Other gases may set off CO detectors

Jeffrey P. Yaroch


Richmond, Michigan

I wish to compliment Chief Frank Montagna (“Responding to CO Detector Activations,” Fire Engineering, January 1996), Chief Chuck Herzog (Letters to the Editor, May 1996), and the International Association of Fire Chiefs (“The IAFC issues additional guidelines for CO detector activation investigations,” News in Brief, May 1996) on their excellent articles on carbon monoxide. As a member of the Health Department and Emergency Management Agency, I was involved in the development of the CO protocol for the City of Pittsburgh and Allegheny County. The articles these individuals have written continue to clarify what at times is a confusing topic.

I would like to comment on the IAFC item. We had a interesting experience involving a CO detector`s activating due to combustible gases. We received a call from a resident requesting assistance in identifying why her CO detector kept alarming and indicating 60 ppm CO. The fire department had responded on several occasions and verified CO with its meters. The gas company and a heating contractor could not identify any source of CO. All appliances were functioning properly.

The next step was to replace the detector. The new detector gave similar responses.

Since the area in question had mines running under it, we suspected the source of the CO was a mine fire. Tests for CO in foundation walls, storm and sanitary sewers, and neighboring residences all gave negative CO readings.

After extensive investigation and luck, we found an acetylene cylinder in the garage that had a slight leak. When placed near a leaking valve, the CO meter alarmed. The lower-explosive-limit function did not. The reason for this was the levels of acetylene were well below the LEL of one percent (10,000 ppm) but high enough to activate the CO detector.

Personnel responding to CO calls need to be aware of the limitations and interferences that can affect CO detectors. CO detectors are electrochemical sensors, which detect a variety of specific gases. In most cases, the gas of interest reacts with the chemical solution in the sensing electrode to produce ions (charged particles) that move through the solution to an electron-accepting electrode, completing an electric circuit. The number of ions produced and the magnitude of the current developed are proportional to the concentration of the gas being measured.

Interfering gases of similar molecular size and chemical reactivity may produce false positive response. Major interferents that can affect CO detectors are acetylene, dimethyl sulfide, ethyl alcohol, ethylene, hydrogen cyanide, hydrogen sulfide, isopropyl alcohol, mercaptan, methyl alcohol, propane, nitrogen dioxide, and sulfur dioxide.

Responders must be aware of the limitations of instrumentation and be on the lookout for interfering substances when doing a CO investigation, particularly on return calls or when no apparent source of CO is found.

I also wish to support Herzog`s comments regarding CO exposure. Exposure to levels of three to 11 ppm CO can result in blood CO levels of three to four percent. A level of 29 ppm will produce a CO blood level of five percent. The lowest level of blood CO that produces significant myocardial changes in humans was six percent. Studies have found that CO blood levels of 10 percent had an effect on driving skills and light discrimination. Other studies have indicated that low level CO exposure can affect visual perception, vigilance, cognitive functions, and time discrimination. These studies suggest that a CO blood level of five percent is a reasonable value in terms of protecting workers against adverse behavioral effects due to CO exposure. Herzog`s belief that a threshold of 25 to 35 ppm is in the best interest of firefighters is supported by the above findings.

Anthony M. Gaglierd, RO

Hazardous Materials Red Team

Allegheny County Emergency Management

Pittsburgh, Pennsylvania

Oil-change shop explosion

Recently in my community an explosion at a 10-minute oil-change shop killed a 16-year-old employee and burned another. It is believed that a customer may have dropped off for disposal waste oil containing an unknown flammable liquid. The waste oil was poured into the waste oil tank in the shop`s pit. Because there was no fluid-level sight glass, the employees are thought to have used a cigarette lighter to look directly into the tank. The vapor ignited, and the tank failed, killing the 16-year-old and touching off a very intense fire. No code violations were found. However, many of these conditions are common to the majority of oil-change shops in the area and probably the entire industry. These factors seem to indicate a lack of awareness of and planning for the potential risks of this new business. Please encourage your readers to take a second look at these facilities in their communities.

Originally ran in Volume 149, Issue 9.

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