First off, anhydrous ammonia is not always used to make meth, however it can be used in making what most users consider to be a purists form of meth. Meth cooks use anhydrous ammonia in the place of red phosphorus to extract methamphetamine from ephedrine by way of a chemical reaction when mixed with lithium (usually via battery innards). Anhydrous meth is used in several types of meth recipes – most popular are the nazi, shake and bake and birch methods.
To explain how dangerous anhydrous ammonia is, I couldn’t put it into words better than the National Ag Safety Database:
Anhydrous ammonia is a hydroscopic compound, which means that it seeks water from the nearest source, including the human body. This attraction places the eyes, lungs, and skin at greatest risk because of their high moisture content. Caustic burns result when the anhydrous ammonia dissolves into body tissue.Most deaths from anhydrous ammonia are caused by severe damage to the throat and lungs from a direct blast to the face. When large amounts are inhaled, the throat swells shut and victims suffocate. Exposure to vapors or liquid also can cause blindness.
An additional concern is the low boiling point of anhydrous ammonia. The chemical freezes on contact at room temperature. It will cause burns similar to, but more severe than, those caused by dry ice.
Under normal temperature and air pressure, anhydrous ammonia is a colorless gas. However, anhydrous ammonia is used and transported under pressure as a liquid. All equipment used for applying or transferring liquid anhydrous ammonia must be designed for use under high pressure to avoid ruptures or breaks.
Anhydrous ammonia has a distinct odor, which humans can detect in concentrations as small as 5 parts per million (ppm). When used in fertilizer, anhydrous ammonia has a concentration of about 1,000,000 ppm. Brief exposure to concentrations of 2,500 to 6,500 ppm can result in death.
Anhydrous ammonia is used to fertilize crops and can be deadly if inhaled over a period of time. Most injuries are caused by thieves inhaling or getting blasted in the face when stealing from a pressurized tank which causes the throat to swell and close. Exposure to vapors or liquid anhydrous ammonia also can cause blindness and severe burns.
Thieves generally steal anhydrous ammonia from large tanks that are found on farms or the co-ops which provide the farms with the ammonia using a hose similar to those found on the vacuum at your local car wash, but may be smaller in diameter depending on the size of the valve used on the tank.
Anhydrous ammonia is able to be stored under pressure for a period of time in pressurized vessels such as propane tanks and fire extinguishers, but can even be stored in small quantity for a very short period of time in a cooler or thermos for use in the shake and bake method of meth making. The corrosive properties of anhydrous ammonia can cause the fittings on the pressurized storage vessels to corrode and turn blue. As such, never use a propane tank if the fittings have turned blue, as the metal has likely been weakened and can potentially cause an explosion.