Seattle Home Inspector's Blog


The dark side of Carbon Monoxide Detectors

Carbon Monoxide detectors have been in the news a lot lately and with good reason.  Given that the gas is colorless, odorless and can kill you while you sleep are all good enough reasons for me to pay attention. The building codes themselves require them, and in addition, many states have made laws requiring them.

There is a dark side to CO detectors that is not discussed very much however, and I have blogged about this issue with Carbon Monoxide Detectors in the past. 

This missing information can leave the consumer with a false sense of security. 

Most of the CO detectors required by state laws and the building codes are quite good at detecting Carbon Monoxide in relatively acute doses.  Acute doses are large levels of exposure over relatively short time periods.  They are not very good at detecting Carbon Monoxide at chronic levels—low levels of exposure over longer periods of time.

In fact, the CO detectors required in your home are NOT ALLOWED to signal a problem at levels below 30ppm in order for them to obtain the UL listing necessary to meet the codes (UL 2034).  It is interesting to note that most of these alarms are designed to sound at or below 70ppm within 60 to 240 minutes (I do not think that this information should make anyone feel protected). 

There is a stream of emerging data (see HUD: Healthy Homes Issues: Carbon Monoxide) that there are health risks associated with low levels of CO exposure—levels below what is considered safe by the EPA.  According to the EPA levels of CO below 9 ppm over an 8 hour period, or 35ppm for one hour are considered “safe.”  Research is beginning to show that the elderly, the very young, the unborn and some other individuals experience negative physical, cognitive and emotional effects with exposures below these levels.

There is technology available for detecting low levels of  CO.  These devices do not meet UL 2034 and therefore cannot be “substituted” for the poorer performing listed devices but must instead be used as supplements to the required detectors.

One such device is the “Defender” CO detector.  It is capable of detecting levels as low as 5 ppm for less than a minute.  It has a wide range of sensitivities with different alarms and visual read-outs to display different levels of concern.  These seem like a prudent device for any home. These devices also attempt to deal with the issue of when they have reached the end of their expected life.  They come with a sealed in place battery so when the battery is dead you simply replace the whole device.  I wonder how many regular detectors will continue to give their false sense of security long after the batteries are dead or when batteries are replaced when the units are well past their expected life.

Also keep in mind that there is no substitute for regular servicing of all combustion appliances in the home.  Proper servicing is an essential part of any CO mitigation/detection system installed in the home.


Charles Buell, Real Estate Inspections in Seattle

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Comment balloon 54 commentsCharles Buell • October 27 2013 01:50PM


I would thin that if you are going to get a detector, or it is required to get, you may as well get one that detects even the lower levels of CO Charles.


Posted by Tom Arstingstall, General Contractor, Dry Rot, Water Damage Sacramento, El Dorado County - (916) 765-5366, General Contractor, Dry Rot and Water Damage (Dry Rot and Water Damage Mobile - 916-765-5366) over 6 years ago

This is absolutely wonderful information and such a useful post!  Thanks for sharing it Charles!

Posted by Kristin Johnston - REALTOR®, Giving Back With Each Home Sold! (RE/MAX Realty Center ) over 6 years ago

Tom, this is a case of regulations being put in place without considering all the available information

Kristin, thanks---I would bet that few people are aware of this information

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Thanks for the heads up.  JUst put up a new carbon monoxide detector at my cabin this past weekend.

Posted by Marc McMaster, Putting my clients before myself (RE/MAX Centre Realty) over 6 years ago
I didn't know this - pretty sobering to realize how many people think they're well protected when that may not be true at all! Thanks for sharing.
Posted by Nancy Conner, Olympia/Thurston County WA over 6 years ago

Hi Charles - Thanks for the heads-up on this, as well as the pointer to the Defender. Like most others, I had just assumed that putting a standard CO detector in place "solved" the problem - at least to the extent of letting us know when there was a problem to be solved. Apparently, not necessarily the case.

