Introduction and Characteristics of Some Common Gases and Toxic Hazardous Gases

1. Nitrogen (N₂)

Nitrogen is the most abundant gas in the air, accounting for about 78% of air volume. It is a colorless, odorless, and tasteless gas. Its chemical properties are relatively stable, making it difficult to react with other substances at room temperature.

In industry, nitrogen has a wide range of applications, such as being used in food packaging to prevent oxidation and spoilage. In chemical production, it can serve as a protective gas to prevent easily oxidizable substances from contacting oxygen.

2. Oxygen (O₂)

Oxygen accounts for about 21% of air volume. It is a colorless, odorless gas that supports combustion and respiration. When the oxygen concentration is below 19.5% VOL, it is hypoxic; above 23.5% VOL is a hyperoxic state. Therefore, conventional oxygen detectors generally have a low alarm set at 19.5% VOL and a high alarm at 23.5% VOL.

In the medical field, oxygen is used for first aid and treating hypoxic diseases. In industry, oxygen is used in steelmaking, reacting with coke to produce carbon monoxide, which then reduces iron ore. In the aerospace field, liquid oxygen serves as an oxidizer for rocket engines, propelling rockets.

3. Carbon Dioxide (CO₂)

Carbon dioxide is a colorless, odorless gas. Its content in the air is about 0.04%. Sources of CO₂ are extensive, including biological respiration and fossil fuel combustion.

In industry, CO₂ is used in the production of carbonated beverages. In firefighting, CO₂ fire extinguishers utilize the fact that CO₂ does not support combustion and is denser than air to extinguish fires. In agriculture, CO₂ is used in greenhouse cultivation to increase CO₂ concentration and promote plant photosynthesis.

Exposure Limits:

• Workplace: According to GBZ 2.1-2019, the PC-TWA for CO₂ is 9000 mg/m³, and the PC-STEL is 18000 mg/m³. Converted, PC-TWA is approximately 5000 ppm, and PC-STEL is approximately 10000 ppm.

• Indoor Environmental Standard: China's indoor air quality standard stipulates that indoor CO₂ concentration should not exceed 1000 ppm.

4. Argon (Ar)

Argon accounts for about 0.934% of air volume. It is an inert gas with extremely inactive chemical properties.

In welding, argon is used as a shielding gas to prevent metal oxidation during welding, widely applied in stainless steel and aluminum alloy welding. In lighting, argon is used to fill light bulbs, reducing filament evaporation and extending bulb life.

5. Helium (He)

Helium is a colorless, odorless, non-flammable gas. It is the most difficult gas to liquefy. Its chemical properties are very stable.

In aerospace, helium is used to inflate airships and balloons because it is lighter than air and safe. In diving, helium is mixed with oxygen for deep-sea diving to prevent decompression sickness. In the electronics industry, helium is used for cooling and protection during semiconductor manufacturing.

6. Neon (Ne)

Neon is a rare gas, colorless and odorless. It emits a bright orange-red light when discharged. Based on this characteristic, it is mainly used in the manufacture of neon signs, giving them various colors. In laser technology, neon can also be used as a filling gas for certain types of lasers.

7. Krypton (Kr)

Krypton is a colorless, odorless gas. It emits white light when discharged and is often used to manufacture high-brightness lighting equipment, such as flash lamps. In the electronics industry, krypton is also used in certain special semiconductor manufacturing processes and device fabrication as a protective gas.

8. Xenon (Xe)

Xenon is an inert gas that emits intense blue-white light when discharged. In lighting, xenon lamps have the characteristics of high brightness and high color temperature, commonly used for automotive headlights and stage lighting. In medicine, xenon has an anesthetic effect and can be used as an anesthetic gas.

9. Methane (CH₄)

Methane is the simplest hydrocarbon and the main component of natural gas. It is a colorless, odorless, flammable gas. The conventional range for combustible gases is generally set at 0-100% LEL.

In energy, methane is used as a clean energy source for power generation and heating. In chemical production, methane can be used as a raw material to produce methanol, acetylene, and other chemical products. Methane mainly comes from natural gas extraction, as well as from biogas produced by anaerobic digestion, which also contains a large amount of methane.

10. Ethane (C₂H₆)

Ethane is a colorless, odorless, flammable gas commonly found in the petrochemical industry. It mainly exists in natural gas and petroleum cracking gas. Ethane can be used to produce ethylene, an important chemical raw material used to manufacture plastics (e.g., polyethylene), synthetic fibers, synthetic rubber, and many other chemical products.

11. Propane (C₃H₈)

Propane is a common fuel gas and one of the main components of liquefied petroleum gas (LPG). At room temperature and pressure, propane is gaseous, but it can be liquefied under appropriate pressure for easy storage and transportation. It is mainly used for household cooking, heating, as well as industrial cutting and welding, producing high heat when burned.

12. Nitric Oxide (NO) and Nitrogen Dioxide (NO₂) (Nitrogen Oxides)

Nitric oxide is a colorless gas; nitrogen dioxide is a reddish-brown gas with a pungent odor. They mainly come from high-temperature combustion processes such as vehicle exhaust and thermal power generation.

Nitrogen oxides are air pollutants that cause environmental problems such as acid rain and photochemical smog. In chemical production, nitric oxide can also be used as a raw material to produce nitric acid and other chemical products.

Nitrogen oxides mainly come from fuel combustion and chemical, electroplating, and other production processes. NO₂ strongly irritates the respiratory organs, can cause acute asthma, and is considered a causative factor for emphysema and lung tumors.

