Overall, when selecting a refrigerant, safety issues cannot be overlooked. The major drawbacks of HFC alternatives are the safety characteristics. Ammonia and some hydrocarbons, whilst they are excellent refrigerants, are toxic, and in case of hydrocarbons extremely flammable. They therefore should be used with care, in conditions of safety demanding maximum attention and strict precautions. The Table hereunder illustrates some of the characteristics of different refrigerants from a safety perspective.
The toxicity rating is given in terms of the Occupational Exposure Limit (OEL). The 1000 ppm limit on non-toxic substances is to prevent any danger of suffocation due to oxygen displacement. Flammability properties are characterised by the upper and lower flammability limits (the concentration limits in which the substance is flammable), the energy released during combustion, and by the auto-ignition temperature. More recently to categorise moderately and marginally substances such as some HFCs and the HFOs, flame speed and ignition energy are being utilised.