Hydrogen Sulphide (H2S) Measurement

Hydrogen sulphide and safety.

Hydrogen sulphide (H2S) is a toxic and acidic gas. It is present to some degree in all natural gas and needs to be removed prior to the gas entering a transmission system. The combination of H2S and water vapour can produce sulphuric acid that corrodes pipeline walls.

High H2S levels in natural gas are closely monitored, because it causes not only corrosion problems, but toxicity is a problem if there is a break in containment.

Hydrogen sulphide levels and their effect:




0.00011 to 0.00033

Typical background.

0.01 to 1.5

A smell of rotten egg is first noticeable. It becomes more offensive at 3 to 5 ppm. Smell is described as sweet or sickeningly sweet.

2 to 5

Prolonged exposure at this level could cause nausea, sometimes eyes water, headaches or loss of sleep. Airway problems can be seen with some asthma patients.


Symptoms include tiredness, loss of appetite, headache, irritability, poor memory and dizziness.

50 to 100

Some people report slight conjunctivitis and respiratory tract irritation after being exposed for about 1 hour. May also cause digestive upset.


This comparatively low level causes coughing and eye irritation. Worryingly, at this level there is a loss of smell after 2 to 15 minutes. Altered breathing, drowsiness after 15 to 30 minutes. After 1 hour there is irritation to the throat and a gradual increase in severity of symptoms over several hours.  Death may occur after 48 hours.

100 to 150

Loss of smell and other symptoms increase.

200 to 300

Pulmonary oedema can occur from prolonged exposure. Conjunctivitis and respiratory tract irritation increases after 1 hour exposure. 

500 to 700

Collapse after 5 minutes exposure. Serious damage to the eyes in 30 minutes. Death after 30-60 minutes.

700 to 1000

Rapid unconsciousness known as "knockdown" after 1 to 2 breaths, breathing stops, death within minutes.

1000 to 2000

Nearly instant death


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