Posted by Dick Greenberg, Northern Colorado Residential Real Estate (New Paradigm Partners LLC) over 6 years ago

Marc, you are welcome

Nancy, exactly--the typical devices installed in homes that meet requirements actually do very little to protect us except in relatively large doses of CO

Dick, it seems that nothing is ever the end of the story


Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

I expline this same issue to people on a regular basis. I have found CO where the furnace was spilling combustion gas into the laundry room. The owner though there was an issue since the window would fog when the furnace was on. He put in a CO detector but it never went off. The detector needed 50ppm for over 30 min to alarm. The only reason I can think of for these detectors to be so high on the scale before they alarm is to prevent faulse alarms that would make some people disable them all together. 

Posted by Rob Ernst, Reno, NV-775-410-4286 Inspector & Energy Auditor (Certified Structure Inspector) over 6 years ago

Wow. I had no idea. Thanks for sharing!

Posted by Wayne and Jean Marie Zuhl, The Last Names You'll Ever Need in Real Estate (Samsel & Associates) over 6 years ago

Interesting.  I had also heard that they really need to be lower on the wall to be effective.  Our codes don't specify this, and I've seen them even on the ceiling. 

Posted by Geoff ONeill (John L. Scott Medford) over 6 years ago

I had no idea. Thank you for the good information. 

Posted by Matt Beck, Going the Distance For You (Berkshire Hathaway Home Services Arizona Properties) over 6 years ago

I could be wrong, but I still have to believe that having something is better than nothing. 

Posted by Richard Iarossi, Crofton MD Real Estate, Annapolis MD Real Estate (Coldwell Banker Residential Brokerage) over 6 years ago

Charles, another great post.  I have one in my basement, probably UL approved, that I'm going to replace with a Defender!

Posted by Patricia Kennedy, Home in the Capital (RLAH Real Estate) over 6 years ago

Wow, good to know.....  I'll check mine.........

Posted by Marlene S Giles over 6 years ago

Charles...Thanks for another informative blog.  We have CO detectors on both levels but I have always told my wife I did not trust them. They are identical to your photo.

I think I will look into the others and have both.  I have lost friends to CO poisoning.  So it is always a concern.

Posted by Mike McCann - Nebraska Farm Land Broker, Farm Land For Sale 308-627-3700 or 800-241-3940 (Mike McCann - Broker, Mach1 Realty Farmland Broker-Auctioneer Serving Rural Nebraska) over 6 years ago

I read about this a while ago but dismissed it for some reason.  Now that you bring it up I feel like the low levels that would not trigger the detector could cause damage over time to people.  My very nonscientific analysis.

Posted by Jay & Michelle Lieberman, Creating Calm in the Buying and Selling Chaos (Keller Williams World Class) over 6 years ago

Charles -- this is a very informative post.  I had no idea.  Where can one purchase this type of CO dectector and what is the cost?

Posted by Joan Whitebook, Consumer Focused Real Estate Services (BHG The Masiello Group) over 6 years ago

Thanks for posting this for us Charles. I live at the Coast and we have not had a problem with Carbon MOnoxide in t he area.  We do have mold issues from damp humid homes that have been shut up for the season..


Posted by Ginger Harper, Your Southport~Oak Island Agent~Brunswick County! (Coldwell Banker Sea Coast Advantage) over 6 years ago

Carbon Dioxide detectors are just a good Idea. Why worry about it.

Posted by Bill Reddington, Destin Florida Real Estate (Re/max By The Sea) over 6 years ago

Rob, yes, these things are poor at short term bursts of large quantities of CO.  I had a oven that was above 600ppm and never set off the CO detector across the room.  I only ran it for a few moments but the client said that it never went off when she used it either.

Wayne and Jean, you are welcome

Geoff, just follow the Manufacturer's instructions---location is not that critical.  Just keep them away from where kids can play with them or where the dog can pee on them

Matt, you are welcome

Richard, for sure

Patricia, don't replace it---supplement it

Marlene, remember though---you need both

Mike, I think we are really only beginning to learn about the issues with low level chronic exposure to CO

Jay & Michelle, there are studies to support those fears as well

Ginger, if you have gas appliances or attached garages then CO can be an issue

Bill, that would be Carbon Monoxide not Carbon Dioxide

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

I just had a new roof put on my house, and the building permit application required that I certify I had smoke detectors and a carbon monoxide detector.  I already had the smoke detectors of course, but it acted as catalyst for obtaining the needed CM detector.  I know it's a good idea,

Posted by Myrl Jeffcoat, Greater Sacramento Real Estate Agent (GreatWest Realty) over 6 years ago

Thank you for this really great information.  I have one in my basement but I am also going to get the defender! I work in my basement for hours upon hours!

