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Hydrogen sulfide is considered a broad-spectrum poison; it can poison several different systems in the body, although the central nervous system is mostly affected. The specific mechanism that causes issues in the body is that hydrogen sulfide forms a complex bond with iron in mitochondrial cytochrome enzymes, thereby blocking oxygen from binding and stopping cellular respiration.

In simple terms, your cells cannot oxygenate and quickly die, resulting in the shutdown of the vital organs in your body. The body is capable of detoxifying low levels of hydrogen sulfide in the body through oxidation to a harmless sulfate molecule. However, at a certain threshold level, the cells cannot oxygenate the hydrogen sulfide and are overwhelmed. This threshold level is considered to be in the range of 300 to 350 parts per million, or ppm.

Exposure to short-term, low-level concentrations can result in symptoms such as eye irritation, shortness of breath, sore throat, coughing, and possibly fluid in the lungs. These symptoms usually go away in a few weeks. Long-term low-concentration exposure may cause loss of appetite, fatigue, headaches, irritability, memory loss, and dizziness.

The exposure limits for hydrogen sulfide vary among jurisdictions and industry activities. The most widely recognized standards for an eight-hour time weighted average, or TWA, is 1 ppm and a 15-minute short-term exposure limit, or STEL, of no more than 5 ppm is recommended by the American Conference of Governmental Industrial Hygienists, or ACGIH. Hydrogen sulfide concentrations above 100 ppm should be considered immediately dangerous to life and health, or IDLH.

Let’s review the hydrogen sulfide levels at which different symptoms and effects are observed. To start, an extremely low recognition threshold of 0.0047 ppm is recognized as the odor at which 50% of the population can detect the characteristic rotten egg odor of hydrogen sulfide. The odor becomes more offensive at 3 to 5 ppm. With prolonged exposure at 2 to 5 ppm nausea, tearing of the eyes, headaches, or loss of sleep may occur. At 20 ppm, symptoms may include fatigue, loss of appetite, headache, irritability, poor memory, and dizziness. At above 30 ppm, the odor is described as sweet or sickeningly sweet. Exposure to 50 to 100 ppm leads to light conjunctivitis and respiratory tract irritation after one hour. This level of exposure may also cause digestive upset and loss of appetite.

At 100 ppm, the olfactory nerve is fatigued after two to fifteen minutes of exposure, resulting in a loss of smell. Additional symptoms, such as coughing, eye irritation, altered breathing, and drowsiness, may occur after fifteen to thirty minutes of exposure. Throat irritation can occur after one hour of exposure. Death may occur after forty-eight hours.

At 100 to 150 ppm, the loss of smell occurs immediately. At 200 to 300 ppm, exposure leads to marked conjunctivitis and respiratory tract irritation after one hour. Pulmonary edema may occur from prolonged exposure. At 500 to 700 ppm, exposure will cause staggering and collapse within five minutes, serious damage to the eyes within thirty minutes, and death within thirty to sixty minutes. Hydrogen sulfide concentrations greater than 700 to 1,000 ppm can result in rapid unconsciousness, knockdown, or immediate collapse within one to two breaths, resulting in death within minutes. At 1,000 ppm, a single breath results in immediate loss of consciousness, followed by cardiac arrest.

An important point must be noted here: a concentration of 800 ppm is the lethal level for 50% of humans with five minutes of exposure. Research has shown that symptoms of hydrogen sulfide exposure vary considerably because of individual physiological make-up. Studies have indicated that persons previously exposed to hydrogen sulfide tend to be hyper-susceptible to further exposure rather than building up a tolerance to it.


Get more information about the Hydrogen Sulfide Safety course here.