Modern HVAC systems are designed with low-toxicity refrigerants such as R-410a, but even these substances can be dangerous if their concentration becomes too high. HVAC engineers must consider the potential amount of refrigerant if there is a leak, ensuring it stays below the refrigerant concentration limit (RCL).
The RCL value indicates an allowable refrigerant weight per cubic foot of room volume (lb/ft3). As long as the concentration stays below the RCL, dangerous effects are avoided: oxygen deprivation, flammability, cardiac sensitization, etc. Since these effects are prevented, occupants can easily escape from the affected area.
To get an idea of how a nontoxic or low-toxicity substance can be dangerous, consider water. Although water is vital, it can cause drowning because it displaces air and is unbreathable – the same principle applies to any other liquid or gas that displaces air.