Question : Urine and perspiration, both are released from the body which is in excess, then why one of it is najis (impure) and the other pure? Whereas, it is said that both have similar chemical composition?
Answer : We should keep in mind that though both urine and perspiration are similar in chemical composition in some aspects, they are also different in some aspects and it is possible that due to their difference in some chemicals two different commands are given about them. Because as we shall see there is a poisonous substance in urine which is absent in perspiration. We shall explain it as follows: The following things are present in Urine: Urea, Calcium, uric acid, Potassium, Creatinine, Phosphate, Ammonia, Sulphates and Sodium Chloride. Whereas the components of perspiration are much less.
Incidentally perspiration that comes out from the pores of the skin, soon evaporates, and this evaporation stops the growth of bacteria on the skin, whereas it is very much possible that different types of bacteria remain in urine. On this basis it is understood why Islam has considered perspiration pure and urine impure. Its reasons are as follows:
The colored matter which is present in the chemical composition of urine, in it the poison is much more than in perspiration and such a substance is not present in perspiration.
The urea in urine, which has poison in it, is much more than urea in perspiration.
As perspiration evaporates from the skin, it does not leave any scope for growth of bacteria.
From many aspects the density of urine is more than that of perspiration. It may be ten times denser than perspiration. That is the reason that one is harmful and the other harmless. It is very difficult to avoid perspiration and it will create a lot of difficulties, whereas it is not difficult to control urinating, and this itself is a difference between them.
Source : Religious Questions Answered Logic for Islamic Rules. By Ayatollah Makarim Al Shirazi & Ayatollah Jafar Al Subhani ( Page No. 62 )