In which scenario should depressurization of a non-habitable airspace not be implemented as a mitigation system?

Depressurization of a non-habitable airspace is a common mitigation strategy for reducing radon levels. However, it is essential to avoid implementing this approach in circumstances where it could introduce safety hazards or compromise system functionality.

In this particular scenario, when the airspace contains a vented combustion appliance, utilizing depressurization could lead to negative pressure conditions that draw combustion gases back into the living space, creating a significant risk of exposure to harmful pollutants, including carbon monoxide. The appliance relies on proper venting and airflow to function safely, and any disruption to this balance due to depressurization can be detrimental, making this choice particularly critical in maintaining safety.

In contrast, the other situations might not carry the same level of risk. Unoccupied airspaces may not pose an immediate threat as there are no occupants to be affected, while scenarios with poor outdoor air quality do require careful consideration but do not inherently prevent depressurization. Similarly, if adequate isolation from adjacent spaces cannot be achieved, it indicates a risk for the movement of contaminants and might not completely mitigate radon levels, but it does not introduce the same imminent safety risks as a vented combustion appliance.