Carbon monoxide (CO) is a deadly, colorless, odorless, poisonous gas. It is produced by the incomplete burning of various fuels, including coal, wood, charcoal, oil, kerosene, propane, and natural gas. Products and equipment powered by internal combustion engines such as portable generators, cars, lawn mowers, and power washers also produce CO.

What is CO?

CO is caused by the combustion process and is actually unburned fuel that, if reignited, will burn and produce more heat. The ignition temperature for CO is 1,128°F.  If the CO is in the proper concretion, it can cause an explosion. 

When CO is breathed into the lungs, it bonds with the hemoglobin in the blood up to 200 more times tighter than oxygen, causing the body to shut down due to a lack of oxygen. This is akin to being chocked or suffocated.

CO poisoning symptoms mimic the common cold or flu. Symptoms include headaches, fatigue, weakness, muscle aches or pains, nausea or vomiting, confusion, memory loss, dizziness, incoordination, difficulty breathing, rapid heart beat, chest pains, and changes to sensory senstivity.

CO poisoning can also aggravate existing medical or genetic problems or cause existing problems to linger. Young children, the elderly, and those that have existing medical problems are more susceptible to the effects of CO poisoning and need to take precautions to ensure their safety.

Action Thresholds

There are certain levels that code and governmental officials use in determining what actions need to be taken to protect the public.

According to ASHRAE standards, 9 ppm is the maximum allowable concentration for continuous (24 hour) CO exposure (averaged over eight hours). This level is usually exceeded during rush hour in many urban areas of the nation due to vehicle emissions, according to the Environmental Protection Agency (EPA).

Concentrations of 10 to 35 ppm present potential risks for the occupants of a building, according to the Building Performance Institute (BPI). This air quality goal is averaged over one hour. At this level, first responders need to utilize self-contained breathing apparatus when      responding and gaining access to the area.

According to the Occupational Safety and Health Administration, 50 ppm is the maximum allowable level for any continuous exposure during an eight-hour period.

The 36 to 99 ppm level is excessive and requires a medical alert. This condition must be mitigated. Ventilation after equipment shutdown is mandated. Any and all repairs need to be performed by qualified personnel. Occupants need to seek medical treatment.

At 100 to 200 ppm, 15 minutes is the maximum allowed exposure time. Occupants are strongly urged to seek medical treatment.

And when concentrations exceed 200 ppm, all government and emergency first responders treat this level as an evacuation action level.

While these levels will help to make us aware of high levels of CO, it does very little for low levels of CO in our living environment. All agency’s currently quote UL 2034 which rates how a detector senses CO in our environment. Sadly, this standard has been changed over the years to “stop false alarms” causing more and more potential for sickness to all of us.

Common Sources of CO in our Living Areas

Exhaust — Auto exhaust or equipment exhaust accounts for 60 percent of all CO poisonings in the U.S. These could be from warming up the car in the garage to running a grill or a generator in an enclosed space. Even if the door is open, it is not enough. The device that has the potential to produce CO needs to be located outside and away from the building.

Unvented appliances — Unvented appliances, such as gas ovens and stoves, fireplaces, and space heaters, account for 20 percent of all CO poisonings. While the homeowner is trying to save energy by only heating the areas of the house they are using, they are actually killing themselves. It has been shown that the two times of the year with the most CO poisonings happen around the holidays of Easter and Thanksgiving. When any fuel-burning appliance is being used to prepare food or to heat an area, it’s important to provide fresh air to that area.

Backdrafting appliances — Due to the recent energy cost increases, people are closing and tightening up their homes and buildings. This has caused a condition that allows air to be sucked into a structure by the most direct route possible. This route is usually the flue pipes that connect furnaces and water heaters to the outside of the structure to allow the products of combustion to be properly vented. When this happens, it is called backdrafting. Backdrafting accounts for 19 percent of all CO Poisonings.

Cracked or damaged appliances — Damaged or misfiring vented appliances only account for less than 1 percent of all CO Poisonings. Newer equipment, with all the safeties working properly and the equipment installed to manufactures’ and code instructions, the potential for CO poisoning has dropped significantly over the past 10 years.

The reason that we are hearing of more and more cases of CO poisonings is because we now have better and more affordable detection equipment, not to mention the public awareness has been raised due to news stories.


Early detectors consisted of birds or other small animals that were taken into confined spaces, and it was the job of one person to watch what the animal did. If their behavior changed in any way, that was a sign that there was some kind of problem in the atmosphere and it was time to leave. The main problem was that we never knew what gas or gases were present or at what levels.

Today, all CO detectors are supposed to be rated by the UL. There is a type of detector that is called a low-level detector that is actually tighter than the rating system from UL. The rating system that most manufactures work toward is UL 2034 (Appendix 1).

Either a standard level stand-alone or monitored detector is now required by code for any new or replacement equipment installation. This was a step in the right direction.

2034 Detectors — CO Detectors that are UL listed must alarm according to the conditions outlined in Table 1. While these levels do provide some protection from poisoning, they are not the best. These detectors are set to stop the “nuisance” false alarms.

Low-Level CO Detectors — Low-level CO detectors start to indicate the presence of CO at a level of 1 to 5 ppm. At this level, they do not alarm, but they display the actual reading so that users can monitor the atmosphere before medical attention is required.


While CO poisoning has been around since mankind started using fire to cook food, provide heat, and improve their lives, it has been little understood. With the push to save energy and money, we are seeing more and more cases of CO poisoning. Therefore, it is absolutely critical to have quality CO detectors installed in every home and building.

Publication date: 4/22/2019

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