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In the aftermath of a fire, the visible danger may be gone, but the threat isn’t. Residual smoke, often referred to as "cold smoke," can linger in a building long after the flames have been extinguished. In hospitals, where safety and hygiene are paramount, effectively clearing this smoke is not just a matter of comfort - it’s a matter of critical operational and patient safety.
Smoke inhalation after a fire event may not cause loss of life, but even small amounts of smoke inhaled can lead to long-term health effects, including respiratory and heart disease. Therefore, the control of smoke is essential to any life safety system controlling the spread of fire and smoke.
What is Cold Smoke?
To clarify the difference between ‘hot’ and ‘cold’ smoke, it is necessary to define what we mean by the terms. Hot smoke will typically be created during the fire event, and in certain scenarios will need to be extracted as part of the fire strategy – for example, in a London basement. Hot smoke has energy and is fast-moving. Cold smoke is the residual smoke created from a fire event; it is slow-moving, can be poisonous and as dangerous as hot smoke. It will be thick and may obscure vision if not cleared.
Cold smoke will be present in fire events where the designer has created compartmentation to contain the fire and smoke from spreading to other areas of the building. Designers will often request whether this smoke can be extracted post-fire. While cold smoke extraction isn’t always mandated in building regulations, it offers substantial operational and health benefits -especially in hospitals where downtime and air quality can directly impact patient care, and expensive machinery should be shielded from the effects of smoke damage wherever possible.
In a ‘hot’ smoke extraction system, the choices are simpler. The smoke in these examples will be extracted through smoke control ductwork, utilising smoke control dampers conforming to EN12101-8 and smoke fans to EN12101-3. The standards are clear, and designers can make informed decisions on strategy and product selection.
Cold smoke clearance is not so simple. In hot smoke control systems, the duct interior forms part of the compartment exposed to fire. Smoke control dampers are employed to isolate branches serving other compartments, thereby preserving fire and smoke separation. A smoke control damper is not a fail-safe device; there is no electro-thermal release mechanism, so after the fire event, the damper, if still functional, may be opened to provide a conduit for cold smoke. A system designed for compartmentation with fail-safe motorised fire dampers provides more challenges.
Why would we need cold smoke extraction?
Cold smoke can be as poisonous and dangerous as hot smoke. Often, the result of partially extinguished fires is the residue of cold smoke, which will inhibit the activities of the firefighters in making the building safe. Moreover, escape routes must be kept clear of cold smoke – and applying strategies in line with Approved Document B needs to be adhered to.
The Hidden Challenge of Hospital Design
In buildings like hospitals, where patient care continues immediately post-fire, managing this residual smoke becomes essential.
The challenge of cold smoke extraction in hospitals where the extensive use of ES/EIS Classified fail-safe fire dampers are used for compartmentation is the electro–thermal release on the motorised fire damper. In a fire event, if the ambient temperature next to the damper or within the duct exceeds 72°C, the damper will close and remain closed. In any system, this can mean that the post-fire extraction of smoke will be impossible. The problem in this example is how to enable the fire safety system to know that it cannot extract.
If operating theatres or MRI suites are in use after the fire event, they will need to be protected from fire and smoke so the medical procedure can be concluded. Pressurising the occupied space will ensure there is no ingress of smoke. Monitoring of the HVAC systems and the ductwork will be necessary to prevent any breaks in compartmentation.
If using an addressable Damper Control Panel (DCP), the damper statuses can be fed directly to the BMS or Fire Alarm system to indicate a fault. It is important to note that if the dampers are in their designated cause-and-effect positions, no fault will be reported.
In summation, a DCP and a well-designed Override panel offer
- The ability to observe damper status in real time
- Detect faults
- Control extraction fans dynamically based on system status
- Actively managing dampers and fans to avoid drawing smoke into clean zones
Using a system in this manner provides a robust design that anticipates the need for cold smoke removal and can significantly ease post-incident revival of the space, protect staff and patients, and ensure faster return to full operational capacity.
Final Thought
Cold smoke may be slow, but its impact is lasting. By designing with extraction and recovery in mind, hospitals can be better prepared - not just for the fire, but for what comes after, helping to ensure continuity of care and minimise disruption to critical services.
