Central air conditioning systems work the same way refrigerators and car air conditioning systems do: they cool down the air by moving heat from the air itself into a refrigerant gas that is cooled into a liquid or warmed into a gas as needed. Freon is actually a specific brand of refrigerant, but air conditioners use many different kinds. While central air systems do not "run out" of refrigerant, there are sometimes signs indicating they need to be refilled.
Signs That Refrigerant Is Leaking
The only time to add more refrigerant to a home air conditioning system is when the system develops a leak. The leak does not have to be noticeable to affect the way your system operates. You can often tell if your system has a problem by listening to how it works. If the conditioner starts and stops frequently without affecting the air as much as it used to, it may have a refrigerant leak. The gas lines in your air conditioner are designed to pump a certain amount of refrigerant. Too much or too little can cause problems. With too little refrigerant, the gas tends to move more quickly and cool things too rapidly.
Examine the evaporator fans inside the air conditioner. If they are covered with frost, this is another sign of a refrigerant leak. The faster-moving refrigerant is cooling the fans too quickly and creating the frost. Shut the system down and call a professional.
While refrigerant is often thought of as a toxic gas, it is actually more inert than many of the gases used around the home. The primary danger is that it will shove the present air out of the way and remove oxygen, causing people to choke, but this only happens in contained spaces with large amounts of escaped refrigerant, so you should be fine. However, you cannot replace the refrigerant yourself. It takes a professional HVAC repair person with the correct equipment to find the leak, fix it, pump more refrigerant back in the system and test the levels.
In some air conditioning systems, especially those in cars, the refrigerant sometimes needs to be flushed out entirely and replaced with new refrigerant. This is done to clean the system, since contaminants can build up in the refrigerant lines and create problems. The refrigerant in a central air conditioner should last the life of the conditioner, but in some cases contaminants can also enter into the system. In this case, a professional may suggest flushing and recharging the system completely.