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LMU study finds 20% of gases from combustion of R1234yf MAC refrigerant consist of highly toxic carbonyl fluoride (correction and update)

Chemists at Ludwig Maximilians Universität München report that 20% of the gases produced by the combustion of R1234yf—the approved low global warming potential refrigerant for mobile air conditioning (MAC) systems, the adoption of which has met with resistance from German automakers (earlier post)—consist of the highly toxic chemical carbonyl fluoride.

Carbonyl fluoride is structurally related to phosgene (which contains chlorine in place of fluorine), which was used as a chemical weapon during the First World War. Kornath and his co-workers have just published the results of their investigation in the journal Zeitschrift für Naturforschung B.

It has been known for some time now that combustion of R1234yf results in production of the toxic hydrogen fluoride. Our analysis has now shown that 20% of the gases produced by combustion of the compound consist of the even more poisonous chemical carbonyl fluoride.

—Andreas Kornath, Professor of Inorganic Chemistry at LMU

The simplest fluoride, hydrogen fluoride (or hydrofluoric acid, HF) is also highly corrosive and so toxic that burns about as big as the palm of one’s hand can be lethal. The agent binds avidly to calcium in body fluids, and this can result in heart failure unless an antidote is rapidly administered.

Carbonyl fluoride is even more dangerous, because it penetrates the skin more easily, and causes severe irritation of the eyes, the skin and the airways. If inhaled, it can damage the alveoli in the lungs, allowing it to reach the circulation and shut down vital functions.

According to guidelines issued by the European Union, automobile manufacturers are legally obligated to use a low global warming potential refrigerant in the air-conditioning systems installed in their cars. Use of the previously approved refrigerant R134a in new models has been forbidden in the EU since 2011, as the agent had been shown to contribute to the global warming in the atmosphere.

However, R1234yf, the recommended replacement (earlier post), has already been the subject of much heated debate in Germany. [Correction: Honeywell, which co-developed and manufactures R1234yf, points out that the EU’s MAC directive is technology-neutral. Thus, it is not correct to state that R1234yf is the recommended replacement.]

Studies carried out by various institutions and by Daimler had pointed to the compound’s flammability, and shown that, in the event of accidents in which vehicles catch fire, combustion of R1234yf leads to the release of hydrogen fluoride.

However, an interim report in 2013 based on independent testing by Germany’s Kraftfahrt-Bundesamt (Federal Motor Transport Authority) found that there is “no sufficient evidence of a serious risk” as defined by the Product Safety Act (ProdSG) related to the use of the low global warming potential (GWP) refrigerant R-1234yf. (Earlier post.)

KBA found that in the most severe crashes (level 3), one of the four models ignited and emitted toxic hydrogen fluoride (HF) gas; “non-negligible amounts” of HF were also found in two other test crashes. However, the level 3 crash testing was outside of the bounds of the statutory scope of product safety regulations—i.e., the level 3 tests could not be associated with the necessary concrete probability of occurrence, but served as a general appraisal of the risk.

More recently, a scientific review of research regarding the safety aspects of the use of refrigerant R1234yf in Mobile Air Conditioning (MAC) systems, published by the European Commission, also shared the conclusion that there is no evidence of a serious risk in the use of this refrigerant in MAC systems under normal and foreseeable conditions of use under product safety guidelines. (Earlier post.)

That review, carried out by Europe’s Joint Research Centre, provided an in-depth analysis of testing and a subsequent report on the refrigerant’s safety by KBA in order to ascertain whether the results stemming from the tests were well founded and supported by a rigorous and scientific methodology.

Based on the LMU results, however, Professor Kornath is urging a re-assessment.

The risk analyses carried out by the manufacturers of the refrigerant so far have not taken carbonyl fluoride into account. In light of our results, we advise that the risks associated with R1234yf should be urgently reassessed.

—Andreas Kornath

Update: Honeywell, however, disagrees with the assessment by Prof. Kornath and colleagues.

Carbonyl fluoride (COF2) is in fact a well-known breakdown product of HFO-1234yf that has been publicly studied by leading experts in the automotive industry. It was studied in the 2007-2009 Cooperative Research Program (CRP) conducted by SAE International, the world’s leading automotive engineering organization. The SAE CRP reviewed the COF2 data, included it in its risk assessment, and concluded that HFO-1234yf is safe for use in automotive air conditioning. In March, the EU’s Joint Research Centre also reviewed this data and again concluded that HFO-1234yf is safe for use in automobile air conditioning.

COF2 is also formed during the burning of the current automotive refrigerant HFC-134a, used in hundreds of millions of vehicles worldwide today. When COF2 does form in such conditions, it only lasts for a fraction of a second, which is not long enough to put bystanders, passengers, or first responders in any danger.

—Rajiv Banavali, Chief Technology Officer for Fluorine Products at Honeywell

The EPA has defined interim Acute Exposure Guidelines (AEGL) for carbonyl fluoride across its three categories of:

  • AEGL-1: airborne concentration of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic, non-sensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure.

  • AEGL-2: airborne concentration of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.

  • AEGL-3: the airborne concentration of a substance above which it is predicted that the general population, including susceptible individuals, could experience life-threatening health effects or death.

Carbonyl fluoride 353-50-4 (Interim) EPA AEGL ppm
  10 min 30 min 60 min 4 hr 8 hr
AEGL-1 Not recommended (NR) due to insufficient data.
AEGL-2 0.35 0.35 0.28 0.17 0.087
AEGL-3 1.0 1.0 0.83 0.52 0.26


  • Michael Feller, Karin Lux, Christian Hohenstein, and Andreas Kornath (2014) “Structure and Properties of 2,3,3,3-Tetrafluoropropene (HFO-1234yf),” Z. Naturforsch. 69b, 379–387 doi: 10.5560/ZNB.2014-4017


Dave R

What are the arguments for using R1234yf over CO2 (Germany's preferred refrigerant)?


Dave R, as I understand it, the CO2 cycle needs significantly higher operating pressures. Additionally, gas characteristics favor its use for much lower temperatures (e.g., freezers). My laissez-faire view is what's the big deal, but it seems that EU bureaucrats have their undies all in a bunch to the point they're threatening Germany and Daimler with various legal actions (see the link in the Millikin Tweet column on "cool wars"). Not exactly an Archduke Ferdinand sort of thing, but damn...


All this to avoid using isobutane.


The true reason behind behind the recommendation of R1234yf as a refrigerant is that past designs of ACs can be used without much ado. Usage of CO2 implicates new AC designs and subsequent investments. CO2 is virtually inherently safe but greed has the advantage as usual.

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