Introduction and Characteristics of Some Common Gases and Toxic Hazardous Gases Part2

19. Sulfur Hexafluoride (SF₆)

SF₆ is a colorless, odorless, non-toxic, non-flammable gas. It is chemically stable with high thermal stability. It has excellent insulation properties, with an arc-extinguishing capability about 100 times that of air.

Exposure Limits: In the workplace, the PC-TWA for SF₆ is 6000 mg/m³. Under standard conditions (0°C, 101.325 kPa), 1 mg/m³ ≈ 0.133 ppm, so 6000 mg/m³ ≈ 798 ppm. This means that under normal working conditions, long-term exposure concentrations should be controlled below this limit to protect worker health.

Applications: Primarily in the power industry, widely used in electrical equipment such as high-voltage switches, transformers, and instrument transformers as an insulating and arc-extinguishing medium. It is also used in metal smelting and the production of electronic-grade pure silicon as a protective gas.

20. Ozone (O₃)

Ozone has strong oxidizing properties and irritates the human respiratory tract and eyes. Inhalation of high concentrations of ozone causes coughing, difficulty breathing, chest pain, etc. Long-term exposure may lead to decreased lung function and increased respiratory diseases. It also adversely affects plant growth, such as damaging leaf tissue and affecting photosynthesis.

In the atmosphere, ozone is mainly generated by photochemical reactions of nitrogen oxides and volatile organic compounds under sunlight. In busy urban traffic areas and industrial clusters, NOx from vehicle exhaust and VOCs from industrial processes interact, easily forming ozone pollution. Additionally, some electrical equipment, such as copiers and laser printers, produce small amounts of ozone during operation.

21. Formaldehyde (CH₂O)

Also known as methanal, it is a colorless gas with a pungent odor that irritates the eyes and nose. It mainly comes from building materials, furniture manufacturing, cosmetics, etc. Long-term exposure may cause cancer.

Exposure Limits:

• Indoor Environment: According to China's GB/T 18883-2002, the indoor formaldehyde concentration limit is 0.1 mg/m³ (approx. 0.08 ppm). According to the Ministry of Housing and Urban-Rural Development, after strong ventilation and 1 hour of sealing in a newly renovated house, formaldehyde release ≤ 0.1 mg/m³ (approx. 0.07 ppm) is considered standard.

• Workplace: The ACGIH threshold limit value for formaldehyde is 2 ppm, meaning that at this concentration, nearly all workers can be repeatedly exposed day after day without adverse effects.

• Recommendation for Sensitive Groups: For pregnant women, children, and other less resistant individuals, the indoor formaldehyde concentration should be below 0.06 mg/m³ (approx. 0.05 ppm).

22. Benzene (C₆H₆)

Benzene is a colorless, transparent liquid with a special aromatic odor, easily volatile into a toxic gas. It mainly comes from petrochemicals, solvents, plastic production, etc. Long-term exposure affects the bone marrow, leading to aplastic anemia and even increasing the risk of leukemia.

Exposure Limits: According to the 1st amendment to GBZ 2.1-2019, the PC-TWA for benzene is 3 mg/m³, and the PC-STEL is 6 mg/m³. Converted, PC-TWA ≈ 0.95 ppm, PC-STEL ≈ 1.9 ppm.

23. Phosgene (COCl₂)

Phosgene is a colorless gas with a smell of moldy hay or rotten apples. It is encountered in phosgene production, high-temperature combustion of chlorinated hydrocarbons, organic synthesis using phosgene, and the manufacture of dyes, pesticides, pharmaceuticals, etc. It mainly causes irritation to the respiratory mucosa, and severe cases can cause bronchospasm, chemical inflammation, pulmonary edema, and asphyxia.

Exposure Limits: According to GBZ 2.1-2007, the MAC for phosgene is 0.5 mg/m³ (approx. 0.29 ppm).

24. Hydrogen Cyanide (HCN)

HCN is a highly toxic gas with a bitter almond odor. It is commonly used in chemical production, metal refining, and plastic manufacturing. It inhibits cellular respiration, causing headaches, respiratory failure, and can be fatal in severe cases.

Exposure Limits: MAC is 0.3 mg/m³ (approx. 0.11 ppm); for confined space operations, the TWA limit is 5 ppm, and the short-term exposure limit is 10 ppm.

