Freon is an odorless, colourless and non-flammable group of liquid chemicals used in air conditioners and refrigerators. It is a chemical compound, technically known as a Chlorofluorocarbon or hydro-chlorofluorocarbon, it is part of a group of chemicals known as Halo-alkanes, it is non toxic to human beings and have been around since the early 20th century.
It is fairly controversial as some forms were phased out by DuPont during the nineties due to being Chlorofluorocarbons (CFC's) and having a negative effect on our atmosphere and environment. Although non-toxic, in 2008 it killed 20 and injured 21 Russian submariners on board the submarine K- 152 Nerpa, when a fire system which was Freon based activated by mistake during trials filling cabins with a high concerntration of the substance.
Inhaling directly from a leak can be dangerous, it is heavier than air and when inhaled it can settle in the lungs which can cause lots of problems, fatally suffocation. Continual exposure to low levels therefore is very unlikely to cause much of a risk, but in large quantities it can displace oxygen in the lungs and can lead to asphyxiation.
In cars certain compounds of freons are used in the air conditioning systems. A story surfaced a few years back claiming to be a researcher for the American Motor Vehicle Safety Organisation (AMVSO) stating that freon is lethal and dangerous, and that every car produced between 1985 - 1998 is dangerous and could lead to Alzheimer's disease.
The truth is that the AMVSO does not even exist, and the hype created a lot of controversial gossip, about what is a simple problem; if your air conditioning brakes, fix it or get it looked at. Also there is not one single company producing air conditioners for every car, and there is no scientific link at all between freon and Alzheimer's.
In conclusion, despite the false worries surrounding the substance in our air conditioners, it is not lethal to humans unless our lungs are flooded with it. It is bad for the environment and the ozone layerr and sustained exposure to it is not recommended. Freon is NOT a dangerous gas, a simple service and even your standard M.O.T will spot a problem with your electrical systems or air conditioning and the amounts used in modern day cars in so negligible that it should make no difference anyway, it just means your car will a bit warm on a hot day unless you open your windows.