Health effects and the dose–response

Hydrogen sulfide affects the body in two main ways. At low concentrations it is an irritant, especially to the eyes and airways. At higher concentrations it is a systemic poison that blocks cell respiration and acts on the nervous system, stopping breathing. Between these two, and depending on how long a person is exposed, it can cause fluid in the lungs, loss of consciousness and lasting damage to the brain. This lesson describes those effects, how they depend on concentration and time, and what this means for setting limits and alarms.

How the gas enters and acts

Inhalation is the main route of exposure. Hydrogen sulfide passes quickly from the lungs into the blood. The body can break down small amounts, mainly by oxidizing it in the liver and blood, which is why low exposures cause irritation rather than poisoning. When the concentration is high, the body's ability to detoxify it is overwhelmed, and the gas binds to cytochrome c oxidase in the mitochondria, the energy factories of each cell. Cells cannot use oxygen, and the tissues that need the most energy, the brain and the heart, fail first.

Hydrogen sulfide also acts directly on nerves. It deadens the olfactory nerve, which carries the sense of smell, and at high concentrations it paralyzes the respiratory center in the brainstem, so breathing stops. This combination explains the classic sudden collapse, often described by survivors as being hit without warning.

Because the body breaks down hydrogen sulfide and does not store it, the gas does not build up over days the way lead or some solvents do. That is why limits focus on peak and short-term concentrations, and why a person removed from exposure who is still breathing often recovers quickly. It also means that blood tests after an incident are of limited use; the gas is gone, though its damage may remain.

Effects by concentration

The effects of hydrogen sulfide depend mostly on concentration, and secondarily on how long a person breathes it. The table summarizes the range of effects reported at different concentrations. Individual responses vary, and the figures should be read as approximate.

Two features of this range are striking. First, it spans five orders of magnitude, from barely smelled at 0.01 ppm to rapidly fatal at 1,000 ppm, which is why a logarithmic scale is used to show it. Second, the steps near the top are small: the concentration that causes loss of smell (around 100 to 150 ppm) is only five to ten times lower than the concentration that causes collapse. In a strong release, a person can move from irritating to deadly in a few steps.

Acute effects

Eye effects are among the most common complaints at low and moderate concentrations. Workers in sewers and pulp mills describe sore, gritty, watering eyes, sometimes called gas eye or sore eyes. Prolonged or higher exposure can inflame the cornea and cause temporary blurred vision and sensitivity to light, with halos around lights.

Respiratory effects range from a sore throat and cough to chest tightness and wheezing. At higher concentrations, hydrogen sulfide can injure the lining of the lungs, letting fluid leak in. This pulmonary edema may develop hours after exposure, so anyone who has had a significant exposure should be assessed by a doctor even if they feel better.

Nervous system effects at moderate concentrations include headache, dizziness, nausea, agitation, confusion and drowsiness. At high concentrations, a person can lose consciousness suddenly. Survivors of severe knockdowns often have no memory of the event.

Heart effects include irregular heart rhythms, and the heart's own energy supply is affected by the same block on cell respiration.

Knockdown and its aftermath

A knockdown is a sudden loss of consciousness from a high concentration. If the person is removed quickly into fresh air and breathing is supported, they may recover within minutes. If breathing has stopped and is not restored, brain damage begins within a few minutes and death follows. Some knockdown survivors recover fully. Others have lasting effects, including memory problems, poor concentration, movement disorders, headaches and personality changes, consistent with brain injury from lack of oxygen and direct toxicity.

Falls are a major secondary hazard. A worker who collapses on a ladder, a tank top, a catwalk or at the edge of a pit may be seriously injured or drown in the liquid below. That is one reason fall protection and retrieval systems are part of hydrogen sulfide planning in confined spaces and on tanks.

Long-term and repeated exposure

Repeated low-level exposure to hydrogen sulfide is associated with eye irritation, headaches, fatigue, poor sleep and irritability. Some studies of workers and communities exposed over long periods have reported effects on the nervous system and breathing, but the evidence is less clear than for acute effects, partly because exposure is often mixed with other gases. Hydrogen sulfide is not classified as a carcinogen. It is not known to build up in the body.

The ACGIH lowered its threshold limit value for hydrogen sulfide in 2010 from 10 ppm to 1 ppm as an eight-hour average, with a 5 ppm short-term limit, based on evidence of eye and respiratory effects at low concentrations and effects on the nose in animal studies. As lesson 5 explains, not every jurisdiction has adopted these lower values.

Who is more vulnerable

People with asthma and other lung disease may react to concentrations that do not bother others. People with heart disease may be more sensitive to the heart effects. Alcohol and some medications may worsen the nervous system effects. Physical work increases breathing rate and therefore dose. In confined spaces, low oxygen and other gases such as carbon dioxide, methane and ammonia often accompany hydrogen sulfide, and the combined effect is worse than any one alone.

Concentration and time

For many toxic gases, the harm depends roughly on concentration multiplied by time: double the concentration and you reach the same harm in half the time. For hydrogen sulfide, concentration matters more than time. Doubling the concentration more than halves the time to serious harm, because high concentrations overwhelm the body's detoxification and act directly on breathing. Emergency planning values reflect this. The table below gives illustrative times to collapse based on the effects table; they are not limits and should never be used to decide how long someone can stay in an atmosphere.

# Illustrative: rough time to serious incapacitation at different concentrations
# (midpoints read from published effect ranges; for teaching only, not limits).
effects = [
    (100, "drowsiness in 15-30 min; prolonged exposure can kill"),
    (300, "lung irritation within an hour; edema possible"),
    (600, "collapse within about 5 minutes"),
    (900, "unconscious within minutes, breathing stops"),
    (1500, "near-instant collapse"),
]
for ppm, note in effects:
    ratio_to_idlh = ppm / 100
    print(f"{ppm:5d} ppm  ({ratio_to_idlh:4.1f} x IDLH): {note}")

  100 ppm  ( 1.0 x IDLH): drowsiness in 15-30 min; prolonged exposure can kill
  300 ppm  ( 3.0 x IDLH): lung irritation within an hour; edema possible
  600 ppm  ( 6.0 x IDLH): collapse within about 5 minutes
  900 ppm  ( 9.0 x IDLH): unconscious within minutes, breathing stops
 1500 ppm  (15.0 x IDLH): near-instant collapse

The output has no exact arithmetic, but the pattern is clear. Anything above the immediately dangerous level of 100 ppm, and especially above 500 ppm, is a situation where the only protection is breathing apparatus or not being there.

Immediately dangerous to life or health

NIOSH sets immediately dangerous to life or health (IDLH) values for many chemicals. The IDLH for hydrogen sulfide is 100 ppm. An IDLH atmosphere is one where a worker without respiratory protection could be killed or suffer irreversible health effects, or be prevented from escaping. Under respiratory protection rules, only pressure-demand self-contained breathing apparatus or a combination supplied-air respirator with an escape bottle may be used in an IDLH atmosphere, as lesson 8 explains, building on HSE-240.

Medical surveillance and records

There is no routine blood or urine test that tracks hydrogen sulfide exposure in the way blood lead is tracked. Medical programs for workers in hydrogen sulfide areas focus instead on fitness to wear respirators, eye and respiratory symptoms, and the response to any exposure incident. Every significant exposure, including any knockdown and any time a worker smelled gas strongly or felt symptoms, should be reported, recorded and investigated. Incidents that seem minor, such as a short episode of dizziness during tank gauging, are warnings that controls are not working.