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Respiratory Hazards Back to SCBA


Respiratory Hazards

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The combustion process consumes oxygen while producing toxic gases that either physically displace oxygen or dilute its concentration. When oxygen concentrations are below 18 percent, the human body responds by increasing its respiratory rate. Symptoms of oxygen deficiency by percentage of available oxygen are shown in Table 1. Oxygen deficiency can also occur in below-grade locations, chemical storage tanks, grain bins, silos, and other confined spaces. Another area of potential hazard would be a room protected by a total-flooding carbon dioxide extinguishing system after discharge.

Table 1

Physiological Effects of
Reduced Oxygen (Hypoxia)
Oxygen in Air
21 None - normal conditions
17 Some impairment of muscular
coordination; increased in
respiratory rate to compensate
for lower oxygen content
12 Dizziness, headache, rapid
9 Unconsciousness
6 Death within a few minutes
from respiratory failure and
concurrent heart failure
      NOTE: These data cannot be considered
absolute because they do not account for diffe-
rence in breathing rate or length of time exposed.

      These symptoms occur only from reduced oxy-
gen. If the atmosphere is contaminated with toxic
gases, other symptoms may develop.

Some departments have the ability to monitor atmospheres and measure these hazards directly. When this capability exists, it should be used. Where monitoring is not possible or monitor readings are in doubt, self-contained breathing apparatus should be worn.

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Exposure to heated air can damage the respiratory tract, and if the air is moist, the damage can be much worse. Excessive heat taken quickly into the lungs can cause a serious decrease in blood pressure and failure of the circulatory system. Inhaling heated gases can cause pulmonary edema (accumulation of fluids in the lungs and associated swelling), which can cause death from asphyxiation. The tissue damage from inhaling hot air is not immediately reversible by introducing fresh, cool air.

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The smoke at a fire is a suspension of small particles of carbon, tar, and dust floating in a combination of heated gases. The particles provide a means for the condensation of some of the gaseous products of combustion, especially aldehydes and organic acids formed from carbon. Some of the suspended particles in smoke are merely irritating, but others may be lethal. The size of the particle determines how deeply into the unprotected lungs it will be inhaled.

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The firefighter should remember that a fire means exposure to combinations of irritants and toxicants whose toxicity cannot be predicted accurately. In fact, the combination can have a synergistic effect in which the combined effect of two or more substances is more toxic or more irritating than the total effect would be if each were inhaled separately.

Inhaled toxic gases may have several harmful effects on the human body. Some of the gases directly cause disease of the lung tissue and impair its function. Other gases have no directly harmful effect on the lungs but pass into the bloodstream and to other parts of the body and impair the oxygen-carrying capacity of the red blood cells.

The particular toxic gases given off at a fire vary according to four (4) factors:

  1. Nature of the combustible
  2. Rate of heating
  3. Temperature of the evolved gases
  4. Oxygen concentration

Table 2 addresses some of the most commonly found gases on the fire scene. The immediately dangerous to life and health (IDLH) concentrations are from the National Institute for Occupational Safety and Health (NIOSH) Pocket Guide to Chemical Hazards. The current NIOSH definition for an IDLH exposure condition is one "that poses a threat of exposure to airborne contaminants when that exposure is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment. " These values were established to ensure that a worker could escape without injury or irreversible health effects from an IDLH exposure in the event of the failure of respiratory protection equipment.

Table 2

Toxic Atmospheres Associated With Fire
Sensibility IDLH * Caused By Miscellaneous
Carbon Dioxide
ppm **
Free-burning Ends product of
complete combustion
of carboniferous
Carbon Monoxide
1,200 ppm Incomplete combustion Cause of most
fire-related deaths
Hydrogen Chloride
Colorless to
slightly yellow;
pungent odor
50 ppm Burning plastics
(e.g., polyvinyl
chloride [PVC] )
Irritates eyes and
respiratory tract
Hydrogen Cyanide
Colorless; bitter
almond odor
50 ppm Burning of wool,
nylon, polyurethane
foam, rubber & paper
Chemical asphyxiate;
hampers respiration at
the cellular & tissue
Nitrogen Dioxide
pungent, acid
20 ppm Given off around
silos or grain bins;
also liberated when
pyroxylin plastics
Irritates nose and
Colorless; odor
of musty hay;
2 ppm Produced when
refrigerants such as
Freon contact flame
Forms hydrochloric
acid in lungs due
to moisture
*   Immediately Dangerous to Life and Health - any atmosphere that poses an immediate hazard to life or produces
    immediate irreversible, debilitating effects on health

** Parts Per Million - ratio of the volume of contaminants ( parts) compared to the volume of air (million parts )

Because more fire deaths occur from carbon monoxide (C 0) poisoning than from any other toxic product of combustion, a greater explanation of this toxic gas is necessary. This colorless, odorless gas is present with every fire. The poorer the ventilation and the more inefficient the burning, the greater the quantity of carbon monoxide formed. A rule of thumb, although subject to much variation, is that the darker the smoke, the higher the carbon monoxide levels. Black smoke is high in particulate carbon and carbon monoxide because of incomplete combustion.

