The silent killer- H2S(Hydrogen sulphide).

Introduction:-

Hydrogen sulphide is a colourless gas that has a sweet taste and a strong odour resembling that of rotten eggs. Hydrogen sulphide is produced from the deterioration of organic materials and is a naturally occurring seam gas is some coal and shale deposits. 

It can be detected by smell at small concentrations as low as 1ppm but this is unreliable as a warning characteristic as nasal sensitivity decreases with exposure and increased concentration levels. Hydrogen sulphide is a highly toxic gas that irritates the mucous membranes and eyes and has narcotic effect on the nervous system. The gas acts on the nervous system causing headache, dizziness, excitement and staggering walk. Massive acute exposure levels produce anoxia (absence of oxygen in Aertrial blood and tissues ) resulting in death. Exposure to concentrations greater than 500ppm can be fatal. Excessive exposure is often characterised by depression at low concentrations, then stimulation, followed by paralysis of the respiratory system at higher levels (1000ppm). 

Hydrogen sulphide has a density of 1.19 relative to normal air and as such, will tend to pool and go bad in wells and poorly ventilated areas. It forms flammable mixtures in air in the range of approximately 4.5 - 45%. At these concentrations human life will not survive. Hydrogen sulphide can be measured using portable gas detector fitted with an electrochemical cell or by using indicator stain tubes. 

Why is it called as "Silent killer"? 

H₂S is known as the "silent killer". It's an insidious and invisible gas which smells a bit like rotten eggs at low concentrations. It quickly paralyzes the olfactory nerves (sense of smell) at about 100 part per million (ppm).

(Tap)Animated Near miss incident due to H2S

Why is it known as a "hidden gas"?

It collects in low-lying and enclosed, poorly-ventilated areas such as basements, manholes, sewer lines, under- ground telephone vaults and manure pits. For work within confined spaces, use appro- priate procedures for identifying hazards, monitoring and entering confined spaces.

The current exposure standards specified for hydrogen sulphide are:

1.Time-weighted average (TWA) of 10ppm.

2.Short-term exposure Limit(STEL) of 15ppm for 15 minutes. 

Chemical specifications:-

Hydrogen sulfide is the chemical compound with the formula H ₂S. It is a colorless chalcogen hydride gas with the characteristic foul odor of rotten eggs. It is very poisonous, corrosive, and flammable. (As per the NFPA toxicity rating of 4/5.)Wikipedia

•Formula: H₂S

•Molar mass: 34.1 g/mol

•Boiling point: -60 °C

•Density: 1.36 kg/m³

IMPACT ON SAFETY (SHORT-TERM)

Gas is a silent threat, often invisible to the body’s senses. Inhalation is the primary route of exposure to hydrogen sulfide. Though it may be easily smelled by some people at small concentrations, continuous exposure to even low levels of H2S quickly deadens the sense of smell (olfactory desensitization). Exposure to high levels of the gas can deaden the sense of smell instantly. Although the scent of H2S is a characteristic, smell is not a dependable indicator of H2S gas presence or for indicating increasing concentrations of the gas.

H2S irritates the mucous membranes of the body and the respiratory tract, among other things. Following exposure, short-term, or acute, symptoms may include a headache, nausea, convulsions, and eye and skin irritation. Injury to the central nervous system can be immediate and serious after exposure. At high concentrations, only a few breaths are needed to induce unconsciousness, coma, respiratory paralysis, seizures, even death.

Tap to view a case study of H2S(In Hindi)

IMPACT ON HEALTH (LONG-TERM)

Those having prolonged exposure to high enough levels of H2S gas to cause unconsciousness may continue to experience headaches, reduced attention span and motor functions. Pulmonary effects of H2S gas exposure may not be apparent for up to 72 hours following removal from the affected environment. Delayed pulmonary edema, a buildup of excess fluid in the lungs, may also occur following exposure to high concentrations.

H2S does not accumulate in the body, but repeated/prolonged exposure to moderate levels can cause low blood pressure, headache, loss of appetite and weight loss. Prolonged exposure to low levels may cause painful skin rashes and irritated eyes. Repeated exposure over time to high levels of H2S may cause convulsions, coma, brain and heart damage, even death.

Impact on facilities:-

Heavier than air, H2S gas accumulates in low lying areas of poorly ventilated spaces. In oil and gas applications, sour gas (products containing H2S gas) in the presence of air and moisture may form sulfuric acid, capable of corroding metals. Facility equipment, including the internal surfaces of various components, faces reduced durability and impact strength, potentially leading to premature failure.

#How to eliminate or diminish the threat of H2S to a considerable extent?(Various PPE's) 

Animated safety training and awareness against H2S

Respirator Protection

OSHA mandates what PPE is required when workers are either directly exposed to H2S or working near a site where the gas is present. Whenever respirators are used, the employer must have a respiratory protection program that meets the requirements of OSHA's Respiratory Protection standard (29 CFR 1910.134). This program must include proper respirator selection, fit testing, medical evaluations, and training (select the right device with 6 Factors to Consider Whe Choosing a Respiratory Protection Device). The exact respirator standards should be discussed with a qualified safety professional , but for the purpose of this Q&A, it should be noted that the respirator must be a positive-pressure, self-contained breathing apparatus (SCBA ).

Eye Protection

If you're using a full-face respirator , the eye protection is already part of the equipment. If the respirator you're using do not cover the eyes, additional eye protectio will be needed, since prolonged exposure ma cause nausea, tearing of the eyes, headache or loss of sleep. For exposures at or above 100 ppm, there is a danger of "gas eye" an conjunctivitis with respiratory tract irritation. Exposures above 100 ppm are considered immediately dangerous to life an health, and employers will need to direct th use of a full face pressure demand self-contained breathing apparatus.

Fire Resistant Gloves and Clothing

Hydrogen sulfide is flammable , so protective gloves and clothing are needed in some case Again, the need for this extra layer of protection will depend on the work being done and the environment it is being done i Weather conditions and temperatures can als have an impact and safety policy must include directives about this. H2S can corrode some materials, so the materials use in any PPE manufactured for the use of a worker exposed to H2S must meet industry standards and any compliance regulations. A always, when working in confined spaces , ventilation should operate continuously.

Proper PPE for use with H2S should be a part of the training and education for any workers and also any visitors who may become exposed to the gas on the work site This training should include emergency plans locations of safety equipment, rescue techniques and location of a safety line for rapid exit from a confined space or remote location (when needed), first-aid, and prop rescue procedures for first responders.

(Tap to view) Detailed video about H2S

How a person exposed to H2S treated? 

Nitrite therapy (found in the cyanide antidote kit) has been suggested as a therapy for hydrogen sulfide exposure. Amyl nitrite is given by inhalation (for 30 seconds every minute until an intravenous line is established) followed by intravenous sodium nitrite (300 mg over absolutely no less than 5 minutes).