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TECH | Carbon Monoxide: What Ambient CO Testing Really Tells You

 November, 22nd 2017     Comments     Views

By David Richardson

The diagnosis of many carbon monoxide (CO) incidents today is based only on ambient CO testing. This measurement is valuable, but it does have limitations. Let’s look at what ambient CO is, what it tells you, and what is missing from the measurement to pinpoint a CO source. 

What is Ambient CO?

The term ‘ambient’ is associated with the immediate surroundings of something. A term many use in HVAC is ambient temperature. This typically refers to the temperature of air surrounding a condenser or inside a building. When ambient is used regarding CO, it relates to the air you are standing in and breathing. 

What Does an Ambient CO Measurement Tell You?

Measuring ambient CO tells you one narrow truth: whether there is carbon monoxide in the air you are breathing. You’ll know if it is safe to be in a building if you monitor the air. Refer to the following ambient CO levels in parts per million (ppm) based on NCI protocols as a guide for recommended action.

1 to 9 ppm: Acceptable to conduct tests and discover the source. These readings are not initially harmful to the tester, but can indicate a larger problem. The source needs to be identified and addressed.

10 to 34 ppm: If small children, elderly, or people with heart or respiratory problems are present, they should be made aware of the levels you’ve measured. They can be susceptible to these amounts that may not affect a healthy adult. It is unacceptable to leave this environment without addressing the problem. 

35 to 69 ppm: At these levels, residents should be asked to step outside and questioned about possible health symptoms they may be experiencing. Turn off all appliances and open windows and doors to ventilate until ambient CO levels drop below 35 ppm. It is unacceptable to leave this environment without addressing the problem.

70 ppm or greater: At these levels, this should be considered dangerous. Evacuate the building immediately and check for health symptoms. Open any door or window that can be accessed quickly without entering the building. Call the local utility company to turn off the fuel if you’re unable to safely access the appropriate controls. Ventilate the building until ambient CO levels fall below 35 ppm. Failure to identify and correct the cause could put the occupants in mortal danger.

So, What’s Missing?

An ambient CO reading does not identify the source of the deadly gas. This is important to know. You cannot pinpoint what is causing excessive CO simply because you measured it in the air. Countless CO problems are improperly diagnosed or missed because the tester is looking in the wrong place.

Understand that when someone diagnoses a CO problem based on ambient readings alone, they are guessing. To truly find the source, all fuel-burning equipment must be tested. The fundamentals of combustion testing -- such as flue gas and draft readings -- hold the key to determining safe operation, not guesses and assumptions.

In addition to combustion testing, you must also consider outside influences such as exhaust fans, duct leakage, and building pressure. To effectively locate any source of CO, a complete approach is needed.

A Recent Example

This past week, nearly 50 students and staff of a school in Rhode Island appeared to suffer the effects of carbon monoxide poisoning. One student was hospitalized, while others had symptoms that included: dizziness, headaches, nausea, and exhaustion. 

Two days passed between the time the incident occurred and when the building was CO tested by two different organizations. Neither found any measurable CO in the air. The school has since been given the “all clear” and will resume business as usual. 

What will happen when the building fills up with students and staff again? Can their safety be guaranteed or is it possible this scenario could repeat itself? 

What Can You Do?

If this scenario sounds familiar, I recommend you start questioning what you normally do. Are you guessing, or can you confidently pinpoint a CO source? Condition yourself to look beyond cracks in heat exchangers as the cause of CO poisoning - there are more sources to explore. Also, understand you’ll have to look past an ambient CO measurement to discover the true cause. 

Put a plan in place to get the proper test instruments, if you don’t already have them. At the minimum you’ll need to have a combustion analyzer, draft gauge, and personal CO monitor to get started. You’ll can’t move past guesses and assumptions, if you don’t have the right gear.

You also need to know how to diagnose the readings displayed on a combustion analyzer and draft gauge. The true value in test instruments is understanding the readings they provide. Proper training and practice are essential to your success – the key is to start.

The answers are out there, together we will find them. If you need any additional information on this material or have any questions, feel free to e-mail me at