Five workers died in Casteldaccia, Palermo region, while maintaining the city’s sewage system. From the first reconstructions, the tragedy occurred due to the inhalation of Hydrogen sulfide (H2S), also known as hydrogen sulfide or dihydrogen sulfide, a toxic and asphyxiating gas which accumulates in sewers, sealed areas and closed environments. This gas with the characteristic odor of rotten eggs is produced naturally by biological and geological activity, but also of artificial origin since it is a industrial process waste. High concentrations can cause asphyxia and death within a few breaths, and even at low concentrations it can cause lung irritation and conjunctivitis.
What is hydrogen sulfide
Hydrogen sulfide is a gas naturally present in the environment. It is very dangerous for humans, colorless, soluble in water and with a density greater than that of air, that is, it weighs more, which is why it tends to accumulate weakly in closed environments. It is flammable at ambient temperature and pressure and has a characteristic smell of rotten eggs at concentrations between 0.70 and 14 µg/m3.
A characteristic that increases the danger of this gas is that the smell of rotten eggs is only noticeable at low concentrationswhile when the hydrogen sulfide concentration reaches approximately 210 µg/m3 the smell is no longer perceptible because the gas “paralyzes” the olfactory nerve. In fact, if you thought the smell of rotten eggs might be a warning sign, at the concentrations that cause poisoning, that smell is no longer present.
How hydrogen sulfide is produced and where it is found
About 90% of hydrogen sulfide in the environment comes from natural sources. It is produced by anaerobic bacteria (i.e. operating in the absence of oxygen) which they break down decaying animal and plant proteins: animal carcasses, leaves and dry plants are a breeding ground for these bacteria. For this reason, high concentrations of hydrogen sulfide are normal in all media. sewers – like that of Casteldaccia – but also in accumulations of manure used in fertilizers and in waste or stagnant water.
It is also released among gases volcanic emissions or in particular geothermal areas, for example those which give rise to the sulfurous waters used in many thermal springs. For example, it is one of the gases released by the fumaroles of Campi Flegrei.
Bacteria in our gastrointestinal tract also produce hydrogen sulfide through the breakdown of proteins we ingest (and contribute to the typical odor of flatulence). He also appears to be involved in inflammatory processes and in those which regulate growth and programmed cell death.
It is also issued by coal mines and in petroleum extraction and refining processes. It’s a waste many industrial processes such as the production of coke, fertilizers, pigments, in leather tanningand in the processes of waste treatment and wastewater treatment and petrochemical companies.
It is used in certain production processes of textile companies and in Stationery which use the Kraft method as an intermediate to produce other sulfur-based reagents or as a catalyst (molecule that promotes and accelerates chemical reactions).
The health effects of hydrogen sulfide
The primary exposure to hydrogen sulfide is through inhalation. It is strongly irritant And stifling and works by inhibiting the enzyme cytochrome oxidase which allows cells to use oxygen. It acts on the respiratory and ocular mucous membranes, on the heart and the brain.
The first symptoms are already visible at 150 mg/m3especially with eye irritation (keratoconjunctivitis). Exposure at higher levels, from 1400 mg/m3, causes nausea, vomiting and respiratory symptoms, accompanied by coughing, asthma attacks and pulmonary edema. On the nervous level, this causes dizzinessconfusion, headaches, drowsinesstremors and loss of consciousness.
Massive exhibitions between 1,500 and up to 18,000 mg/m3 they are rapidly absorbed by lung tissue and cause immediate asphyxia, even after a few breaths. This type of acute exposure to such high concentrations is only achieved in rare cases, such as industrial gas leaks, or accumulation of hydrogen sulfide in closed environments (such as cisterns), for example when generated during wastewater disposal processes, cesspools and sewers.
Chronic occupational exposure or that of people living near sources of hydrogen sulphide and exposed for long periods to relatively low concentrations (70 to 140 mg/m3) may cause lung and eye irritation, difficulty concentrating, migraines and increase the risk and incidence of lung infections.
Safety and security
As reported by ARPA Toscana, there are no limit values for hydrogen sulfate emissions, but we generally refer to those indicated by the WHO: 7 μg/m3 (short-term concentration) and 150 μg/m3 (average daily concentration).
As for safety, this insidious gas is placed in the list of occupational diseases (DM April 9, 2008) among other “deleterious gases”, without however any precise indication for hydrogen sulphide.
According to the decree, the protection of workers must be ensured by ensuring adequate training of operators, the use of appropriate PPE (individual protection equipment) and gas detection systems.
