Major Concerns with Carbon Monoxide in Homes

Vintage Double Oven


As a home inspector and and indoor air quality professional I’m interested in all things related to residential safety. This article will touch on several points related to carbon monoxide.

Nothing here should be taken to be health advice. Consult with your doctor if you think you’ve been affected by carbon monoxide (or anything else, for that matter).

What is carbon monoxide?

Carbon monoxide is a gas made up of carbon and oxygen.  You can’t smell it or taste it.  

It’s produced by the incomplete combustion of fuels that contain carbon, including natural gas, wood, gasoline, fuel oil, propane, kerosene and diesel fuel.  Almost anything that you might burn will produce carbon monoxide. 

Since complete combustion doesn’t typically occur outside of a lab, you should expect that household appliances produce it.

High levels of carbon monoxide are not only deadly, but also contribute to poor indoor air quality.

What are the risks associated with it?

According to the CDC, “Carbon monoxide (CO) causes the most non-drug poisoning deaths in the United States.”

Carbon monoxide displaces oxygen in the bloodstream when you breathe, and can essentially suffocate your vital organs.  If the levels of CO are high enough, then you can be overcome in only a few minutes.  

Breathing too much carbon monoxide will lead to poisoning, and according to the Mayo Clinic, the symptoms include headache, weakness, dizziness, nausea or vomiting, confusion, blurred vision and unconsciousness. 

Carbon monoxide poisoning causes about 400 deaths per year in the U.S., so everyone should take the stuff seriously.In cold climates such as Buffalo, where my home inspection company is based, the risk can be considered higher because of our closed-up homes during the snowy months.

Is carbon monoxide the same thing as CO2?

NO!  CO2 is carbon dioxide, which is the cause of fizziness in beer and soda.  It’s also the gas produced by plants.  

CO2 is not normally dangerous.  You don’t need to fear for your life when you see a plant. 

But carbon dioxide is a greenhouse gas that is a major contributor to global warming.  That’s another topic entirely, but you don’t need to fear its presence in the household.

What can produce carbon monoxide (CO) in a home?

Essentially anything that burns will release carbon monoxide.  Since houses tend to be closed up for much of the year (at least in colder places), everyone should be aware of the risk.  

Let’s look at some sources of carbon monoxide and tips for how to make sure they are as safe as possible

Furnaces & Boilers

Natural gas, propane or oil-burning furnaces all will produce CO.  My biggest recommendation is to have them inspected annually by an HVAC technician.  The inspection should check for proper operation of the system, but more importantly it should include an inspection of the exhaust system.  As a home inspector I find corroded furnace exhaust piping quite often, which can be a big danger.

Water Heaters

The most common source exhaust (and therefore CO) leaks with water heaters is also the exhaust piping.  Based on experience, it seems that many water heater installers don’t care too much about the exhaust system of these things. Look at the piping leading up and away from your water heater, and if the joints are taped or if there are holes in the pipe, then have those things corrected.

Wood-burning Fireplaces

Wood stoves and fireplaces rely on their chimneys and vent piping to carry exhaust gases (including CO) up and away from the house.  Making sure that the chimney or piping is safe and intact is the number one way to keep fireplaces safe.  Chimneys can develop cracks and other internal damage over time, which can lead not only to exhaust leaks but also fire risk to the structure of your home.

Gas Fireplaces

There are two types of gas fireplaces: vented and ventless. Vented ones obviously have piping or other ways to allow exhaust gases to escape.  Ventless gas fireplaces don’t have this basic feature, and in my opinion are an invention by the fireplace industry that is an unnecessary risk.  Regardless of the claims of safety, carbon monoxide is still released.  Personally I would avoid ventless fireplaces. discusses concerns with unvented gas fireplaces here.

Vehicles in Garages

Cars and trucks obviously release carbon monoxide(except if they are 100% electric, of course).  If they are parked in a garage while running, then this is an obvious danger.  There are a few things you can do to reduce the risk of carbon monoxide in garages and attached homes:

  1. Make sure that the home entry door is self-closing.  This is required by building code in most areas and can be accomplished by installing special hinges.
  2. Install a carbon monoxide alarm in the garage
  3. Install exterior vents in the garage.  They aren’t required by code, but common sense dictates that they can help reduce risk
  4. Make sure that HVAC supply or return vents aren’t installed in the garage.  I’ve actually seen this during home inspections.  It’s a dangerous condition that will allow car exhaust to be sucked into the home’s ductwork.

Cooktops and Ovens

Cooking appliances release CO while they are operating.  A great way to reduce the danger is to have a vent hood that sends air to the outside of the house.  Building codes don’t always require this, but it’s good not only to keep CO levels down but also do suck out cooking smells.  Who wants to wake up at 7:00am only to smell salmon from last night’s dinner???

Clothes dryers

Gas dryers of course emit carbon monoxide.  It’s not generally an issue so long as the dryer exhaust doesn’t have holes or openings, and it’s not clogged up with lint.  I always check for clogged dryer exhausts during home inspections.  It’s a simple process of finding the exterior vent opening, and feeling around to make sure that air can escape while the dryer is running.  If it can’t then you may need to temporarily disconnect the hose and clean it out.  There are special cleaning brushes that you can find to make this easier.

