The air conditioner's refrigerant is responsible for absorbing heat inside the house and dumping it outside. This means your AC won't be able to cool your house if it doesn't have adequate refrigerant. Below are some of the reasons an AC might leak refrigerant.

Wear And Tear

Wear and tear affect all parts of the AC system, including the refrigerant lines. The materials degrade over time and weaken, increasing the susceptibility of refrigerant leakage. Therefore, an old AC that is nearing the end of its life is likely to experience refrigerant leaks than a relatively new AC. Note that central ACs last for about 12 to 15 years, so you might start to see some problems after a decade or so.

Corrosion

Corrosion is a major cause of refrigerant leaks. Exposure to moisture and corrosive chemicals, such as household cleaners, accelerate the rate of corrosion. Corrosion eats away at the metal and reduces their effective thickness. That can trigger small cracks or pinholes on the metal and allow refrigerant to escape.

Accidents

Physical blows to various parts of the AC can also cause mechanical damage that can trigger refrigerant leaks. For example, if something heavy accidentally falls on the AC coils, it can puncture them or disconnect the refrigerant lines and cause leakage. A classic example is a tree that falls on the outside unit of the AC.

Vibrations

A typical AC emits minor vibrations as it operates. Extrinsic vibrations can also originate from nearby construction sites (think heavy construction machinery), blast sites, and even highways where big trucks pass all the time. All these vibrations can affect the AC and loosen some of its parts over time. The disconnections can eventually cause refrigerant leakage.

Negligent Installation or Maintenance

Even the manner in which you install or service an AC system can trigger refrigerant leaks. If you don't know how to install or service an AC, you can make mistakes that can allow the refrigerant to escape from the system. For example, improper application of sealants on joints, missing O-rings, and making the wrong joints can all lead to refrigerant leaks. A professional AC installer is unlikely to make such mistakes; DIY installers without adequate skills or experience are more likely to make such mistakes.  

Note that impaired cooling is not the only effect of low refrigerant levels. The AC can also overheat and malfunction due if you continue running it with inadequate refrigerant levels. Contact an air conditioning repair technician as soon as possible if you suspect low refrigerant levels in your AC.