Firefighters are well aware that smoke is the deadliest hazard at fires and that carbon monoxide (CO) is the deadliest component of smoke. In recent years, Miami-Dade (FL) Fire-Rescue has responded to two fires that emphasize the hazards of CO and the need to remain alert for this hazard, even at fires where conditions seem to be clear of visible products of combustion.


This fire in rack storage in a sprinklered warehouse represents a “routine” fire. [Photo by Bill Gustin, Miami-Dade (FL) Fire-Rescue.]

At the first incident, my company responded, at 2 a.m., as a single unit to a report of a fire alarm’s ringing in a commercial occupancy. On arrival, a police officer told us he smelled paper burning. My size-up revealed a two-story office building, approximately 50 feet 2 200 feet, with an attached one-story warehouse of approximately 50,000 square feet. At the entrance to the office area, water was cascading from beneath the aluminum stile and glass doors, and there was an odor of paper burning. A waterflow motor gong could be heard in the distance.

I requested an assignment for a fire in a commercial building and proceeded to investigate. I had my company force entry into the office, where we discovered a light haze at ceiling level. Conditions below the ceiling were clear, and we entered searching for the fire’s location. All firefighters were wearing SCBA with facepieces in the standby position. None of the firefighters perceived the need to don their SCBA facepieces, since there were no appreciable visible products of combustion. There was approximately one inch of water throughout the ground floor of the office. Our search revealed a vacant office that was void of furnishings.

On the second floor, a padlocked door accessed the adjacent warehouse area. What sounded like a sprinkler flowing could be heard on the other side of the door. A light haze at the ceiling was also present on the second floor; however, we again saw no apparent need to don SCBA facepieces.

This information was reported to the battalion chief, who was now on the scene with a full structure fire assignment. He directed companies to force entry into the warehouse through a side entrance to search for the seat of the fire. A rescue and engine company forced entry and reported locating a fire that involved several stacks of cardboard shipping containers. The fire was controlled by two sprinkler heads, and two inches of water were on the floor. There was no visible smoke of any consequence in the warehouse.

In the fire area, the ceiling was approximately 25 feet high. As noted, there was a light haze at that level, some 20 feet above the firefighters’ heads. The labels on boxes just below the ceiling could be read from a distance, indicating that there was no appreciable smoke present. Our standard operating procedures (SOPs) are very specific about the use of SCBA in smoke conditions. Nevertheless, none of the firefighters operating in the area felt the need to don SCBA facepieces.

Heating, ventilation, and air-conditioning (HVAC) systems spread the products of combustion throughout a fire structure. [Photo by Bill Gustin, Miami-Dade (FL) Fire Rescue.]

With the fire controlled by the sprinklers, an officer from another engine company and I attempted to locate the riser to shut down the system. The warehouse was very large and was filled with rows of stacked cardboard boxes. We spent approximately 10 minutes looking for the sprinkler riser without success. After locating a doorway that had been sealed with concrete blocks, we realized the building had been altered and that the riser was on the other side of the wall. It was not accessible from the fire building. I reported this information to the chief, and we proceeded to head out of the warehouse.

On the long walk out from the back of the warehouse, the engine company officer who was with me soon outdistanced me. I did not think much of it at the time. On exiting the structure, I noted that this officer was now staggering and was being supported by other firefighters. At this time, I noticed that I felt as though I were intoxicated; the entire scene in front of me looked surreal.

The proverbial lightbulb went off in my head: The accumulation of water on the floor indicated that the fire had been burning for a long time, possibly for hours. The sprinklers controlled the fire, which must have been smoldering since it started, generating large volumes of CO. What fooled me was the fact that there was no smoke in the building. I have responded to dozens of fires in sprinklered buildings. Fires in these structures usually generate heavy, wet smoke that banks down to the floor, is light in color, and is typically difficult to vent. This fire appeared to generate no smoke whatsoever.

I told my battalion chief that I believed we had been exposed to high levels of CO. He immediately requested our hazardous-materials unit and additional medical rescue units. Ultimately, nine firefighters were transported to the hospital and treated for CO exposure. The levels of carboxyhemoglobin (HbCO) of the firefighters ranged between six and 31 percent (the normal range is zero to one percent). The CO readings taken by the haz-mat unit inside the warehouse pegged the meter out at 1,000 parts per million (ppm) at the entrance to the fire area. The lieutenant in charge of haz mat estimated that CO levels in some parts of the warehouse were as high as 2,000 ppm.1


The lessons learned from this fire are striking. Nine firefighters almost lost their lives in an atmosphere that was free of visible smoke. Any company responding to a fire that may have been smoldering for a prolonged time must consider the possibility of encountering high CO levels, even though there is no visible smoke.

At this fire, the tip-off was the accumulation of water on the floor. To accumulate levels of one to two inches of water throughout a large area, the two sprinkler heads must have been flowing for hours. Even though the sprinklers controlled the fire, the areas they could not reach continued to smolder and generate CO. The results were nearly fatal.


At the second incident, we responded first-due at 1:30 p.m. to a reported apartment fire. On arrival, I noted a two-story apartment building with no visible signs of fire. A security guard directed me to a ground-floor apartment, where I observed soot stains pushing from around the doorjamb but no real smoke condition. I reported a working fire in the ground-floor apartment.

