The LWF Blog

Fire Engineering for Healthcare Premises – Evacuation Time – Part 27

March 22, 2021 11:31 am

In LWF’s blog series for healthcare professionals, our aim is to give information on best practice of fire safety in hospitals and other healthcare premises. In part 26 of Fire Engineering for Healthcare Premises, LWF considered the human factors relevant when considering evacuation from a building. In part 27, we will look at the effects of smoke on movement, tenability limits and exit choice.

In some healthcare buildings, it can be considered overly-cautious to have a design approach which treats any person’s encounter with smoke as a failure of the system. However, the possibility of having a system which might permit building occupants to come into contact with smoke means that building occupants might have to be able to predict and avoid the movement of smoke, which is unrealistic.

For example, a person moving through a healthcare building in which there is a fire may be exposed to smoke and the toxic products of combustion. Exposure to a hazardous substance or toxin may be viewed as a fractional effective dose (abbreviated to FED in HTM 05-03). The dose depends on the toxic make-up of the fire gases and how long a person may be exposed. When a person reaches their personal FED, they will be overcome by the smoke and collapse.

In the case of some combustion products (for example irritants), the effects can be instantaneous rather than cumulative; therefore, concentration is the key parameter.

Of course, individuals do not have the same fractional effective dose and will be able to withstand different amounts of the smoke and associated gases. Young children and older people are generally more susceptible than fit adults. However, the condition of patients in a healthcare building means that generalisation is not possible based on age, for instance. A previously fit person with a serious health condition could have no more resilience to smoke than an older person.

A tenability limit in relation to smoke relates to indicators such as breathability, visibility and radiant heat. There are no standard values for tenability limits for particular combustion products.

Stratification of the smoke layer above a clear layer of breathable air should be above a certain height – a safety margin should be built in as the layer cannot be at head height or below. Layer height is not sufficient to cause loss of tenability, one of the other limits of smoke properties must also be exceeded.

In Part 28 of LWF’s blog series, LWF will look at exit choice in an emergency fire situation and egress modelling. 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 for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact LWF on freephone 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.