Mung Beans are traditionally touted as being good for cooling heat. But this is when you ingest them.
Traditional training with medium not covered in bags but in pans/barrels/woks can be detrimental to your health. Breathing in any form of dust over a period of time is bad news for your lungs.
Bag your mediums and you will NEVER have an issue.
Anyone who tells you Mung Beans ONLY, or your not training the REAL iron palm is just plain silly.
I have used soy beans, kidney beans, chick peas, tapioca pearls, etc though I use these for finger and claw training.
My bags are full of metal shot from day one, as this is how I learned from Shifu Gene Chicoine.
Mouth Boxers have not the testicular nor the spinal fortitude to be known.
Hence they hide rather than be known as adults.