I ordered my company to stretch a line and forced the locked apartment door. The battalion chief informed me that the windows in the apartment were soot-stained but cool. After the door was forced, my company advanced a handline into the fire apartment. There was no discernible heat; visibility was zero because of extremely heavy, black smoke. I reported to the chief that we could not locate the fire.

After groping our way through three bedrooms, performing horizontal ventilation as we advanced, I realized we were indeed in the fire room and that the fire had burned itself out. The temperature in this room gave no indication that it was the fire room. After the apartment cleared of smoke, the scene that was revealed was one of heavy, oily soot accumulations “cooked” onto the walls down to within a foot of the floor. Sooty residue on every surface made footing slippery throughout the apartment.

The fire apparently burned itself out in a closed bedroom and continued to smolder for hours. One neighbor reported smelling smoke up to four hours prior to the call reporting the fire. The apartment’s energy-efficient windows apparently contributed to containing and concealing the fire and allowing it to burn unnoticed for hours.

The fire was under control, and the battalion chief released most of the companies. The adjacent apartment and the two upstairs apartments were clear of visible smoke, so the occupants were allowed to return.

Since the warehouse fire discussed above, all our battalion chiefs carry a CO meter in the battalion vehicle. Our SOP allows releasing a residential occupancy when the CO level is below nine ppm. CO levels in the fire apartment were two to three ppm, but the occupants were not allowed to return yet because the fire scene investigation was in progress. No CO readings were taken in the adjacent apartments because they never had any smoke in them.

My company assisted the fire investigator with digging through the fire room to determine the cause of the fire. The investigator narrowed the logical point of origin to an area that contained numerous electrical cords. He released my company and said he would take another half hour or so to determine which wire started the fire, which was apparently electrical in origin.

Five minutes after leaving the scene, I heard the investigator request a unit with a positive-pressure ventilation (PPV) fan and a medical rescue for a “child passed out.” I assumed it was for the distraught occupant of the fire apartment who had just lost everything she owned. We had already been dispatched to another call en route to quarters. Monitoring the radio while on this call, I heard that the rescue from my station was transporting a child from the fire scene to the hospital. I thought this was unusual for such a routine fire.

On clearing our call, I notified Fire Alarm that my unit was responding back to the fire scene. We arrived to find the apartment occupants still on the scene and in no apparent distress. A neighboring engine company had responded and already vented the apartment adjacent to the fire apartment with its PPV fan.

The investigator informed me that the occupants of this adjacent apartment had called him and told him that their little boy was not feeling well and that his sister was ill, too. When he noted that the child’s mental status appeared altered, he called for a rescue company.

He requested that my company monitor the apartment for CO. I told him that the readings might be skewed since the apartment had been vented with a gasoline-powered fan without an exhaust extension (the fitting for the extension was broken). From my experience, these fans can put 100 to 150 ppm of CO into the structure being vented.

The readings in the apartment indicated a CO level of 100 ppm, which was consistent with what I expected. The apartment located above had CO readings of 15 ppm. None of these apartments had any smoke in them during the fire. After all the apartments were vented by natural, horizontal ventilation, they were released to the occupants. The CO level in each was less than three ppm.

Follow-up investigation with the physician who treated the two children revealed that one had a carboxyhemoglobin level of nine percent and the other had a level of 27 percent. These readings were almost as high as the levels of the firefighters at the warehouse fire (1,000 to 2,000 ppm)! These children were occupying an apartment that was free of any visible products of combustion. We will never know the exact CO levels to which they were exposed, but I am sure they were significant.


Although the incidents appear to be totally different in nature, the lessons learned from this fire are similar to those of the sprinkled warehouse fire. The fire apparently burned for hours, generating large volumes of CO. Even though no visible smoke entered the exposed apartments, high levels of CO evidently did. These high CO levels were present in areas that were clear of smoke. When there is reason to believe that a fire has burned for a long time, as indicated by several inches of water on the floor, or unusually dense smoke with no appreciable heat, certain rules must be followed:

  • SCBA use is mandatory until the area has been monitored for CO, even though there is no visible smoke.
  • Any space directly adjacent to (next to, below, or above) a fire area must be monitored for CO, even though the area may be totally free of visible products of combustion.
  • Firefighters must anticipate that energy-efficient windows may conceal a fire for hours, allowing CO to accumulate to lethal levels.
  • HVAC (heating, ventilation, and air-conditioning) systems may allow CO to spread throughout a structure, even when visible smoke is absent or minimal.
  • Any fire that smolders for a long period of time is generating high levels of CO.

The apartment fire described in this article was a typical room-and-contents fire. [Photo by Omar Torres.]

Firefighters must be diligent to observe potential situations at routine fires that seem minor yet may be lethal if they take a nonchalant attitude. It can’t be stressed enough: Any signs that indicate that a fire has been smoldering for a prolonged period of time warrant the mandatory use of SCBA and the monitoring of CO levels-even if no smoke is visible.


1The immediately dangerous to life and health (IDLH) level of carbon monoxide is 1,200 ppm, meaning that a person exposed to this concentration would be able to self-rescue up to a half hour from the onset of exposure.

DAVID WOOD, a 25-year veteran of the fire service, is a captain with Miami-Dade (FL) Fire Rescue, where he has served for 19 years and is an instructor in confined space rescue, trench rescue, and marine firefighting. He was also a member of the Fire Department of New York. He is a rescue squad officer and instructor for Miami-Dade’s USAR team and a FEMA-certified collapse rescue technician instructor.

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