Car air conditioning is designed to pull the heat from the air within the interior area of the car outside, leaving only cooled or 'conditioned' air to be redistributed. This is achieved using a special compound called refrigerant, capable of readily absorbing heat as a gas and quickly losing heat as a liquid.
The compressor needs the right amount of oil in the refrigerant to prevent it from seizing up. Should the air filters be jammed with dirt and bacteria, it will affect the entire system's performance, and it can even cause it to stop working altogether. On top of inhibiting air conditioner efficiency, dirty air filters can cause health problems.
Being exposed to the elements and very diverse operating conditions and being used intermittently rather than being 'Always On' can cause decay of the hoses, connections and seals. A vehicle air conditioning system should be examined regularly to ensure there are no problems or refrigerant loss due to failure or leakage from these points.
Simply by cooling the air in your car under normal running conditions, the air filters get dirty and intermittent use can cause seals to dry and crack, allowing them to leak. Even without leaks, the gas in a car air conditioning system will eventually begin to degrade or evaporate over time. It should be recharged as required to keep the air con system in peak running condition.