Portable Generators

These awesome inventions help people keep their homes warm and food cold during long power outages caused by snowstorms and other big weather events.  The thing is, that people sometimes run out and buy a portable generator and want to put it in their garage or even inside of the homes.  DO NOT DO THIS!  Hundreds of people die because of carbon monoxide poisoning from portable generators.  

If you need to run a portable generator:

  1. Keep it no less than 20 feed from all outside walls and porch or deck roofs.  
  2. Make sure you have CO detectors installed in the home 

Outdoor grills

These emit CO just like any other carbon-burning thing.  Keep windows near them closed when they are running.

How to protect yourself from carbon monoxide?

Let’s quickly summarize how to protect your family from CO poisoning

  1. Install CO detectors in the right places
  2. Make sure the door connecting the garage to your house is self-closing
  3. Check your garage to makes sure there are no HVAC registers installed
  4. Look at the vent pipes from your furnace and water heater for any unsealed joints, holes or other openings.
  5. Check your dryer exhaust for holes and clogs
  6. Get your chimney swept and inspected every year
  7. Make sure that your range hood fan exhausts to the outside
  8. Have your furnace or boiler inspected annually 

Types of Carbon Monoxide Detectors

A quick Google search on the topic might reveal some confusing results.  To round out this article I wanted to quickly go over the available types of carbon monoxide (CO) detectors.  Not all of these make sense for a residential setting.


This is probably the best-selling type of CO detector out there.  They can be found at home centers, hardware stores and many other places.  

Models are available with 10-year batteries, or with replaceable batteries.  I recommend 10-year battery CO detectors since it’s easy to either forget to replace batteries, or to pull the detector off the wall when it warns you of a low battery.

Hardwired Carbon Monoxide Alarm

This is just what it sounds like – the CO alarm is connected directly to your home’s wiring, which gets around the problem of dead batteries.  Hardwired alarms are usually installed when the home is built, or during major renovations.  Some municipalities require them as part of their building code.  

Plug-In Alarms

These alarms are designed to plug into an electrical outlet.  They have the same advantage as hardwired alarms, except that you’ll need to make sure they don’t fall out of the outlet.

Combination fire & CO Alarm

Combination detectors contain the electronics needed to warn you of either fire, smoke or high carbon monoxide levels.  This is my go-to type of detector, for the simple reason that for a marginal price increase over a single-function alarm, you get better protection.

Interconnected CO Alarms

This type of detector is designed so that two or more of them are linked together, either via wired or wireless connections.  When one detector senses a high level of the deadly gas, all of the detectors will unleash their audible alarms so that everyone jumps up and freaks out (and leaves the house for safety).

Portable or “Personal”  Carbon Monoxide Alarms

These are typically used by those who work in areas where the risk of CO exposure is high.  They’re designed to constantly measure CO levels and will alert the wearer to high levels, and usually have a digital readout so that the user can be aware of rising, but not dangerous, levels.

Where in the home should CO detectors be installed?

According to the National Fire Protection Association, carbon monoxide detectors should be installed “in a central location outside each sleeping area and on every level of the home and in other locations where required by applicable laws, codes or standards.” 

They go on to recommend that detectors are interconnected so that when one alarm goes off, they all go off so that everyone in the house can be alerted to the dangerous situation. 

How often should detectors be replaced?

Unless you’re able to find the owner’s manual for your detectors, it’s best to assume that after seven years the detector will be unreliable.  Some units, such as the Nest combination fire and CO detector, are designed to last ten years and won’t need to be replaced before then.  

I generally recommend to home buyers here in the Buffalo, NY area that they replace the detectors as soon as they move in, unless they appear to be brand-new.

What level of CO is Dangerous?

The exact point of danger isn’t clear to me, but the Consumer Product Safety Commision states that “Most people will not experience any symptoms from prolonged exposure to CO levels of approximately 1 to 70 ppm but some heart patients might experience an increase in chest pain.”

Example Carbon Monoxide Levels Found in a Home

Using my Sensorcon Inspector carbon monoxide meter, I took a few quick measurements around my home.  I’ll update this post as I collect more data, but for now here are a few data points:

On a Simple Gas Range

After running two burners for roughly 20 minutes, the CO level rose to 5 parts per million (PPM).

Major Concerns with Carbon Monoxide in Homes 1
The CO monitor is at the back of the range

Burning a Candle

I lit this candle, waited about 10 minutes, and held the monitor over the flame.  The monitor actually showed over 50 PPM.  But when the monitor was resting next to the candle, it only read 10PPM.

Major Concerns with Carbon Monoxide in Homes 2

Running a Gas Clothes Dryer

The dryer was running for about 30 minutes, and the monitor registered zero PPM of CO.

Major Concerns with Carbon Monoxide in Homes 3

Do home inspectors test for carbon monoxide levels?

At least in New York State, testing for CO levels isn’t part of the home inspector code of ethics.  It’s also not included in the standard operating procedures of the American Society of Home Inspectors (ASHI), which I’m a member of.  

Here at Alto Home Inspection, LLC we don’t check CO levels, but as part of our detailed home inspection process we assess carbon-burning items such as furnaces, fireplaces and water heaters for visual clues that might indicate an increased risk of carbon monoxide exposure.

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About the Author:

Bradley Beck

Bradley is owner of Alto Home Inspection, LLC.  He lives just south of Orchard Park, in Western New York’s Southtowns, and inspects homes throughout the Buffalo area, and is NY Licensed Home Inspector #16000086029, NY Certified Mold Assessor #MA01313.