The excerpt below is from "Refrigerant Safety," originally printed in the ASHRAE Journal (July 1994, pp. 17-26). It is presented by permission of the author and the ASHRAE Journal.
"The alternative refrigerants are as safe or safer than those they replace, but more care is needed with all refrigerants."
Dictionaries define safety as being free from harm or the risk of injury or loss. The concerns usually associated with refrigerants are toxicity, flammability, and physical hazards. Are refrigerants completely safe? No, all pose one or more of these concerns. But can refrigerants, and especially the new refrigerants, be used safely? Yes, and generally more so than in the past.
The first practical refrigerating machine was built by Jacob Perkins in 1834; it used ether in a vapor-compression cycle. The first absorption machine was developed by Edmond Carr, in 1850, using water and sulfuric acid. His brother, Ferdinand Carr, demonstrated an ammonia/water refrigeration machine in 1859. A mixture called chemogene, consisting of petrol ether and naphtha, was patented as a refrigerant for vapor-compression systems in 1866. Carbon dioxide was introduced as a refrigerant in the same year. Ammonia was first used in vapor-compression systems in 1873, sulfur dioxide and methyl ether in 1875, and methyl chloride in 1878. Dichloroethene (dilene) was used in Willis Carrier's first centrifugal compressors, and was replaced with methylene chloride in 1926.
Nearly all of the early refrigerants were flammable, toxic, or both, and some also were highly reactive. Accidents were common. The task of finding a nonflammable refrigerant with good stability was given to Thomas Midgley in 1926. He already had established himself by finding tetraethyl lead, to improve the octane rating of gasoline.
With his associates Henne and McNary, Midgley observed that the refrigerants then in use comprised relatively few chemical elements, clustered in an intersecting row and column of the periodic table of elements. The element at the intersection was fluorine, known to be toxic by itself. Midgley and his collaborators felt, however, that compounds containing it should be both nontoxic and nonflammable.
Their attention was drawn to organic fluorides by an error in the literature. It showed the boiling point for tetrafluoromethane (carbon tetrafluoride) to be high compared to those for other fluorinated compounds. The correct boiling temperature later was found to be much lower. Nevertheless, the incorrect value was in the range sought and led to evaluation of organic fluorides as candidates. The shorthand convention, later introduced to simplify identification of the organic fluorides for a systematic search, is used today as the numbering system for refrigerants. The number designations unambiguously indicate both the chemical compositions and structures.
Within three days of starting, Midgley and his collaborators had identified and synthesized dichlorodifluoromethane, now known as R-12.
The first toxicity test was performed by exposing a guinea pig to the new compound. Surprisingly, the animal was completely unaffected, but the guinea pig died when the test was repeated with another sample. Subsequent examination of the antimony trifluoride, used to prepare the dichlorodifluoromethane from carbon tetrachloride, showed that four of the five bottles available at the time contained water. This contaminant forms phosgene (COCl2) during the reaction of antimony trifluoride with carbon tetrachloride. Had the initial test used one of the other samples, the discovery of organic fluoride refrigerants might well have been delayed for years.
The development of fluorocarbon refrigerants was announced in April 1930. To demonstrate the safety of the new compounds, at a meeting of the American Chemical Society, Dr. Midgley inhaled R-12 and blew out a candle with it. While this demonstration was dramatic, it would be a clear violation of safe handling practices today.
Commercial chlorofluorocarbon (CFC) production began with R-12 in early 1931, R-11 in 1932, R-114 in 1933, and R-113 in 1934; the first hydrochlorofluorocarbon (HCFC) refrigerant, R-22, was produced in 1936. By 1963, these five products accounted for 98% of the total production of the organic fluorine industry. Annual sales had reached 372 million pounds, half of it R-12. These chlorofluorochemicals were viewed as nearly nontoxic, nonflammable, and highly stable in addition to offering good thermodynamic properties and materials compatibility at low cost. Close to half a century passed between the introduction of CFCs and recognition of their harm to the environment when released. Specific concerns relate to their depletion of stratospheric ozone and to possible global warming by actions as greenhouse gases. Ironically, the high stability of CFCs enables them to deliver ozone-depleting chlorine to the stratosphere. The same stability prolongs their atmospheric lifetimes, and thus their persistence as greenhouse gases.
