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The available safe escape time (ASET) represents the onset of untenable conditions within a space, and includes calculations of air temperatures, visibility through smoke and the toxicity of the products of combustion.

Limits of tenability have to be set with reference to results from physiological research, such that the maximum upper limit for air temperature is 110degreesC, or less in a humid atmosphere, with a limit for radiative heat exposure of 2.5kW/m 2.When the visibility through smoke is less than10m, it can be assumed that occupants will be unable or unwilling to move to an exit and the cumulative effects of smoke inhalation can be assessed by determining the cumulative 'dose' of hot smoke and toxic gases that will be inhaled during the course of events.

By summing the fractions of the dose inhaled each minute, the time to incapacitation - through asphyxiation or poisoning - can be calculated. These factors can be affected by the size and geometry of the compartment or building, the fire load and load density, the mass and constituents of the smoke produced and the size and heat output of the fire.

In a similar way, the required safe escape time (RSET) can be estimated by taking into account the physical and behavioural characteristics of the occupants.

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