25. Silane (SiH₄)

Silane is a toxic, flammable gas that can spontaneously ignite in air, producing silicon dioxide and water as combustion products. It strongly irritates the respiratory tract, eyes, and skin. Inhalation of high concentrations causes headache, dizziness, nausea, difficulty breathing, and in severe cases, pulmonary edema and chemical pneumonia.

In the photovoltaic industry, silane is an important gas, mainly used in chemical vapor deposition (CVD) processes for semiconductor and solar cell manufacturing, such as preparing polysilicon thin films.

26. Phosphine (PH₃)

Phosphine (again, see also #17) – The text provides additional details. It is a highly toxic gas with a garlic-like odor, affecting the respiratory and nervous systems. Inhalation causes coughing, difficulty breathing, dizziness, and fatigue. High concentrations cause severe poisoning such as pulmonary edema, convulsions, and death.

In photovoltaic cell manufacturing, phosphine is used in doping processes, especially for preparing N-type semiconductor materials.

Exposure Limits:

• PC-TWA: 0.3 mg/m³ (approx. 0.22 ppm)

• PC-STEL: 0.6 mg/m³ (approx. 0.44 ppm)

• IDLH: 200 ppm (immediately dangerous to life and health).

27. Chlorine (Cl₂)

Chlorine is a highly toxic gas with a strong, irritating odor. It severely irritates the respiratory tract, eyes, and skin. Inhalation causes coughing, difficulty breathing, chest tightness, and eye stinging. High concentrations can lead to pulmonary edema and respiratory failure.

In the photovoltaic industry, chlorine may be used in some cleaning and etching processes, e.g., for removing impurities from silicon wafers or as an oxidizing agent.

Exposure Limits: According to GBZ2.1-2007, the MAC for chlorine is 1 mg/m³ (approx. 0.315 ppm).

28. Hydrogen Chloride (HCl)

HCl is an acidic gas with a pungent odor, strongly irritating to the respiratory tract and eyes. Inhalation of high concentrations causes coughing, sore throat, difficulty breathing, and in severe cases, chemical pneumonia and pulmonary edema.

In photovoltaic cell manufacturing, HCl can be produced during etching processes using chlorine-containing reagents. For example, when hydrochloric acid reacts with other substances to remove specific material layers from silicon wafers, one of the byproducts is HCl gas.

Exposure Limits: According to GBZ 2.1-2007, the MAC for HCl is 7.5 mg/m³ (approx. 4.5 ppm).

29. Hydrogen Fluoride (HF)

HF is a highly toxic gas that strongly corrodes the respiratory tract, eyes, and skin. It binds to calcium and magnesium in the body, causing physiological abnormalities such as hypocalcemia. Inhalation of high concentrations causes coughing, difficulty breathing, chest pain, and skin contact causes severe burns.

In the photovoltaic industry, hydrofluoric acid is a common chemical reagent used for cleaning and etching silicon wafers to remove impurities such as silicon dioxide. During use, HF volatilizes to produce toxic hydrogen fluoride gas.

Exposure Limits: According to GBZ 2.1-2019, the MAC for HF (as F) is 2 mg/m³ (approx. 1.19 ppm).

30. Carbon Disulfide (CS₂)

CS₂ is a poison that damages nerves and blood vessels. Long-term exposure to low concentrations causes neurasthenic syndrome and autonomic nervous system dysfunction. High concentration inhalation leads to acute poisoning, with symptoms including headache, dizziness, nausea, vomiting, and in severe cases, loss of consciousness and respiratory failure.

It mainly comes from chemical production, e.g., in the manufacture of viscose fibers, CS₂ is used to dissolve cellulose. It is also used to produce carbon tetrachloride, rubber vulcanization accelerators, etc. During these processes, CS₂ can be released into the air due to equipment leaks or improper operation.

Occupational Exposure Limits: PC-TWA: 5 mg/m³, PC-STEL: 10 mg/m³. Under standard conditions, 1 ppm ≈ 4.1 mg/m³, so PC-TWA ≈ 1.22 ppm, PC-STEL ≈ 2.44 ppm.

31. Vinyl Chloride (C₂H₃Cl)

Vinyl chloride is a carcinogen; long-term exposure increases the risk of liver cancer and other cancers. It also damages the central nervous system and liver, causing dizziness, drowsiness, nausea, and in severe cases, coma and death.

It is an important monomer for producing polyvinyl chloride (PVC) plastic. In chemical production, vinyl chloride is synthesized by the addition reaction of acetylene and hydrogen chloride. In PVC plants, if equipment seals are poor or leaks occur during transport/storage, vinyl chloride is released into the air.

