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Fire Engineering: Smoke Properties and Intoxication

Smoke Development


As the old saying goes: Where there’s smoke, there’s fire!


In our case, as our focus is the study of fire, we could say, “where there is fire, there is smoke”. Understanding the effects of smoke is vital in fire safety, as smoke causes more fire-related deaths than fires themselves.

Smoke is a collection of gases and particles containing hundreds of chemicals. Visible smoke is mostly carbon (soot).


Figure 1 - Sequence of smoke development (Source: Principles of Fire Behaviour, J.G. Quintiere, 2017)


a) plume rise – When a fire is in a confined space, its turbulent plume will begin to rise toward the ceiling. It will continue growing until it hits the ceiling as long as its temperature remains higher than room temperature.

b) Ceiling jet forms – When the smoke reaches the ceiling, it spreads horizontally along with the ceiling at a relatively fast velocity. This ceiling motion is referred to as a ceiling jet.

c) Start of layer – Once the ceiling jet reaches the confining walls, the flow’s momentum wants to drive it back down, but buoyancy keeps it up. This is the reason that smoke detectors are commonly installed on ceilings.

d) Layer descends – As this cycle repeats, the smoke layer moves down, getting thicker.

e) Layer reaches vent – At the moment the smoke comes to the vent (i.e. doors, windows) it will fall below it, and the pressure of the fire pushes the smoke out of the room.

f) Bidirectional flow – As the smoke flows out, the space it occupied is filled with airflow from the lower portion of the vent.


 

How dangerous is the smoke?


The smoke generated in a fire can affect the visibility for escaping occupants and their ability to see signs and lights and navigate around obstacles. The smoke can also reduce their ability to see because of the effect of irritants to their eyes. The main villain during a fire event is smoke which actually kills more than the fire itself.


Figure 2 – Smoke development


Toxic Products: The danger posed by the smoke starts with its composition. The toxic products within the smoke are acid gases, organic irritants and/or asphyxiant gases that, when inhaled, will have pathological and physiological effects. It may also affect the nose and eyes. The effects vary depending on the exposure time to the smoke, the amount inhaled and how far it penetrates into the lungs.


Temperature: Despite the fire itself, the smoke also has high temperatures, and the temperature can cause burns to the body, and if inhaled, can cause respiratory tract burns.


Radiation: As with any hot surface, both the smoke and the fire radiates heat, and this radiation can also cause burns or heat exhaustion.


There are many effects on the human body caused by smoke inhalation. The reason is that smoke contains hundreds of toxic elements (gases, particles and chemicals) that can affect human health. Some effects of exposure or inhalation of smoke are short-term while others may have a long-term result. See below some examples:


Short-term effects are the ones that happen within seconds and minutes, which means the person would feel these effects during the evacuation from the fire. Some examples are:

  • Reduced speed of movement

  • Mental Confusion

  • Poor choices made about the escape route

  • Poor vision

  • Heat or Radiation Injury (other than from direct flames)

  • Sensory irritation (lungs, eyes, nose, etc.)

Long-term effects are the ones that would affect a person after the evacuation, which can take hours, days, or even years. See below some examples:

  • Serious long-term illness or death

  • Heart Attack

  • Cancer

  • Serious illnesses such as emphysema, asthma, or other chronic lung diseases

  • Long-lasting health problems or physiological responses in the human body

 

The composition of the smoke is also an important fact for the consequences it can cause.


The most dangerous substances in smoke are:

  • Soot particles,

  • Halogen Acids (e.g. hydrogen chloride) – the irritant effect is potent but can also cause lethal effects

  • Organic Irritants (e.g. formaldehyde) – irritate eyes, nose, and throat.

  • Carbon Dioxide (CO2) – Breathing influence. Inhaling CO2 increases the rate of breathing, which increases the uptake of asphyxiant gases.

  • Carbon Monoxide (CO) – prevents oxygen from being carried with the blood. It can develop from gas, oil, coal and wood. It is colourless, tasteless, odourless, non-corrosive and highly poisonous.

  • Hydrogen cyanide (HCN) – influences breathing, causes tissue hypoxia and prevents oxygen metabolism in the mitochondrion.

  • Low Oxygen (O2) – It can be challenging to work depending on how low the oxygen level is. On an even lower dose, it can increase heart rate and respiration, cause dulled senses, slow mental process down and even causes unconsciousness or death.


 

Remember:


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The best way to prevent smoke inhalation is by creating smoke separations in your building, which is often a requirement.


Nelligan Consulting Engineers has the expertise to help with the specification and detailing of Smoke separations on your Fire Safety Design, including Fire Engineering solutions and Passive Fire Protection systems.

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