If a carbon monoxide alarm goes off, or if several people in a room suddenly get headaches, nausea, or dizziness, get everyone outside before calling for help

Cost: No cost at all.
In plain terms: When the alarm sounds, or when multiple people in one room start experiencing headaches, nausea, and dizziness, move everyone outdoors first and then call for help. Do not stay inside trying to figure out what’s happening, and do not go back inside to turn off any valves. The fact that symptoms improve once outside is a very strong indicator of carbon monoxide exposure. In 2018 alone, China reported 11,523 deaths caused by carbon monoxide poisoning; most of these occurred in winter, and the majority of victims were at home.
Benefit: According to China’s disease surveillance system, 11,523 people died from carbon monoxide poisoning in 2018. The highest numbers were recorded in January (2,845 cases), February (2,321 cases), and December (2,010 cases). During those three winter months, 66% to 73% of all deaths happened at home. These figures come directly from official reports, not from population-based estimates. In the United States, over 400 people die each year from accidental carbon monoxide poisoning unrelated to fires; more than 100,000 people seek emergency treatment, and over 14,000 are hospitalized.
Evidence grade: B
Sources:You J, Liu J, Zhou M (2020). Number of Deaths due to Carbon Monoxide Poisoning by Month and by Place of Death — China, 2018. China CDC Weekly 2(2):32：「In 2018, there were 11,523 deaths caused by carbon monoxide poisoning reported in China」. https://doi.org/10.46234/ccdcw2020.008；美国疾病控制与预防中心. 一氧化碳中毒：「More than 400 Americans die from unintentional CO poisoning not linked to fires」「More than 100,000 visit an emergency department」「More than 14,000 are hospitalized」，常见症状「headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion」. https://www.cdc.gov/carbon-monoxide/about/index.html；美国消费品安全委员会. 一氧化碳信息中心：「If you suspect CO poisoning, get outside to fresh air immediately, and then call 911」. https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Carbon-Monoxide-Information-Center
Notes: Symptoms of carbon monoxide poisoning often resemble those of a cold or mild intoxication. When several people in the same space experience these symptoms and feel better immediately after leaving the area, it’s a strong sign of exposure. Make sure everyone exits the building before calling for help, and do not attempt to turn off any valves while still inside. Instructions on how to install a carbon monoxide alarm can be found in Section 1, Item 3.