Refrigerant is the substance used inside air conditioning systems. It is often called FREON, although that is a brand name, like Kleenex. For decades, auto A/C systems used R-12. It was cheap to produce, very well suited to its task, and had no significant drawbacks. In the 1970's, it was theorized that the chemical class that included R-12 was largely responsible for the growing hole in the upper atmosphere's protective ozone layer, and in 1987 the Montreal Protocol was implemented, outlining a phase-out and eventual elimination of producing these chemicals (chloroflourocarbons--CFC's). Over 140 countries have signed this agreement, and CFC production was officially ceased at the end of 1995. A new refrigerant, R-134a, was developed for automotive use. (The numbers follow a code for the chemical formula.) All 1995 and later cars use R-134a; some began using it a few years before then. In the 1990's, many regulations were enacted regarding use and treatment of refrigerants. It is illegal to vent them into the atmosphere, which was routinely done previously whenever a system had to be emptied for repairs. Now systems can be emptied only using approved recovery equipment, and service technicians must be certified to do this work. Most recovery equipment also reclaims and recycles the refrigerant, allowing it to be reused. Separate gauge sets and reclaiming machines ($3000+ each) must be used for each type of refrigerant, and refrigerants cannot be mixed in a system. A/C systems must be labeled and use service fittings unique to the refrigerant type. Furthermore, "floor taxes" were assessed on all stocks of R-12 at the same time that production was being limited. This, of course, caused the price to rise. In the 1970's, R-12 retailed for $1 a pound; now it's around $80/lb and rising still. (Most cars and light trucks hold 2 1/2 - 4 pounds.)  Most R-12 systems can be converted (retrofitted) to use R-134a. The cost to do this varies depending on many factors. R-12 and R-134a have somewhat different physical properties which affect the design of A/C system components. The two do not mix well, and use significantly different types of oils in the system. These oils are incompatible with the other refrigerants. There are also various alternative refrigerants, created in an attempt to find an inexpensive "drop-in" replacements for R-12. Although some are approved by the EPA, none are approved by car manufacturers or A/C parts manufacturers. There are many reports of systems with other than pure R-12 or R-134a suffering component damage.  Some retrofits are not properly done, resulting in their having a mixture of refrigerants. Often alternative blends are installed without the required labeling and changing of fittings. There are stories of R-22 being used in cars. (R-22 is used in home window units, which have all-metal piping. Auto A/C uses rubber hoses and seals, which R-22 destroys in short order.) There are even reports of propane & butane being used to replace R-12 (propane is used as a refrigerant is some commercial stationary systems, but of course they don't have engine ignition systems generating sparks). What all this means is that any car A/C system could have Lord-knows-what inside it. Since R-134a is relatively affordable, it is mostly R-12 systems that are found to be contaminated.

When shops reclaim refrigerant, it goes into a large recovery tank. The refrigerant then will be used to recharge various cars. As most contaminated systems do not have the required labels or changed fittings, shops can easily recover "junk" into their tanks and then pass it on to several other cars. The ONLY way to know for sure what is in a given car is to test it with a refrigerant identifier. This is a sophisticated infrared spectrometer. There are some that test to 90% purity, but the industry standard is 98% pure. The better identifiers run $1500+. Any shop that does not use a good identifier does not know what is in the cars it services, in its recovery tanks, or even its new jugs of refrigerant (there are some new jugs that are not 100% pure, and stories of black market R-12 that is R-22 with false labeling). If your car needs A/C service, you would be wise to make certain that the shop has and uses a top-quality refrigerant identifier. Otherwise you run the risk of having your system contaminated, possibly causing further problems. As the equipment required to do a good and legal job is costly, be especially wary of low-priced A/C service.