Five workers died in Casteldaccia, in the Palermo area, during maintenance of the city’s sewage system. From the first reconstructions, the tragedy occurred due to the inhalation of hydrogen sulphide (H2S), also known as hydrogen sulfide or dihydrogen sulfide, a toxic and asphyxiating gas which accumulates in sewers, watertight areas and closed environments. This gas with the characteristic smell of rotten eggs is produced naturally by biological and geological activity, but also artificial origins as it is one waste product of industrial processes. High concentrations can cause asphyxia and death in 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 highly dangerous for humans, colorless, water-soluble and with a higher density than air, i.e. it weighs more, which is why it tends to accumulate low in closed environments. It is flammable at ambient temperature and pressure and has a characteristic smell of rotten eggs at concentrations between 0.70-14 µg/m3.
A feature that increases the danger of this gas is that the smell of rotten eggs is only noticeable at low concentrationswhile when the concentration of hydrogen sulphide reaches around 210 µg/m3 the smell is no longer perceptible because the gas “paralyzes” the olfactory nerve. In fact, if you thought that the smell of rotten eggs could be a warning sign, at the concentrations that cause intoxication this smell is no longer present.
How hydrogen sulfide is produced and where it is found
Approximately 90% of the hydrogen sulfide present in the environment comes from natural sources. It is produced by anaerobic bacteria (i.e. operating in the absence of oxygen) which they degrade decomposing animal and plant proteins: animal carcasses, leaves and dry plants are fertile ground for these bacteria. For this reason, high concentrations of hydrogen sulfide are normal in all sewers – like that of Casteldaccia – but also in the 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 that give rise to the sulphurous waters used in many thermal springs. For example, it is one of the gases released by the fumaroles of the Campi Flegrei.
The bacteria present in our gastrointestinal tract also produce hydrogen sulfide through the degradation of the proteins we ingest (and contributes to the typical smell of flatulence). He also appears to be involved in the inflammatory processes and in those that regulate growth and programmed cell death.
It is also issued by coal mines and in oil extraction and refining processes. It’s a waste product of many industrial processes such as the production of coke, fertilizers, pigments, in leather tanningand in the processes of waste treatment and wastewater and petrochemical companies.
It is used in some production processes of textile companies and in paper mills who use the Kraft method as an intermediate to produce other sulfur-based reagents or as a catalyst (a molecule that promotes and speeds up chemical reactions).
The health effects of hydrogen sulfide
The primary exposure to hydrogen sulfide is inhalation. It is strongly irritating And suffocating and works by inhibiting the enzyme cytochrome oxidase which allows cells to use oxygen. It acts on the respiratory and eye mucous membranes, on the heart and brain.
The first symptoms can already be seen at 150 mg/m3especially with eye irritation (keratoconjunctivitis). Exposure at higher levels, from 1400 mg/m3, causes nausea, vomiting and respiratory symptoms, with cough, asthma attacks and pulmonary edema. On a nervous level it causes dizzinessconfusion, headache, drowsinesstremors and loss of consciousness.
Massive exposures between 1500 and up to 18,000 mg/ m3 they are rapidly absorbed by the lung tissue and cause immediate asphyxiation, even after a few breaths. This type of acute exposure to such high concentrations is achieved only in rare cases, such as industrial gas leaks, or accumulation of hydrogen sulphide in closed environments (such as cisterns) for example when it is generated during wastewater, cesspool and sewer disposal processes.
Chronic occupational exposure or that of people who live near sources of hydrogen sulphide and are exposed for long periods to relatively low concentrations (from 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 protection
As reported by ARPA Toscana, there are no limit values for hydrogen sulphate emissions, but reference is usually made 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 9 April 2008) among other “deleterious gases”, without however a specific indication for hydrogen sulphide.
According to the decree, the protection of workers must be carried out by ensuring adequate training of operators, the use of appropriate PPE (personal protective equipment) and gas detection systems.
