Introduction
Fire protection engineering is largely based on the definition of tenability criteria for people in a fire-affected enclosure. These criteria establish, among other parameters, the minimum visibility and maximum thermal radiation admissible for occupants to evacuate or remain safely for a determined period.
Various performance-based design guides and standards, both national and international, have historically used values such as 10 m visibility and 2.5 kW/m² maximum radiation to define the acceptability of fire conditions for the general public and 5 kW/m² for firefighters. However, Catalonia’s performance-based guides have introduced more conservative criteria:
- 20 m as minimum visibility for evacuation.
- 1.7 kW/m² as maximum tolerable thermal radiation for evacuation.
- 3.0 kW/m² as maximum tolerable thermal radiation for firefighters.
This article presents the technical foundations that justify why these stricter values offer greater safety and align with other international studies and recommendations. Modern fire detection systems work in harmony with these criteria to ensure maximum protection.
The impact of radiation on survival
Heat from the smoke layer and flames is transmitted fundamentally by radiation and convection. Some standards or guides, such as UNE 23585, assume up to 200 °C in the smoke layer at 2.5 m above the floor to ensure no contact with occupants, with the idea that the resulting radiation —around 2.4 kW/m²— is still considered tolerable under certain conditions and exposure times.
However, studies exist (for example, those referenced in PD 7974 or in Simms & Hinkley trials) that indicate that 2.5 kW/m² can produce pain (erythema) on skin in less than 60 seconds.
This reveals a significant risk for people with reduced mobility or in scenarios with prolonged evacuations. Therefore, reducing the radiation threshold to 1.7 kW/m² provides more time for evacuation especially if complications occur (high humidity, direct skin contact, etc.).
Scientific evidence on radiation limits
The exposure time from which pain occurs at different radiation levels on exposed skin can be clearly observed in figure 68.6 that appears in the SFPE Handbook 5th edition, showing that with 2.5 kW/m² pain occurs between 30 and 50 seconds and in figures 68.4 and 68.5 first and second degree burns occur at 100 seconds.
So it is not very sensible to admit this situation occurs during evacuation which will surely last longer. The text states that a study considers 1.7 kW/m² the limit below which nobody experiences pain regardless of exposure time and which appears in this graph as “critical heat flux limit”.
Comparison with international standards for firefighters
It is also noteworthy that the NFPA 921 standard on fire investigation when addressing the issue of radiant flux effect on different elements considers 2.5 kW/m² as the maximum value for firefighter exposure, as it explains that exposed skin experiences second-degree burns in 79 seconds.
So one wonders why some use the value considered maximum for firefighters in the US as admissible for the general public. Also the Australian Fire Authorities Council according to the Society of Fire Safety (2014) has a lower value for firefighters as it considers 3 kW/m² as the maximum exposure value for them.
Relevance of 20 meters visibility
Traditionally, many documents have established 10 m visibility as acceptable for evacuation. However, Catalan guides recommend 20 m in situations where occupants depend on reading signs or reflective elements to guide their exit. Why reconsider this value?
Difference in visibility calculation from extinction coefficient
Simulation tools, such as FDS, use the relationship factor between visibility and extinction coefficient of C = 3 for reflective signs, but the real value is between 2 and 4.
Experimentally, an extinction coefficient of 0.15 1/m has been established as a reference value for people evacuation and for this the visibility calculated in FDS by default is 20 m, indicating that “10 m” may fall short for the general public. In fact, applying a factor of C = 2 the necessary calculated visibility would be 30 m.
In literature, optical density is sometimes expressed (in log10 base) or as extinction coefficient (in Napierian base). The following table clarifies the relationship between them and visibility according to constant C:
Occupant profile
If dealing with general public, elderly people or those with reduced mobility, it is preferable to give greater visibility allowance, especially considering eventual evacuation delays.
Greater variability in smoke density
Visibility at 10 m can be quickly reduced if the fire grows minimally or if smoke becomes denser than estimated. By requiring 20 m, a margin is given so that other aspects such as smoke tenacity, possible signage corrosion or darkening by soot do not compromise evacuation.
Alignment with other international references
Although there are still standards that accept 2.5 kW/m² and 10 m visibility, precedents and studies exist that support more prudent values:
PD 7974-6:2019
Differentiates between “zero exposure to smoke and heat” (criterion 10.2) and “minimal exposure” (criterion 10.3) depending on whether there is an effective smoke control system or not. In both cases, the standard emphasizes that, if smoke descends to occupant height, radiation should be <2.0 kW/m² and temperature <115 °C.
It is cited that in situations of high heat and humidity, people’s tolerance decreases even more.
NFPA 130 (Railway transportation systems)
Requires strict visibility and temperature conditions in underground platforms, recognizing the need for conservative thresholds for the safety of a large number of users, expressing that the tenability limit is 1.7 kW/m².
Simms & Hinkley studies
Place around 2.5 kW/m² the point where skin pain occurs in few seconds. Lowering to 1.7 kW/m² offers a wider safe exposure window.
Consequently, the Bombers Guide of the Generalitat of Catalonia does nothing but align with the most conservative positions, by requesting greater visibility and lower radiation.
Conclusions
Greater safety in fire conditions
The adoption of 20 m visibility and 1.7 kW/m² maximum radiation reduces the probability that occupants suffer early irritations, burns or disorientation.
Safety coefficients and human variability
Simulation models are never exact; human physiology varies according to age, health or stress. Using more conservative values minimizes the risk of error or unforeseen events.
Reflection of studies and external guides
Although 2.5 kW/m² and 10 m visibility are consolidated values in some documents, there are increasingly more references —including PD 7974— that recommend lower radiation values and higher visibility, especially in environments with many occupants or with prolonged evacuation times.
The role of humidity and sprinklers
The presence of sprayed water or steam can aggravate the situation from the skin and breathing perspective, although it also helps cool the smoke. This underlines the importance of not limiting oneself only to temperatures in the smoke layer or to simplified visibility and radiation criteria.
In summary, Catalonia’s decision to require more restrictive conditions for visibility and radiation in fire scenarios is consistent with the latest research advances and practical experience accumulated in different sectors. With this measure, it is intended to strengthen occupant protection and strengthen safety margins that, in real cases, can make the difference between a successful evacuation or an accident.
Summarized references
- PD 7974-6:2019, “Application of fire safety engineering principles to the design of buildings — Part 6: Human factors: Life safety strategies — Occupant evacuation, behaviour and condition (Sub-system 6)”.
- SFPE Handbook of Fire Protection Engineering, various chapters on tenability criteria (visibility, radiation, toxicity).
- UNE 23585:2017. “Requirements and calculation and design methods for designing a temperature control and smoke evacuation system (SCTEH)”.
- Bombers Guide of the Generalitat of Catalonia and Fire Safety Cluster.
- Simms & Hinkley (various studies on thermal tolerance and radiation).
