For those of you that use refillable propane tanks the industry would like you to take just a moment to look at the tank before using it. Propane tanks are used by meth cooks to store anhydrous ammonia which contaminates the tank. Once the tank is empty it is returned to the store and could possibly be refilled with propane and put back on the market for sale. The contaminated tank once refilled and sold to consumers could lead to someone getting hurt or becoming sick from the toxic gas. The ammonia corrodes the valve and weakens the tank resulting in the possibility of dangerous leaks.
Here are the things that Randy Cole, State Fire Marshal says you should look for;
A putrid smell, any blue-green or purple stain on any brass portion of the service valve.
Jeff Atkinson with the propane tank inspection company Blue Rhino says, If there is any doubt "don't think twice." Don't take any chances call your local fire department along with the store where you purchased the tank.