Exposure Limits:

• Workplace Occupational Exposure Limits: According to the 2nd amendment to GBZ 2.1-2019 (effective May 1, 2025), the PC-TWA for NO is 15 mg/m³; for NO₂, the PC-TWA is 5 mg/m³ and the PC-STEL is 10 mg/m³.

• Ambient Air Quality Standard: According to GB 3095-2012, the annual average limit for NO₂ is 40 μg/m³, and the 24-hour average limit is 80 μg/m³, which converts to approximately 0.017 ppm and 0.035 ppm.

• Industrial Emission Standard: According to GB 16297-1996, for enterprises established after January 1, 1997, the NOx emission concentration limit is 240 mg/m³; for those established before, it is 420 mg/m³. For aluminum smelting, secondary aluminum enterprises must comply with GB 31574-2015, with a NOx emission limit of 200 mg/m³. There are also some local standards.

13. Carbon Monoxide (CO)

CO is a colorless, odorless, toxic gas. It is mainly a product of incomplete combustion of carbon-containing materials, such as vehicle exhaust and incomplete coal combustion. CO binds to hemoglobin in the blood, reducing its oxygen-carrying capacity, leading to tissue hypoxia and poisoning.

In industry, CO can be used in metal smelting, such as in ironmaking, where CO acts as a reducing agent to reduce iron from iron ore.

Effects of Different CO Concentrations on the Human Body:

• Mild Discomfort: At 50-100 ppm, mild headaches and dizziness may occur. Healthy adults in such an environment for a short time (e.g., 1-2 hours) may feel discomfort but generally can tolerate it. Therefore, the conventional range for CO is 0-100 ppm.

• Obvious Discomfort: At 200-300 ppm, there will be obvious headaches, nausea, vomiting, and fatigue. People will feel very uncomfortable, and symptoms will gradually worsen over time (e.g., 2-3 hours).

• Severe Poisoning: At 800-1200 ppm, severe symptoms such as coma and convulsions occur. Without prompt removal from the environment, there is a risk of death. In such high concentrations, it is difficult for people to tolerate for long; loss of consciousness may occur within 30 minutes to 1 hour.

• Life-threatening: Above 3200 ppm, respiratory depression and cardiac arrest can occur within minutes, rapidly endangering life. The human body can hardly withstand such high CO concentrations.

14. Ammonia (NH₃)

Ammonia is a gas with a pungent odor. It is an important chemical raw material, mainly used to produce nitrogen fertilizers such as urea and ammonium nitrate. In refrigeration, ammonia was once a common refrigerant, but its use is now restricted due to its toxicity and flammability. In the textile industry, ammonia is used to manufacture artificial fibers.

Exposure Limits:

• MAC: 30 mg/m³ (approx. 41.7 ppm)

• PC-TWA: 20 mg/m³ (approx. 27.8 ppm)

• PC-STEL: 30 mg/m³ (approx. 41.7 ppm)

15. Sulfur Dioxide (SO₂)

SO₂ is a colorless, strongly irritating gas with a sulfuric acid-like odor, easily soluble in water. It mainly comes from combustion products of sulfur-containing mineral fuels, as well as from processes such as roasting of metal ores, bleaching of wool and silk, chemical pulping, and acid production. It irritates the respiratory tract, causing bronchitis and asthma, and also forms acid rain, harming the ecological environment.

Exposure Limits:

• MAC: 15 mg/m³ (approx. 5.3 ppm)

• PC-TWA: 5 mg/m³ (approx. 1.8 ppm)

• PC-STEL: 10 mg/m³ (approx. 3.5 ppm)

16. Hydrogen Sulfide (H₂S)

H₂S is a colorless gas with a rotten egg odor. It mainly comes from oil and gas extraction and decomposition of sulfides in chemical production. H₂S is a highly toxic gas that causes severe damage to the human respiratory and nervous systems. In industry, H₂S can be used to produce sulfuric acid and other chemical products.

Exposure Limits (China):

• MAC: 10 mg/m³ (approx. 6.5 ppm)

• Threshold Limit Value: 15 mg/m³ (10 ppm)

• Safety Critical Concentration: 30 mg/m³ (20 ppm)

• Dangerous Critical Concentration: 150 mg/m³ (100 ppm)

17. Phosphine (PH₃)

Phosphine is a highly toxic gas. Inhalation causes damage to the respiratory system, nervous system, and heart. High concentrations can cause death within a short time. Even long-term exposure to low concentrations can cause chronic poisoning, such as dizziness, nausea, and fatigue.

It mainly comes from the hydrolysis of metal phosphides such as aluminum phosphide. In grain storage, aluminum phosphide is used as a fumigant to prevent insects. Aluminum phosphide reacts with water vapor in the air to generate phosphine. Additionally, improper treatment of industrial waste containing phosphorus may also produce phosphine.

Exposure Limits: China's MAC for phosphine is 0.3 mg/m³ (approx. 0.22 ppm).

18. Arsine (AsH₃)

Arsine is a potent hemolytic poison. When it enters the body, it binds to hemoglobin in red blood cells, causing red blood cell rupture and acute hemolysis. Symptoms include headache, nausea, vomiting, lower back pain, and hemoglobinuria. Severe cases can lead to kidney failure and even death.

During non-ferrous metal smelting, especially of arsenic-containing ores, arsine may be generated when arsenic in the ore reacts with hydrogen. For example, in lead, zinc, and other metal smelting processes, if the ore contains arsenic impurities, arsine can be produced under high-temperature, reducing environments. Additionally, in the electronics industry, compounds like gallium arsenide may produce small amounts of arsine during processing.

Exposure Limits: According to GBZ 2.1-2019, the MAC for arsine is 0.03 mg/m³ (approx. 0.08 ppm).