or airway compromise
  • IV access, continuous cardiac monitoring

Antidotal therapy

Hydroxocobalamin (preferred)

  • Hydroxocobalamin (Cyanokit): 5 g IV over 15 minutes; second dose may be given PRN[5]
  • Mechanism: binds sulfide directly, forming sulfhydroxocobalamin
  • Animal models demonstrate prevention of PEA cardiac arrest when given early[5]
  • Emerging as the preferred antidote over nitrites due to safer side-effect profile (does not induce methemoglobinemia) and efficacy against both H₂S and cyanide
  • Cobinamide (vitamin B₁₂ analog) shows promise in animal studies as a potentially superior antidote with greater sulfide-binding capacity[6][7]

Nitrite therapy (alternative)

  • May use the nitrite component of the cyanide antidote kit — do NOT give the thiosulfate portion (thiosulfate is the product of H₂S detoxification and does not help)[8]
  • Mechanism: nitrites induce methemoglobinemia; methemoglobin (Fe³⁺) has high affinity for sulfide → converts it to the less toxic sulfmethemoglobin
  • Amyl nitrite inhaled (pearls crushed and inhaled) — may be used as a bridge
  • Sodium nitrite (3% NaNO₂): 10 mL (300 mg) IV over 2-4 minutes in adults; pediatric dose 0.33 mL/kg (max 10 mL)
  • Obtain methemoglobin level 30 minutes after dose — desired level <30%
  • Caution: nitrite-induced methemoglobinemia can worsen oxygen-carrying capacity in already hypoxic patients; not first-line if hydroxocobalamin is available

Supportive care

  • Aggressive IV fluid resuscitation
  • Bronchospasm: inhaled beta-2 agonists (albuterol), consider racemic epinephrine for stridor in children
  • Seizures: benzodiazepines first-line
  • Pulmonary edema: positive-pressure ventilation (CPAP/BiPAP or mechanical ventilation); diuretics generally not effective for noncardiogenic pulmonary edema
  • Metabolic acidosis: treat with oxygen and supportive care; IV sodium bicarbonate if severe
  • Cardiac arrest: standard ACLS protocols; prolonged resuscitation may be warranted (potentially reversible etiology)
  • Hyperbaric oxygen therapy: has been used; no proven benefit; consider on a case-by-case basis for severe neurologic injury[8]

Things to avoid

  • Do NOT give thiosulfate (the third component of the traditional cyanide antidote kit) — thiosulfate is the end product of endogenous H₂S detoxification and provides no benefit
  • Do NOT rely on odor as a gauge of ongoing exposure — olfactory nerve fatigue renders it unreliable at dangerous concentrations

Disposition

  • Asymptomatic patients with possible low-level exposure: observe for minimum 4-6 hours; if asymptomatic with normal labs and CXR, may discharge with return precautions for delayed pulmonary edema (up to 72 hours)[3]
  • Any symptomatic patient or significant exposure: admit, likely to ICU
    • Continuous cardiac and pulse oximetry monitoring
    • Serial ABGs, lactate, CXR
    • Watch for delayed pulmonary edema (may develop up to 72 hours after exposure)[3]
  • "Knockdown" patients (even with apparent rapid recovery): admit for observation — high risk for persistent or delayed neurologic sequelae[2]
  • Outpatient follow-up:
    • Neuropsychiatric evaluation — cognitive impairment, amnesia, personality changes, and peripheral neuropathy may develop or persist weeks to months after exposure
    • Pulmonary function testing if respiratory symptoms persist
    • Ophthalmologic evaluation for corneal injury
  • Toxicology consultation for all significant exposures
  • Contact Poison control (1-800-222-1222 in the US) for all cases

Medication Dosing

Hydroxocobalamin 5g IV over 15 min, may repeat x1 PRN IV

See Also

External Links

References

  1. ↑ 1.0 1.1 1.2 1.3 Tintinalli et al.
    • Hydrogen Sulfide.
    • In: Tintinalli et al.
    • Emergency Medicine A Comprehensive Study Guide.
    • New York, NY: McGraw Hill. 2011. 1320.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Hydrogen Sulfide Toxicity. StatPearls. 2024.
    • PMID: 32119270
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Hydrogen Sulfide Medical Management Guidelines. Agency for Toxic Substances and Disease Registry (ATSDR). CDC.
  4. ↑ Chou S, et al. Hydrogen sulfide exposure in an adult male. Ann Saudi Med. 2010;30(1):76-80. doi:10.4103/0256-4947.59379
  5. ↑ 5.0 5.1 Haouzi P et al. High-dose hydroxocobalamin administered after H₂S exposure counteracts sulfide-poisoning-induced cardiac depression in sheep. Clin Toxicol (Phila). 2015;53(1):28-36.
  6. ↑ Jiang J et al. Hydrogen Sulfide — Mechanisms of Toxicity and Development of an Antidote. Sci Rep. 2016;6:20831.
  7. ↑ Brenner M et al. The vitamin B₁₂ analog cobinamide is an effective hydrogen sulfide antidote in a lethal rabbit model. Clin Toxicol (Phila). 2014;52(5):490-497.
  8. ↑ 8.0 8.1 Goldfrank et al. Hydrogen Sulfide Poisoning. In: Goldfrank et al. Goldfrank's Toxicologic Emergencies. New York, NY: McGraw Hill. 2002. 1504-1507.
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