What makes it scary is that carbon monoxide is tasteless, odorless and invisible so one can be breathing it and not even be aware that it's in the space until symptoms begin to show like shortness of breath, mild headaches, nausea and fainting. In serious cases of CM poisoning, brain damage and death can occur. Not something you want to experience for an aging parent nor family friend of any age.
As a safe and sound precaution, add a carbon monoxide detector to an interior just like you would for smoke or fire. The should be installed at least 5 feet above the finished floor or on the ceiling as this gas is lighter than air and will rise. CM Detectors can be hard-wired or battery operated and the investment will be less the $50.00 in most cases but the return is the very "life" of those who occupy the space. There are many models available and ones that combine smoke and carbon monoxide detection into one battery-operated unit including this one from First Alert for $35.00. So worth the investment, don't you think?