Before entering septic tanks, manholes, sewage pits, vegetable cellars, basements, fermentation tanks or other confined spaces, ventilate, pass gas testing and arrange protection and supervision; do not enter without all conditions. If someone collapses, do not follow them in: call 119 and 120

Ventilation, testing, protection and supervision require equipment, personnel and time, all of which cost money. For household cleaning, prioritize an equipped, trained team and ask local providers about fees. Do not enter yourself without adequate conditions. The difficult part is resisting the urge to jump in when someone collapses.

“Confined spaces” are enclosed or partly enclosed areas such as septic tanks, manholes, sewage pits, cellars, and tanks that are not intended for continuous occupancy. China's Ministry of Emergency Management analyzed serious confined-space accidents in industrial and trade enterprises since 2013. Poisoning and suffocation accounted for 96.8% of incidents and 96.9% of deaths. The main hazardous gases were hydrogen sulfide, carbon monoxide, nitrogen, and carbon dioxide. Incidents involving inadequately protected, improvised rescue accounted for 87.4% of incidents and 86.8% of deaths. The ministry's 2020 safety handbook says improvised rescue increased casualties in nearly 80% of incidents. More than 50% of those who died in confined spaces were rescuers. In 2018, the Office of the State Council Work Safety Committee reported 8 incidents. Each began with only 1 person in danger, but improvised rescue brought the final death toll to 3–4 in each incident. In 2019, 3 people cleaning a sewage pit at a Dongguan paper mill collapsed after inhaling hydrogen sulfide. Six others entered in succession to rescue them; 7 people ultimately died. In 2020, 1 worker fell into a materials tank in the fermentation area of a noodle-products factory in Jiaozuo. Improvised rescue by others brought the death toll to 6. The US National Institute for Occupational Safety and Health investigated 70 fatal confined-space incidents from 1983–1993, involving 109 deaths. Of these, 39 people, or 36%, had entered to rescue someone. Only 4 of those 39 were professional responders such as police officers or firefighters. Its 1986 alert said about 60% of those killed were would-be rescuers. The Chinese handbook says hydrogen sulfide smells like rotten eggs, but at high concentrations the nose can no longer detect it. Concentrations above 1000 milligrams per cubic meter can kill within seconds. High carbon dioxide concentrations can also cause unconsciousness and collapse within seconds. The handbook requires calling 119 and 120 promptly when on-site conditions do not allow self-rescue, stating that a rescue attempt must never be forced through. Pulling someone out from outside requires the trapped person to be wearing a full-body harness connected to an external safety line. Ministry of Emergency Management Order No. 13 requires industrial and trade enterprises to “ventilate first, test next, then work.” Enterprises must provide testing, ventilation, protection and rescue equipment appropriate to the risks, with an attendant present throughout. Ventilation and testing must continue during work; entry is prohibited when safety conditions are not met. Attendants must stop improvised rescue attempts without adequate safety measures. Anshun's health authority warns that vegetables in long-sealed cellars consume oxygen and release carbon dioxide. Before entering a cellar, open the vents and ventilate thoroughly. This ventilation advice does not replace gas testing, protection and supervision (China / United States).

应急管理部 (2023). 《工贸企业有限空间作业安全规定》解读. https://www.mem.gov.cn/gk/zcjd/202312/t20231212_471584.shtml；应急管理部 (2023). 《工贸企业有限空间作业安全规定》专题新闻发布会. https://www.mem.gov.cn/xw/xwfbh/2023n12y12rxwfbh/wzsl_4260/202312/t20231212_471598.shtml；应急管理部令第 13 号 (2023). 工贸企业有限空间作业安全规定（第三、十三至十五条）. https://www.mem.gov.cn/gk/zfxxgkpt/fdzdgknr/gz11/202312/t20231208_471426.shtml；应急管理部办公厅 (2020). 有限空间作业安全指导手册（第 2.1、5.1、5.3 节及典型事故案例）. https://www.mem.gov.cn/gk/tzgg/tz/202010/W020201102332723879957.pdf（通知页 https://www.mem.gov.cn/gk/tzgg/tz/202010/t20201030_371153.shtml）；国务院安委会办公室 (2018). 国务院安委办通报 8 起有限空间盲目施救导致伤亡扩大事故. https://www.mem.gov.cn/xw/bndt/201806/t20180608_229498.shtml；应急管理部 (2021). 应急管理部公布一批有限空间作业生产安全事故典型案例. https://www.mem.gov.cn/xw/yjglbgzdt/202103/t20210326_382087.shtml；安顺市卫生健康局 (2021). 进入地窖当心二氧化碳中毒. http://wjj.anshun.gov.cn/wjgz/zsyd/202102/t20210223_66808351.html；NIOSH (1994). Worker Deaths in Confined Spaces: A Summary of NIOSH Surveillance and Investigative Findings (DHHS (NIOSH) Publication No. 94-103), p. 20, 22. https://www.cdc.gov/niosh/docs/94-103/pdfs/94-103.pdf（原页面已移入 CDC 存档站，链接为 2023 年 3 月 8 日的网页存档：https://web.archive.org/web/20230308204907/https://www.cdc.gov/niosh/docs/94-103/pdfs/94-103.pdf）；NIOSH (1986). Request for Assistance in Preventing Occupational Fatalities in Confined Spaces (DHHS (NIOSH) Publication No. 86-110). https://www.cdc.gov/niosh/docs/86-110/default.html（原页面已移入 CDC 存档站，链接为 2023 年 6 月 11 日的网页存档：https://web.archive.org/web/20230611112004/https://www.cdc.gov/niosh/docs/86-110/default.html）

The evidence grade is C because these are accident statistics and safety regulations; no study quantified how many deaths following them would prevent. The benefit is judged medium: the outcome is death, often several deaths at once, but the risk applies only to people entering these spaces. Household situations include cleaning septic tanks or biogas digesters and entering vegetable cellars. Do not judge safety by smell: no odor does not mean no hydrogen sulfide. At work, you can refuse entry into wells or pits without ventilation, gas testing, an attendant, and a safety harness. For the legal basis, see Section 19, Item 10 (you may refuse work without protective measures). If someone collapses inside, first call out for help and call 119 and 120. What you can do from outside is open the cover and blow air in. Once the person is brought out, move them to fresh air; if they are not breathing normally, start chest compressions. See Section 13, Item 1 (someone collapses and is not breathing normally). The response is the same whether the person is a coworker, neighbor, or stranger, for the same reason as Section 13, Item 25 (do not enter the water yourself when someone is drowning). For carbon monoxide from indoor coal burning, see Item 3 in this section (carbon monoxide alarms). The main beneficiary is you, because you may be the person tempted to enter, followed by family members working alongside you. Site note: Opening a cover or ventilating does not establish that entry is safe. Industrial and trading enterprises must follow the required sequence of ventilation, atmospheric testing, then work, with supervision, protection and rescue measures. At home, do not enter a pit or tank based on smell or ventilation alone. Call for help from a safe place; do not enter to attempt an unprotected rescue. Source