In addition to having the desired thermodynamic properties, an ideal refrigerant would be nontoxic, nonflammable, completely stable inside a system, environmentally benign even with respect to decomposition products, and abundantly available or easy to manufacture. It also would be self-lubricating (or at least compatible with lubricants), compatible with other materials used to fabricate and service refrigeration systems, easy to handle and detect, and low in cost. It would not require extreme pressures, either high or low. There are additional criteria, but no current refrigerants are ideal even based on the partial list. Furthermore, no ideal refrigerants are likely to be discovered in the future.
A fundamental tenet of toxicology, attributed to Paracelsus in the 16th century, is "dosis solo facit venenum", the dose makes the poison. All substances are poisons in sufficient amounts. Toxic effects have been observed for such common substances as water, table salt, oxygen, and carbon dioxide in extreme quantities. The difference between those regarded as safe and those viewed as toxic is the quantity or concentration needed to cause harm and, in some cases, the duration or repetition of exposures. Substances that pose high risks with small quantities, even with short exposures, are regarded as highly toxic. Those for which practical exposures cause no harm are viewed as safer.
There are multiple reasons that toxicity concerns have surfaced with the introduction of new refrigerants. First, they are less familiar. Second, public consciousness of health hazards is growing. Manufacturer concerns with liability also have increased. Third, few refrigerant users fully understand the measures and terminology used to report the extensive toxicity data being gathered. And fourth, the new chemicals are somewhat less stable when released and exposed to air, water vapor, other atmospheric chemicals, and sunlight. This increased reactivity is desired to reduce atmospheric longevity, and thereby to reduce the fraction of emissions that reaches the stratospheric ozone layer or that persists in the atmosphere as a greenhouse gas. While toxicity often increases with higher reactivity, atmospheric reactivity is not necessarily pertinent. The most toxic compounds are those with sufficient stability to enter the body and then decompose or destructively metabolize in a critical organ. As examples, most CFCs are very stable in the atmosphere, generally less stable than either HCFCs or hydrofluorocarbons (HFCs) in refrigeration systems, and generally have comparable or greater acute toxicity than HCFCs or HFCs.
Concerns with refrigerant safety have been heightened by negative marketing by competing equipment vendors. Frequent overstatement, to influence customer perceptions, coupled with contradictions have fueled discomfort in refrigerant choices for all of the alternative refrigerants.
Acute versus Chronic Risks
Acute toxicity refers to the impacts of single exposures, often at high concentrations. It suggests the possible risk levels for the consequences of accidental releases, such as from a spill or rupture. It also is a gauge for service operations in which high exposures may be experienced for brief periods, such as upon opening a compressor or removing a gasket that may have refrigerant trapped under it.
Chronic toxicity refers to the effects of repeated or sustained exposures over a long period, such as that experienced in a lifetime of working in machinery rooms. Few technicians actually spend their full day in machinery rooms and concentrations may fluctuate. Most chronic exposure indices, therefore, are expressed as time-weighted average (TWA) values.
The nature of chronic effects is such that most can be anticipated and/or monitored, and occupational safety measures can be taken to minimize their impacts. As an example, refriger- ant concentrations can be lowered by designing equipment with reduced leakage and promptly repairing leaks that do occur. Refrigerant sensors can be used to sense and warn technicians of concentration increases. Further measures are identified below, in the discussion of safety standards.
From a safety perspective, the goal is to reduce both acute and chronic risks.