Posted by Evelyn Johnston, The People You Know, Like and Trust! (Friends & Neighbors Real Estate) over 6 years ago

Charles, thanks for this important information everyone needs to be aware of. I notice the device Defender is NOT available for sale in California. Funny, since it's been law in CA carbon monoxide detectors must be installed when selling or leasing homes. 

Posted by Pamela Seley, Residential Real Estate Agent serving SW RivCo CA (West Coast Realty Division) over 6 years ago

Never go more than 6 months without checking detectors is my rule of thumb.

Posted by Laura Cerrano, Certified Feng Shui Expert, Speaker & Researcher (Feng Shui Manhattan Long Island) over 6 years ago

Charles, I am a huge proponent of having more than one carbon monoxide detector in the house.  I almost died from carbon monoxide poisoning when my detector failed.  I was home injured and taking lots of bed rest for a few days.  An exhaust pipe from the furnace had a crack in it.  If the phone didn't ring to wake me out of my stupor, I wouldn't be here today.  The phone was right on the headboard of the bed, but I could barely maneuver to answer it and told the person on the line to call 911 right away because I was so weak. Carbon monoxide detectors are so very important!

Posted by Lisa Friedman, 30 Years of Real Estate Experience! (Great American Dream Realty) over 6 years ago

Myrl, just remember that the "required" alarm is going to be one that meets the requirements of UL 2034 and will not be useful and chronic low levels of CO--get both types to be safest.

Eveyln, good idea

Pamela, yes, and the required ones are going to be the ones that meet UL2034.  Not sure why the Defender is not allowed as a supplement to the others at least.

Laura, good plan

Lisa, do you have any of the low level CO detectors?

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Charles, thanks for sharing this important information with us all. I was unaware of the low level exposure risks... would be worthwhile to add a Defender as a supplement.

Posted by Ralph Gorgoglione, Hawaii and California Real Estate (310) 497-9407 (Maui Life Homes / Metro Life Homes) over 6 years ago

Charles - Interesting and a little bit scary to think the cabon monoxide detectors do not actually protect people.

Posted by Christine Donovan, Broker/Attorney 714-319-9751 DRE01267479 - Costa M (Donovan Blatt Realty) over 6 years ago

Ralph, I think so

Christine, it is not that they do not protect people it is that people need to be aware of the kind of protection they provide so they can be proactive about additional protection that is likely warranted.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Excellent information. We're always learning every day about the little hidden dangers that abound and it's certainly important to take the time to ensure those that we love are protected.

Posted by Aaron Hofmann, aka Mr. Smyrna Vinings (Atlanta Communities) over 6 years ago

Good information for everyone.  What you can't see can kill you.

Posted by Richard Burge Realty/ Burge Homes, Broker in Charge/Owner (Richard Burge Realty/Burge Homes) over 6 years ago
Charles, Very interesting information. Interesting that approved levels of detection are so high.
Posted by Bill and MaryAnn Wagner, Jersey Shore and South Jersey Real Estate (Wagner Real Estate Group) over 6 years ago

Charles, the UL approved sensors give a false sense of security. Most people are not aware of this.

Posted by Michael Setunsky, Your Commercial Real Estate Link to Northern VA over 6 years ago

Aaron, thanks---a pretty simple addition to the arsenal

Richard, sometimes for sure

Bill, I would love to know the reasoning behind it.  It may have just been the state of technology at the time

Michael, I am sure that is so

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Thank you for the information and for the discussion from everyone

Posted by Drick Ward Property Management / Broker Assoc, "RealtorDrick" - Experienced Representation (NEPTUNE REALTY) over 6 years ago

I had no idea.   I'm going to have to get a low-level detector!

Posted by Kat Palmiotti, The House Kat (406-270-3667,, Broker, Blackstone Realty Group - brokered by eXp Realty) over 6 years ago

Thanks for the information.  I have a CO detector but have not read about low level exposure before.

