Proficiency in nirvichar samadhi, the state of being thoughtless, leads to etheric purification.

Of the five elements, agni is the one that cannot be contaminated and has the power to purify others. Pollution of earth, water, air we are all aware of, space or ether is contaminated by thoughts.

The thoughts are not created by us, they come from outside. The kind of thoughts we catch from the environment depends on our level of evolution. Grosser thoughts pertain to self, subtler thoughts pertain to creation.

But all the thoughts are foreign, they contaminate us in the sense of tying us to the physical creation. When one is able to still oneself such that no thought enters, it cleanses the body – physical as well as subtle.