Carbon monoxide: Dangers and devices for detection

June 16, 2016 By    

Carbon monoxide is colorless, odorless and not detectable by human senses, and it shouldn’t be confused with carbon dioxide.

With a specific gravity of .97, as compared to air at 1.0, carbon monoxide is lighter than air and mixes well with it.

Carbon monoxide is a toxic gas. It can cause death if enough of it is inhaled. It’s also flammable. It can produce an explosion in a wider range of mixtures with air than does natural gas. Its flammability limits extend from about 12.5 percent to 74 percent of the gas in the air. Many explosions that occur during fires are the result of a buildup of carbon monoxide. Carbon monoxide is highly toxic in a small fraction of 1 percent air. Its toxic effect thus is a much more serious problem than the chance of an explosion.

One can also expect hydrogen gas to be present in flue gases if a fuel gas is not completely burned. Hydrogen can burn or explode in air when it makes up about 4 percent to 74 percent of the mixture. Hydrogen is not toxic – the most serious result of incomplete combustion is the production of carbon monoxide.

Inhaled carbon monoxide is absorbed into the blood. It combines with the hemoglobin in the blood to a much greater extent than oxygen and remains in the blood longer than oxygen. In doing so, it acts to reduce the oxygen-carrying function of the blood. Thus, a person exposed to carbon monoxide can die of a lack of oxygen.

The amount of carbon monoxide absorbed into the blood depends chiefly on two things:
1. How much carbon monoxide is in the air
2. The time of exposure

Adverse effects of carbon monoxide on humans are reduced by periods of breathing fresh, uncontaminated air. The degree of recovery depends on the number and length of those periods. Over 200 parts of oxygen are needed to replace one part of carbon monoxide in the blood. The exposed person’s general state of health and degree of physical activity are other factors involved in the effects of carbon monoxide on the body.

All gas appliance designs tested under American National Standards Institute (ANSI) have met rigid tests for combustion, along with other safety tests. For example, ANSI Standard Z21.1 for gas ranges states a unit shall produce no carbon monoxide. ANSI standards for other appliances contain similar limits on carbon monoxide produced during testing.

These are some of the facts. The question is how do you protect yourself from being overcome by carbon monoxide. Certainly your nose does not know, so testing has to be done. If you do not test, you do not know.

Testing can be done with a combustion analyzer. Technicians typically use these instruments to determine equipment efficiencies by taking samples from the flue gases of oil and gas equipment. However, not everyone has a combustion analyzer (those who service equipment should have one). Even if they do have one, a combustion analyzer is not something typically carried into buildings on every visit.

Needed instead is a device – classified as personal protective equipment (PPE) – that can attach to your belt or be placed in your pocket (belt is best). There are several types on the market. I have used a Testo 317-3 for years. It alarms at 9 parts per million (ppm); the alarm is audible or can be silenced if you do not want to alarm your customer.

Some of the different detectors available on the market and which range in price between $200 and $300 include:

  • Sensorcon Inspector
  • Sensit Technologies P100
  • Drager Pac 5500
  • Industrial Scientific T40 Rattler

If you detect carbon monoxide upon entry into a dwelling, then evacuate the dwelling until the cause can be safely determined by those trained to do so.

Possible causes of carbon monoxide in a dwelling include fossil fuel burning equipment, electrical generators used when power failures take place and construction heaters used in confined spaces at construction sites.
Be safe; protect yourself and your customers; and wear carbon monoxide PPE at all times.

Concentration: The typical levels of carbon monoxide allowed
9 ppm – Environmental Protection Agency allowable average over eight hours for residential living space
50 ppm – Occupational Safety & Health Administration (OSHA) maximum for eight-hour period for workspaces
200 ppm – OSHA maximum at any one time
400 ppm – The allowable level from ANSI for a flue sample. (We, however, strive to have the level below 100 ppm.)
1,500 ppm – Immediate danger to life

Timmie McElwain is president of Gas Appliance Service Training & Consulting and the Gas Training Institute in Warren, R.I. He can be reached at or 401-437-0557.

Photo credit: Clint__Budd via / CC BY

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