Posted by Jerry Lucas, Mobile Notary Colorado Springs, CO Notary Training (ABC Legal Docs LLC) over 6 years ago

Drick, you are welcome

Kat, I think a lot of people will be now

Jerry, now you know

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Walls used to be for decorative items.  Mine are being filled with detectors.  NFPA certified rep was here last week to explain several detectors.  Most fires that kill people in their sleep are smoldering fires.  Detector of choice is a photo electric sensor.  This is different than the standard ionic chamber smoke detector that is so good at telling me I am burning the bacon.  We also need the heat detector set to the right temperature for its location.  Don't forget the heat rise detector that senses changes in temperature.Add the detector for radon and another for smelling gas.  This does not include the two carbon monoxide detectors.  WOW!

Posted by Dwight Puntigan, Dwight Puntigan (DRP Realty, LLC) over 6 years ago

Good read. Thanks for posting this. Will pass this along to everyone....

Posted by Greg Nino, Houston, Texas (RE/MAX Compass, formerly RE/MAX WHP) over 6 years ago

Dwight, so true about the photo-electric smoke detectors.  The American Society of Home Inspectors, ASHI, has come out publiclly to recommend installation of ONLY photo-electric type detectors.  I have never heard of a stationary Radon Detector---not an issue where I am.  Warning sensors will become even more a part of our lives in the future I am afraid.

Greg, thanks---get the word out.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Life and health saving information here Charles. Thank you. I remember a listing agent grumble about an inspector who wanted to test for radon levels - the agent called me to complain that radon was NOT an issue and that the guy shouldn't be allowed to do inspections. How curious...we've since learned that in our county, there are many pockets of neighborhoods built in areas with alarmingly high radon levels.

Posted by Debb Janes EcoBroker and Bernie Stea JD, REALTORS® in Clark County, WA (ViewHomes of Clark County - Nature As Neighbors) over 6 years ago

Debb, Clark County is indeed one of the "hot spots" for WA State.  However, I never like to pass up an opportunity to tilt at one of my favorite windmills, so please check out the following links:

Radon, is another example of where legislation likely got ahead of science.


Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Wow, I thought all detectors were the same.  Definitely would need to do some research before purchasing and decide how much risk you are willing to take.  


Posted by Kim Peasley-Parker (AgentOwned Realty, Heritage Group, Inc.) over 6 years ago

Definitely didn't know about the risk of low levels of carbon monoxide and that there was a device to detect these lower levels.

Posted by Morgan Evans, LICENSED REAL ESTATE SALESPERSON (Douglas Elliman Real Estate) over 6 years ago

Thank you for the information. I will share it with others.

Posted by Gita Bantwal, REALTOR,ABR,CRS,SRES,GRI - Bucks County & Philadel (RE/MAX Centre Realtors) over 6 years ago

Great information. I was unaware that the standard for these are not as good as they should be to protect the occupants. 

Posted by Gerard Gilbers, Your Marketing Master (Higher Authority Markeing) over 6 years ago

Charlie good info as always. Low level chronic CO is just really starting to get some traction. The British seem to be ahead of in this area. CO prevents proper Oxygen absorption and release in the blood and builds up over time.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 6 years ago

Kim, I guess there will always be risk---but it is good to keep up with the science if we can

Morgan, yes---now we will be seeing both and in time either the codes will adjust or combination ones will become the norm

Gita, thanks

Gerard, it is a case of the codes not keeping up with the science I think

Don, why are we always playing catch-up with som many countries?

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Did not know the limitations of the technology behind the more inexpensive devices sold in most retail stores.  The science behind these carbon monoxide detectors is good for everyone to know.  Thanks for researching and posting!

Posted by Michael Dagner, Your Denver Homes Realty Expert (Brokers Guild Classic) over 6 years ago

Interesting stuff Charles.  Thanks for enlightening us. 

Posted by Steve Mun, Silicon Valley Realtor (Keller Williams Silicon Valley) over 6 years ago

Michael, you are welcome

Steve, all in a days work, playing in the rain

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Good info. A false sense of security is right. People install these devices and believe they are fully protected when in fact they are left partially risk exposed. 

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 6 years ago

Jim, for sure.  I think in the coming years we are going to hear a lot more about issues with low level CO exposure

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago