The LWF Blog
Fire Safety Engineering for Design – Smoke Toxicity – Part 194
July 29, 2024 10:36 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 193, LWF discussed visibility in smoke. In part 194, we talk about the toxicity of smoke and tenability limits.
The majority of deaths attributed as a fire fatality are caused by carbon monoxide exposure. Smoke also contains other toxic gases and therefore conditions may cause hyperventilation due to high levels of carbon dioxide, or hypoxia caused by oxygen deprivation. Toxicity is sometimes expressed as fractional effective dose (FED).
One approach is to provide a simple tenability limit for 5 minute and 30 minute exposure durations, based on the concentrations of carbon monoxide, carbon dioxide, oxygen and hydrogen cyanide in the fire effluent.
Based on a 0.3 FED tenability limit for conditions ascribed to typical fires in buildings, these are proposed limiting exposure times for asphyxiants:
Design tenability limit exposure concentrations for asphyxiant gases expressed as carbon monoxide for
5-minute and 30-minute exposures (BSI, 2004)
Fuel type Maximum asphyxiate concentration as CO / ppm
5-minute exposure 30-minute exposure
Retail/residential
(>2% nitrogen by mass of fuel) 800 125
Offices
(<2% nitrogen by mass of fuel) 1200 275
A fire containing greater than 2% nitrogen by mass of fuel is likely to be one involving furniture or clothing, most commonly found in residential or retail premises. Fires low in nitrogen (less than 2% by mass of fuel) are mainly cellulosic or other materials low in nitrogen and are mainly in offices.
The Handbook of Smoke Control Engineering (PDF) (Klote et.al.) used an alternative method, by comparing the concentrations against the concentration that is lethal to 50% of animal subjects for a given period of time (usually 30 minutes)
Approximate lethal exposure dose LC50 for common materials
Material Approximate LC50 dose / g · m–3 · min–1
Non-flaming fire Fuel-controlled fire Fully developed fire
Cellulosic (e.g. wood) 730 3120 750
C, H, O plastics 500 1200 530
PVC 500 300 200
Wool/nylon 500 920 70
Flexible polyurethane 680 1390 200
Rigid polyurethane 63 100 54
For an exposure at a constant concentration, the FED (Klote et al., 2012) is:
Where FED is the fractional effective dose, mf is the mass concentration of fuel burned (g · m–3), t is the exposure time (min) and LC50 is the lethal exposure dose from the test subject (g · m–3 · min–1).
A FED of 1 (unity) is considered to be fatal and various values from 0.5 (Klote et al., 2012) to 0.3 (NFPA, 2009) may represent levels which can incapacitate.
In part 195 of LWF’s series on fire engineering we will talk about the noise levels for design values when considering smoke extraction. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.