Refrigerants for the 21st Century
16. The Fate of HFC-134a

Certainly, none of the CFC alternatives that have become commercial would have found a market if it had an adverse ecological impact.

All of them are chemically quite stable. Yet they are designed to be labile or prone to attack by OH radicals in the atmosphere. Just as fluorocarbon manufacturers had come together to address the toxicity of these new compounds, so they banded together to try to establish the compound's ultimate fate in the environment. The effort is called, Alternative Fluorocarbon Environmental Acceptability Study (AFEAS). In contrast to safety studies which employed toxicology, a well-established science, AFEAS required new research techniques.

All of the CFC alternatives eventually introduce highly toxic and corrosive HF into the environment. The HCFC's also generate HCl on breakdown. In the atmosphere, these acids quickly pick up water to become hydrofluoric and hydrochloric acids, respectively. These become part of acid rain. However, there is a variety of basic materials on the earth's surface - particularly limestone - which quickly neutralize these acids. This is not to say that these acids have no impact on the ecology. However, their impact would seem significantly less than the sulfuric acid (H2SO4) produced by combustion of high sulfur fuels.

We know that HFC-134a initially degrades, after OH radical attack, to trifluoroacetic acid (= TFA; CF3CO2H). Here again, this organic acid picks up water. TFA is quite a persistent molecule. Fortunately, it has been shown to be essentially innocuous to plant and animal life.

Recall that there is one CFC alternative, CF3CHCl2 (HCFC-123), which has proven more toxic to animals than all the other alternatives. This compound is converted to trifluoroacetyl chloride (CF3COCl) by the action of a common animal enzyme. The chloride is capable of trifluoroacetylating liver proteins, causing a toxic effect. A similar pathway to protein damage from degradation of HFC-134a and HFC-125 apparently doesn't occur.

One might conclude that fluorocarbon alternatives are not "ideal" replacements for CFC's. However, they certainly are not as bad as CFC's in their environmental impact.