Why do confined spaces become natural "production workshops" for these deadly gases? The root cause lies in their enclosed structure and poor ventilation. Sewers, septic tanks, wastewater pools, fermentation pits, and similar locations contain large amounts of organic matter. Under anaerobic fermentation and microbial metabolism, toxic gases such as hydrogen sulfide are generated. This is the most common endogenous formation mechanism. Beyond natural generation, chemical volatilization, residual industrial materials, welding and cutting operations, and gas leaks from connected pipelines can also introduce carbon monoxide, nitrogen oxides, and other toxic gases into confined spaces.

Among these lethal gases, hydrogen sulfide is undoubtedly the "number one killer." Not only because it is highly toxic, but because it exploits human senses as a deadly trap. Hydrogen sulfide is colorless but carries the characteristic smell of rotten eggs. Since it is heavier than air, it easily accumulates in low-lying areas such as sewage pits, septic tanks, fermentation pools, and underground drains. At low concentrations, people can clearly detect its pungent odor. However, once concentrations rise to around 100 ppm, the olfactory nerves become rapidly paralyzed. A person may already be standing in a lethal environment, yet can no longer smell the warning odor. In other words, the brain begins sending a false signal that "there is no danger."

Hydrogen sulfide primarily attacks the central nervous system and respiratory system. It inhibits the activity of cytochrome oxidase, preventing cells from utilizing oxygen for aerobic respiration, effectively causing "cellular-level internal suffocation." At high concentrations, hydrogen sulfide can directly paralyze the respiratory center, causing sudden unconsciousness and respiratory arrest without warning. China's maximum allowable workplace concentration is set at 10 mg/m³, while the immediately dangerous to life or health threshold is only 142 mg/m³. When concentrations reach 700 to 1000 mg/m³ and exposure lasts from 15 minutes to one hour, life-threatening poisoning may occur. Once concentrations exceed 1000 mg/m³, "lightning-type" death can occur within seconds, leaving no time even to call for help.

On April 13, 2024, hydrogen sulfide concentrations in a septic tank at a livestock farm in Sichuan severely exceeded safety limits. Workers entered the pit without proper precautions, collapsed from poisoning, and multiple others rushed in blindly to rescue them. The accident ultimately killed seven people and caused direct economic losses exceeding RMB 5.65 million. Later that year, on October 12, 2024, hydrogen sulfide accumulated in a corn slurry tank at a biotechnology company in Shandong. Workers illegally entered the bottom of the tank for operations, resulting in seven deaths, four injuries, and economic losses exceeding RMB 14 million. On January 17, 2026, hydrogen sulfide built up inside a sludge concentration tank at a marine biological products company in Zhoushan, Zhejiang. Workers entered without authorization to clear a filter screen, leading to three deaths. Just half a month later, at a textile company in Mianyang, Sichuan, microbial fermentation processes generated large quantities of hydrogen sulfide. Maintenance workers recklessly entered a high-concentration area, causing four deaths and three injuries. These tragedies repeatedly sound the same warning: when facing hydrogen sulfide, any negligence is disregard for human life.

If hydrogen sulfide is the "lightning killer" that strikes within seconds, then carbon monoxide is known as the "silent killer" because it is colorless, odorless, and difficult to detect. Carbon monoxide is a colorless, odorless, non-irritating gas produced by the incomplete combustion of carbon-based fuels. In confined spaces, it is commonly generated in large quantities during welding, heating, and similar operations. Carbon monoxide binds to hemoglobin with an affinity 200 to 300 times greater than oxygen. Once it forms carboxyhemoglobin, hemoglobin permanently loses its ability to carry oxygen, leading to tissue hypoxia and suffocation. Even more terrifying, the cerebral cortex, which controls human movement, is among the first areas affected. Victims may remain conscious, yet become physically incapable of performing even a simple self-rescue action such as opening a door to escape. In enclosed spaces, exposure to carbon monoxide concentrations of 0.05% for one hour can already produce poisoning symptoms. At 0.1%, exposure may become life-threatening within two hours, while concentrations of 4% can kill rapidly.

In January 2025, at a gas-production workshop operated by an energy company in Xinjiang, an employee illegally repaired a pressure relief valve, causing a large release of high-concentration carbon monoxide. The accident killed one person and caused direct economic losses of RMB 1.942 million. In April 2024, workers at an energy and chemical company in Inner Mongolia recklessly entered a confined space and suffered poisoning and suffocation, resulting in four deaths and one injury.

Beyond these gases, many other deadly toxins lurk inside confined spaces. Though less notorious than hydrogen sulfide, they can kill just as silently. Benzene and benzene-series compounds are commonly found in painting and rust-removal operations. They are colorless, transparent liquids with aromatic odors and have anesthetic effects on the central nervous system. Acute inhalation can cause dizziness, headaches, nausea, and vomiting, while severe exposure may lead to coma and respiratory or circulatory failure. Hydrogen cyanide is even more lethal than hydrogen sulfide. At room temperature, it exists as a colorless liquid that vaporizes easily and carries a bitter almond smell. It inhibits cytochrome oxidase, blocks cellular oxygen utilization, and causes cellular hypoxia and tissue necrosis. Pickling pits and preserved-food tanks are common environments where it may accumulate. Phosphine is another deadly gas. It is colorless, has a garlic-like odor, and is slightly heavier than air, making it prone to accumulation in wastewater treatment tanks and sealed grain silos. It primarily damages the nervous system, respiratory system, heart, kidneys, and liver, and can rapidly become fatal in severe cases.

Fortunately, advances in technology are now strengthening safety protection. Modern intelligent confined-space monitoring systems can continuously detect changes in concentrations of toxic and hazardous gases in real time. Once levels exceed alarm thresholds, the system immediately issues warnings, helping cut off the chain of accidents at the source.

To ensure confined spaces no longer become "death spaces," every worker must maintain genuine respect for life itself and take every ventilation procedure, every gas test, and every work permit seriously. These seemingly tedious steps are, in reality, the barrier that separates life from death.