Why can carbon monoxide become dangerous aboard without being noticed?
Short answer
It is colourless, tasteless and odourless, while early symptoms can resemble fatigue or seasickness
CO cannot be reliably detected by human senses, and headache, nausea, tiredness or confusion can be misread as ordinary conditions aboard. Use a marine-suitable alarm and treat an activation or shared symptom pattern as exposure evidence: move to open air, control the source when safe and seek medical help.
Quick check
Why can carbon monoxide become dangerous aboard without being noticed?