Carbon Monoxide (CO) is a killer. It is colorless and odorless, and it is very very different from the smoke that results from burning the toast. Since you can’t see or smell CO, you need an instrument to alert you to its presence. No doubt, it is handy to have both the smoke detector and the CO alarm in one product. BUT they are not the same thing. Maybe you set the smoke detector off every night so you take the batteries out because you are tired of waving a towel at the ceiling to move the smoke away. You can’t do that with a CO alarm.

I’m sure that most people who have CO alarms don’t read the accompanying information so they probably don’t know that the CO level has to be at 40 parts per million for 10 continuous hours before the alarm will go off.

Carbon Monoxide Level	Alarm Response Time
40 PPM	10 hours
50 PPM	8 hours
70 PPM	1 to 4 hours
150 PPM	10 to 50 minutes
400 PPM	4 to 15 minutes

So what’s the harm in a little carbon monoxide?

• 9 ppm is the ASHRAE standard for allowable spillage from vented appliances;
• 10 ppm in the outdoors is associated with a significant increase in heart disease death and hospital admissions;
• 15 – 20 ppm is the first level the World Heath Organization lists as causing impaired performance;
• 30 ppm earliest onset of exercise induced angina;
• 35 ppm US NIOSH recommended 8 hour maximum workplace exposure.

The CO alarms that are installed in most homes are alarms. They are not monitors. They don’t make shrieking noises until it is time to get out of the house. They won’t tell you if there is CO in the air. I served as an expert witness in a case where a woman died from CO poisoning in her house. There were serious signs that the boiler had been back-drafting and the batteries had been removed from the CO alarm.

And another thing: the sensor in a CO alarm has a limited life – 5 to 7 years is being optimistic. You can’t just replace the batteries. The whole unit should be replaced. A good low level CO alarm like the Defender, costs more and it is not UL approved because it is too sensitive. But I like to think my life and the lives of my grandchildren are worth a few extra bucks.

Strangely, CO alarms can be activated by ground coffee. You can put your alarm in a Zip Lock bag with some freshly ground coffee, and it will alarm or at least indicate something on the display.

I used CO as a murder weapon in my novel, Death at the Edge of Diamond. Building science can lead to unique weaponry!