Grand Aytollah Khamenei's view about a mutanajjis (an item which has come into contact with an essential impurity) as to how many intermediaries are required in the number of transmission of impurity  in order for us to consider something as either impure or impure, is as under:  
The first mutanajjis item would make the item that comes into contact with it impure; similarly, the second mutanajjis would make the item that comes into contact with it impure, if one of them is wet. Again, if this thing which has come into contact with a mutanajis and has been made impure come into contact with a pure object, the obligatory precaution is that it makes that object impure; but the third mutanajjis can no longer make other items impure."[1]
According to grand Ayatollah Sistani (may Allah him long life), If a pure thing touches a najis thing and if either or both of them are so wet that the wetness of one reaches the other, the pure thing will become najis. Similarly, if the wetness of the thing which has become najis, touches a third thing, that third thing will also become najis. It is commonly held by the scholars, that a thing which has become najis transmits its najasat (impurity), but indefinite number of transmissions is improbable. In fact, after certain stage it is pure.
For example, if the right hand of a person becomes najis with urine, and then, while still wet, it touches his left hand, the left hand will also become najis. Now, if the left hand after having dried up, touches a wet cloth, that cloth will also become najis, but, if that cloth touches another wet thing, it cannot be said to be najis. [2]
Hence, if there are three intermediaries, all the objects are treated as impure and when it comes to the fourth intermediary, it is considered as pure. Meanwhile, it has to be noted that according to Ayatollah Sistani, if the initial moisture with the intermediaries do not get dry and the intermediaries come into contact  with other objects, the number of transmission do not increase.
Also, the fourth intermediary is considered to be pure only when an essential impurity has not been transmitted from the first intermediary. If it is transmitted with the intermediaries, then they definitely make other things impure because it is necessary to remove the essential impurity (not the color or smell) before purifying something and it does not make any difference whether the essential impurity is on the first item or on the second one.[3]

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