Occupational Exposure Limits: PC-TWA: 10 mg/m³. Under standard conditions, 1 ppm ≈ 4.1 mg/m³, so PC-TWA ≈ 2.44 ppm.

32. Chlorine Dioxide (ClO₂)

ClO₂ is a strong oxidant that irritates the respiratory tract, eyes, and skin. Inhalation of high concentrations causes coughing, wheezing, difficulty breathing, and in severe cases, pulmonary edema. Its toxicity is slightly lower than chlorine, but still requires careful handling.

It is mainly used in water treatment, pulp bleaching, and other industrial processes. In drinking water disinfection, it is used to kill bacteria and viruses. In the paper industry, it is used to bleach wood pulp. If handled improperly, such as equipment leaks or damaged containers, ClO₂ can be released into the air.

Exposure Limits: PC-TWA: 0.3 mg/m³, PC-STEL: 0.8 mg/m³. Under standard conditions, 1 ppm ≈ 4.1 mg/m³, so PC-TWA ≈ 0.073 ppm, PC-STEL ≈ 0.195 ppm.

33. Hydrogen Selenide (H₂Se)

H₂Se is a highly toxic gas, more toxic than hydrogen sulfide. It irritates the respiratory tract, causing coughing and difficulty breathing. Once in the body, it damages multiple organs, especially the liver, kidneys, and nervous system, and can be fatal.

It may be generated during the processing of metal selenides, such as cadmium selenide or zinc selenide, in acidic environments. In the electronics industry, selenium is used in photoelectric cells and rectifiers, and H₂Se may be produced during processing.

Exposure Limits: TWA: 0.15 mg/m³ (approx. 0.036 ppm), STEL: 0.3 mg/m³ (approx. 0.073 ppm).

34. Volatile Organic Compounds (VOCs)

The World Health Organization (WHO) defines VOCs as organic compounds with a boiling point between 50-260°C at room temperature. These compounds are volatile and can exist as gaseous molecules in the air at room temperature and pressure.

• Chemical Industry: Many organic solvents used in production, storage, and application processes release VOCs. For example, paint and coating production uses large amounts of solvents like toluene, xylene, etc., which volatilize during mixing, storage, and application.

• Printing Industry: Solvents in inks are also a major source of VOCs, released as the ink dries.

• Petrochemical Industry: Refining units and petrochemical production generate large amounts of VOCs, including light hydrocarbons from crude oil refining and intermediates from various chemical products.

• Electronics Industry: Cleaning processes use organic solvents (e.g., trichloroethylene, isopropanol) that volatilize after cleaning circuit boards and other equipment.

• Indoor Decoration: A major source of VOCs in daily life. Building materials (e.g., engineered wood products using urea-formaldehyde resins, insulation, wallpaper) and furniture (paints, coatings, adhesives) release various VOCs. For example, formaldehyde can be continuously released from new wood products and furniture surfaces for years.

• Daily Household Products: Cleaning products, cosmetics (some lotions, perfumes containing volatile components like ethanol), and automotive interior materials (seats, plastic dashboard parts) release VOCs during use.

• Vehicle Exhaust: Contains various VOCs, such as hydrocarbons, from incomplete combustion and evaporative emissions from fuel systems.

Exposure Limits:

• Indoor Environment: GB/T 18883-2022 sets a TVOC limit of 0.6 mg/m³. GB 50325-2020 sets Class I civil buildings (residences, hospitals, schools, etc.) TVOC limit of 0.45 mg/m³; Class II (offices, shops, hotels, etc.) TVOC limit of 0.5 mg/m³.

• Ambient Air: General areas: daily average VOC limit 0.3 mg/m³, hourly average 0.2 mg/m³; Key areas: daily average 0.2 mg/m³, hourly average 0.1 mg/m³.

• Industrial Production Environment: GB 16297-1996 specifies maximum allowable emission concentrations and rates for benzene, toluene, xylene, etc. For example, benzene maximum emission concentration: 17 mg/m³ (existing sources) and 12 mg/m³ (new sources); toluene: 60 mg/m³ (existing) and 40 mg/m³ (new).

In summary, these toxic and hazardous gases are widely present in industry, transportation, daily life, and other fields, posing serious threats to human health and the environment. Understanding their characteristics and hazards helps in taking effective prevention and control measures.