If a carbon-monoxide alarm sounds or several people develop headache and nausea together, get outside before calling
Move everyone outdoors first, then call for help. Do not stay inside searching for the cause or return to close a valve. Symptoms can resemble flu or intoxication; improvement outside is a strong clue. In the US, unintentional non-fire-related carbon-monoxide poisoning kills over 400 people annually and sends over 100,000 to emergency departments.
What it takes
Free.
What you may gain
US annual figures for unintentional carbon-monoxide poisoning unrelated to fire are over 400 deaths, over 100,000 emergency visits and over 14,000 hospital admissions.
Context & considerations
Several people becoming ill together and improving outdoors are strong clues. Do not investigate indoors or return for valves; leave first. See alarm installation. TODO: a national Chinese carbon-monoxide death estimate was not obtained from official statistics.
Research & references
US CDC. About Carbon Monoxide: the above annual estimates and symptoms including headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. https://www.cdc.gov/carbon-monoxide/about/index.html; US Consumer Product Safety Commission. Carbon Monoxide Information Center: get outside to fresh air immediately if poisoning is suspected, then call emergency services (911 in the US). https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Carbon-Monoxide-Information-Center