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Question:

According to ASHRAE’s 1992 refrigerant safety classification, which of the following designations would be the safest?

A A-1
Explaination

Each ASHRAE safety classification contains two parts: The letter designation (A or B) indicates toxicity, and the number designation (1 thru 3) indicates flammability. So an A-1 classification indicates the lowest toxicity and no flame propagation.
A-1 = Lower toxicity / No flame propagation
A-2 = Lower toxicity / Lower flammability
A-3 = Lower toxicity / Higher flammability
B-1 = Higher toxicity / No flame propagation
B-2 = Higher toxicity / Lower flammability
B-3 = Higher toxicity / Higher flammability