The blood's hemoglobin combines with and carries oxygen in a loose chemical combination called oxyhemoglobin. The most significant characteristic of carbon monoxide is that it combines with the blood's hemoglobin so readily that the available oxygen is excluded. The loose combination of oxyhemoglobin becomes a stronger combination called carboxyhemoglobin (COHb). In fact, carbon monoxide combines with hemoglobin, creating carboxyhemoglobin, about 200 times more readily than does oxygen. The carbon monoxide does not act on the body, but crowds oxygen from the blood and leads to eventual hypoxia of the brain and tissues, followed by death if the process is not reversed.

Concentrations of carbon monoxide in air above five hundredths of one percent (0.05 percent) (500 parts per million [ppm]) can be dangerous. When the level is more than 1 percent, unconsciousness and death can occur without physiological signs. Even at low levels, the firefighter should not use signs and symptoms for safety factors. Headaches, dizziness, nausea, vomiting, and cherry-red skin can occur at many concentrations, based on an individual's dose and exposure. Therefore, these signs and symptoms are not good indicators of safety. Table 3 shows the toxic effects of different levels of carbon monoxide in air. These effects are not absolute because they do not take into account variations in breathing rate or length of exposure. Such factors could cause toxic effects to occur more quickly.

Table 3

Toxic Effects Of Carbon Monoxide
( CO )
( ppm *)
( CO ) in air
( percent )
100 0.01 No symptoms - no damage
200 0.02 Mild headache; few other symptoms
400 0.04 Headache after 1 to 2 hrs
800 0.08 Headache after 45 min.;
nausea, collapse and
unconsciousness after 2 hrs
1,000 0.10 Dangerous - unconsciousness after 1 hr
1,600 0.16 Headache, dizziness, nausea after 20 min.
3,200 0.32 Headache, dizziness, nausea after 5
to 10 min.; unconsciousness
after 30 min.
6,400 0.64 Headache, dizziness, nausea after 1
to 2 min.; unconsciousness
after 10 to 15 min.
12,800 1.28 Immediate unconsciousness;
danger of death in 1 to 3 minutes
* Parts Per Million - ratio of the volume of contaminants ( parts)
compared to the volume of air (million parts )

Measurements of carbon monoxide concentrations in air are not the best way to predict rapid physiological effects because the actual reaction is from the concentration of carboxyhemoglobin in the blood, causing oxygen starvation. High oxygen user organs, such as the heart and brain, are damaged early. The combination of carbon monoxide with the blood is greater when the concentration in air is greater. An individual's general physical condition, age, degree of physical activity, and length of exposure all affect the actual carboxyhemoglobin level in the blood. Studies have shown that it takes years for carboxyhemoglobin to dissipate from the bloodstream. People frequently exposed to carbon monoxide develop a tolerance to it, and they can function asymptomatically (without symptoms) with residual levels of serum carboxyhemoglobin that would produce significant discomfort in the average adult. The bottom line is that firefighters may be suffering the effects of CO exposure even though they are asymptomatic.

Experiments have provided some comparisons relating air and blood concentrations to carbon monoxide. A 1-percent concentration of carbon monoxide in a room will cause a 50-percent level of carboxyhemoglobin in the bloodstream in 21/2 to 7 minutes. A 5-percent concentration can elevate the carboxyhemoglobin level to 50 percent in only 30 to 90 seconds. A person previously exposed to a high level of carbon monoxide may react later in a safer atmosphere because the newly formed carboxyhemoglobin may be traveling through the body. A person so exposed should not be allowed to use breathing apparatus or resume fire control activities until the danger of toxic reaction has passed. Even with protection, a toxic condition could be endangering consciousness.

A hardworking firefighter may be incapacitated by a 1percent concentration of carbon monoxide. The stable combination of carbon monoxide with the blood is only slowly eliminated by normal breathing. Administering pure oxygen is the most important element in first aid care. After an uneventful convalescence from a severe exposure, signs of nerve or brain injury may appear any time within three weeks. This is why an overcome firefighter who quickly revives should not be allowed to reenter a smoky atmosphere.

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