Refrigerant safety comparison: Which of the following refrigerants is both highly toxic and flammable under typical service conditions?

Introduction / Context:
Refrigerant selection weighs thermodynamic performance against safety and environmental criteria. Toxicity and flammability classifications (e.g., ISO/ASHRAE) guide application choices.

Given Data / Assumptions:

• Standard industrial interpretations of toxicity and flammability.
• Normal HVAC&R service pressures and temperatures.
• Generalized safety classes (e.g., ammonia A3/B2L type considerations).

Concept / Approach:
Ammonia (R-717) is efficient but toxic and mildly flammable; it requires stringent safety measures (ventilation, detectors, restricted occupancy). CO2 (R-744) is non-toxic and non-flammable but operates at high pressure. SO2 is toxic but not commonly classified as flammable in HVAC usage. R-12 and R-134a are non-flammable and of low acute toxicity (environmental concerns aside).

Step-by-Step Solution:

Verification / Alternative check:
Safety standards place ammonia systems in machinery rooms with ventilation and leak detection; CO2 systems focus more on pressure management and asphyxiation risks, not flammability.

Why Other Options Are Wrong:
CO2 is non-flammable; SO2 is toxic but not flammable; R-12 and R-134a are non-flammable and lower acute toxicity (though they have environmental impacts).

Common Pitfalls:
Confusing toxicity with asphyxiation risk; CO2 can displace oxygen but is not flammable.

Final Answer:
Ammonia (R-717)