The Programme for Alternative Fluorocarbon Toxicity Testing (PAFT) is a cooperative effort sponsored by the major producers of CFCs from nine countries. PAFT was designed to accelerate the development of toxicology data for fluorocarbon substitutes, as refrigerants and for other purposes. Examples of the other uses include as blowing agents, aerosol propellants, and solvents. The PAFT research entails more than 100 individual toxicology tests by more than a dozen laboratories in Europe, Japan, and the United States. The first tests were launched in 1987, to address R-123 and R-134a (PAFT I). Subsequent programs were initiated for R-141b (PAFT II), R-124 and R-125 (PAFT III), R-225ca and R-225cb (PAFT IV), and R-32 (PAFT V). The cost of testing for each compound is $1-5 million and the duration is 2-6 years, depending on the specific tests deemed necessary or indicated by initial findings.
These PAFT studies investigate acute toxicity (short-term exposures to high concentrations, such as from accidental releases), subchronic toxicity (repeated exposure to determine any overall toxicological effect), and chronic toxicity and carcinogicity (lifetime testing to assess late-in-life toxicity or potential to cause cancer). The experiments also gauge genotoxicity (effects on genetic material, an early screen for possible cancer-inducing activity), reproductive and developmental toxicity (teratology, assessment of the effects on the reproductive system and of the potential for causing birth defects), and ecotoxicity (assessment of potential to affect living organisms in the environment).
A new program, initiated in 1994, is addressing the mechanistic causes of tumors and other effects observed in other programs. PAFT M was spurred by findings of benign tumors in earlier tests of R-123, R-134a, and R-141b. Although the tumors occurred late in life and were neither cancerous nor life threatening, a better understanding of causal effects is being sought.
Table 1 explains key toxicity and safety terminology to assist readers in understanding the following summaries.
Tests of R-123 indicate that it has very low acute inhalation toxicity, as measured by the concentration that causes 50% mortality in experimental animals, a 4-hour LC50 of 32,000 ppm in rats. A cardiac sensitization response was observed at approximately 20,000 ppm. This response was measured in experimental screening with dogs, with simultaneous injection of epinephrine to simulate human stress reactions. Anesthetic-like effects (e.g., weakness, disorientation, or incoordination) were observed at concentrations greater than 5,000 ppm, or 0.5%. R-123 has very low dermal toxicity (by skin application) and is only a mild eye irritant. Long-term inhalation caused an increase in the incidence of benign tumors in the liver, pancreas, and testis of rats. None of the tumors attributable to the exposures were malignant or life-threatening; all occurred near the end of the study, late in the lives of the test specimens. The exposed animals actually exhibited higher survival rates at the end of testing than those in the control group. The rats exposed to higher concentrations also experienced slight reductions in body weight and decreases in cholesterol and triglyceride levels. Studies are continuing to investigate the biological relevance of the tumors to humans. The tests completed to date indicate that R-123 is neither a developmental toxicant nor a genotoxin.
Based on the findings of extensive testing, R-123 has been deemed to have low toxicity. Refrigerant manufacturers recommend that long-term, occupational exposures not exceed limits of 10 and 30 ppm, on eight-hour time-weighted average (TWA) bases. One manufacturer suggests a limit of 100 ppm, also TWA, but is expected to revise this recommendation to somewhere in the 10-30 ppm range. The differences in recommended limits stem from conservative interpretation of the data. As discussed below, occupational exposures can be held well below even the most stringent of these recommendations.
The exposure limits are based on chronic toxicity concerns and are below those at which toxic effects were observed in the laboratory tests. Higher concentrations are allowable for short-periods, but exposures still should be kept to the minimum practicable, as for all chemicals.
