Only a MVAC-trained, EPA-certified, automotive technician should service the A/C system or its components. When servicing the A/C system, please note the following precautions:

R-134a liquid refrigerant is highly volatile. A drop on the skin of your hand could result in localized frostbite. When handling the refrigerant, be sure to wear gloves
It is standard practice to wear goggles or glasses to protect your eyes, and gloves to protect your hands. If the refrigerant splashes into your eyes, wash them with clean water immediately
The R-134a container is highly pressurized. Never leave it in a hot place, and check that the storage temperature is below 126°F (52°C).
A electronic leak detector should be used to check the system for refrigerant leakage. Bear in mind that the R-134a, upon coming into contact with flame, produces phosgene, a highly toxic gas
Use only recommended the lubricant for R-134a systems. If lubricants other than the recommended one used, system failure may occur

PAG lubricant absorbs moisture from the atmosphere at a rapid rate, therefore the following precautions must be observed:

When removing refrigerant components from a vehicle, cap immediately the components to prevent from the entry of moisture
When installing refrigerant components to a vehicle, do not remove the cap until just before connecting the components
Complete the connection of all refrigerant tubes and hoses without delay to prevent the A/C system from taking on moisture
Use the recommended lubricant from a sealed container only
If an accidental discharge in the system occurs, ventilate the work area before resuming service

When replacing parts on an A/C system:

Never open or loosen a connection before discharging the system
Seal the open fittings of components with a cap or plug immediately to avoid moisture or dust from entering the system
Do not remove the sealing caps from a replacement component until it is ready to be installed.
Before connecting an open fitting, always install a new sealing ring. Coat the fitting and seal with refrigerant oil before making the connection.