Table 1 A Glossary of Safety Terminology
- acute toxicity
- the effect of a single, short term exposure, as might occur during an accidental release
- a test for mutagenicity on bacteria, designed as a screen for possible carcinogens
- not malignant; not likely to cause death or deterioration
- a substance that causes cancer
- an effect in which the heart is rendered more sensitive to the action of adrenalin and similar drugs, possibly resulting in cardiac arrythmia and arrest (heart attack)
- an exposure level (as in PEL-C, REL-C, or TLV-C) that should not be exceeded during any part of the day, assessed as a 15-minute TWA unless otherwise specified
- long-term toxicity effects, generally assessed over the lifetime of test animals to gauge late-in-life signs of toxicity
- of or relating to the skin
- effects on genetic material, an early screen for possible cancer-inducing activity
- Immediately Dangerous to Life and Health (set by the U.S. National Institute of Occupational Safety and Health, NIOSH), the maximum concentration of airborne contaminants, normally expressed as parts per million (ppm), from which one could escape within 30 minutes without a respirator and without experi- encing any escape impairing (e.g., severe eye irritation) or irre- versible health effects
- taken into the body by mouth or swallowing
- taken into the body by breathing
- a measure of acute, inhalation toxicity representing a lethal con- centration for 50% of exposed test animals
- Lower Flammability Limit, the minimum concentration in air at which flame propagation occurs
mouse micronucleus assay
- a test for changes in chromosomes in a mouse, designed as a screen for possible mutagens and carcinogens
- No Observed Effect Level, the maximum dose at which no signs of harm are observed
- a substance that causes a change in the amount or structure of genetic material
- Programme for Alternative Fluorocarbon Toxicity Testing a cooperative effort to accelerate the development of toxicology data for fluorocarbon substitutes
- Permissible Exposure Levels (set by the U.S. Occupational Safety and Health Administration, OSHA), A PEL is the TWA concentration that must not be exceeded during any eight-hour work shift of a 40-hour work week. Chemical manufacturers pub- lish similar recommendations (e.g., acceptable exposure level, AEL; industrial exposure limit, IEL; or occupational exposure limit, OEL depending on company), generally for substances for which a PEL has not been established.
- parts per million (generally in air at 25 °C, 77 °F, and 1 atmo- sphere of pressure, 14.7 psia), may be converted to percentages by dividing by 10,000
- Recommended Exposure Limit (set by NIOSH), a recommended occupational exposure limit generally on a TWA basis for up to ten hour/day during a 40 hour work week; may also be on a STEL or Ceiling basis
- Short-Term Exposure Limit
- the effects obtained after repeated exposures to a chemical, usually for 90 days
- Threshold Limit Value (set by the American Conference of Gov- ernment Industrial Hygienists, ACGIH), the airborne concentration of a substance to which nearly all workers may be exposed without adverse health effects
- Time-Weighted Average for concentrations
- Upper Flammability Limit, the maximum concentration in air at which flame propagation occurs
- Workplace Environmental Exposure Limit (set by the American Industrial Hygiene Association, AIHA)
R-134a also has very low acute inhalation toxicity. The lowest concentration that causes mortality in rats, the 4-hour Approximate Lethal Concentration (ALC), exceeds 500,000 ppm. The cardiac sensitization response level for R-134a is approximately 75,000 ppm. Anesthetic-like effects are observed at concentrations greater than 200,000 ppm, or 20%. Long term exposures with very high concentrations, 50,000 ppm, caused an increased incidence of benign tumors in the testis of rats. Again, none of the observed tumors were life-threatening, and all occurred near the end of the study. The evidence from all tests in cultured cells or organisms, as well as in laboratory animals, indicates that R-134a in not genotoxic and that the increased incidence in benign tumors is not due to an effect on genetic material.
The test findings indicate that R-134a has very low acute and subchronic inhalation toxicity, is not a developmental toxicant, and is not genotoxic. Most refrigerant manufacturers recommend that TWA occupational exposures not exceed 1,000 ppm; this also is the level recommended by the American Industrial Hygiene Association, Workplace Environmental Exposure Limit (WEEL) Committee. Again, exposures still should be kept to the practicable minimum.
It is important to note that the tumors attributable to the R-123 and R-134a exposures were not cancerous. The findings reflect an increase in tumor incidence compared to rats in the experimental control group, those not exposed to the refrigerants. Some tumors also were observed in this control group, but not as many. Also, the recommended occupational exposure limit for each refrigerant is below the level at which toxic effects were observed in laboratory animals. The use of rats, dogs, and other animals is based on accepted scientific procedures and sensi- tivities to specific concerns by species. The lower exposure limit affords both a margin of safety and a conservative reflec- tion of potential differences, between responses in individual humans and between humans and test animals.
Information on the toxicity of other refrigerants is available from chemical manufacturers, published literature, and chemical and safety databases. The toxicology findings for other refrigerants covered by PAFT also are available in reference . R-123 and R-134a were summarized here as the newest of the widely used refrigerants for chillers. Other chiller refrigerants are addressed in less detail below, and a survey of toxicity data is underway, by the author, on alternative refrigerants for additional applications.
Table 2 summarizes safety data, including acute, subchronic, and chronic toxicity indicators, for the refrigerants most commonly used in chillers. It also presents the lower and upper flammability limits (LFL and UFL) of refrigerants in air, calculated heats of combustion, and the safety classifications assigned by ASHRAE Standard 34. The table enables comparison of data for the alternative refrigerants with corresponding information for R-11 and R-12, which are being phased out to protect the environment. Safety specialists, including toxicologists, industrial hygienists, and fire prevention authorities, should be consulted for interpretations, since other data and specific conditions may be pertinent for individual applications. Also, chemical manufacturers provide frequently updated Material Safety Data Sheets (MSDSs) that summarize risks, recommend first aid measures, and give other safety guidance. Individuals who work with refrigerants should be familiar with these documents and have access to them for reference. R-11 and R-12 have been in wide use for more than sixty years. While some accidents have occurred with them, both are regarded as fairly safe substances. Both have been used as aerosol propellants for consumer products, including cosmetics likely to be inhaled or sprayed on exposed skin. Both are used as propellants in metered dose inhalers, intended for inhalation with medications. As indicated in the table, and further discussed below, the alternative refrigerants introduced toa replace R-11 and R-12 are safer in many respects, especially those involving acute toxicity concerns.
|Acute (short term)
LC50, 4 hr rat (ppm)
|Cardiac sensitization, dog (ppm)||5,000 c||20,000||50,000||75,000||50,000||5,000|
|Anesthetic effect (ppm)||10,000||5,000||>200,000||>200,000||200,000||500|
|NIOSH IDLH (ppm)||10,000 d||4,000 e||50,000||50,000 e||50,000 e||500|
|Short-term exposure limit (ppm)||1,000||1,000 f||50,000||75,000||50,000||35|
NOEL, rat (ppm)
|Mutagenicity or Carcinogicity|
|Mouse micronucleus assay||negative||negative||negative||negative||negative||g|
|Carcinogenic||no||no h||no||no h||weakly i||unknown|
rats or rabbits
|Chronic (long term) toxicity|
|Occupational exposure limit (ppm)||C1000 (PEL, TLV-C)||10-30 (manufacturer)||1,000 (PEL, TLV-TWA)||1,000 (manufacturer)||1,000 (PEL, TLV-TWA)||50 (PEL j/sup>)|
|LFL-UFL (%vol in air)||none k||none k||none k||none k||none k||15-25 l|
|Heat of combustion (MJ/kg)||0.9||2.1||-0.8||4.2||2.2||22.5|
|Safety classification m||A1||B1||A1||A1||A1||B2|
The alternative refrigerants are as safe or safer than those they replace, but more care is needed with all refrigerants.
Dictionaries define "safety" as "being free from harm or the risk of injury or loss." The concerns usually associated with refrigerants are toxicity, flammability, and physical hazards. Are refrigerants completely safe? No, all pose one or more of these concerns. But can refrigerants, and especially the new refrigerants, be used safely? Yes, and generally more so than in the past.