I was reading some where that before meditation we should make sure that the place is clean of impurities like hair, dust and all unwanted things so my question is why do entities like hair, saliva, nails, excreta, urine are not considered impure if they are part of your body with which you have been doing meditation but once they get detached from your body they are termed as impure? Is there any specific reason behind this logic? Just curious to know what impact these entities have on meditation.
Manu Smriti 5.135. Oily exudations, semen, blood, (the fatty substance of the) brain, urine, faeces, the mucus of the nose, ear-wax, phlegm, tears, the rheum of the eyes, and sweat are the twelve impurities of human (bodies).
So your question is quite valid. For example, why blood, which always is flowing in our veins, is not making the body eternally impure then?
And, the reason i think is, these substances, in order to make us impure, should get in touch with our sense of touch, that is with the skin.
See the following verse:
Maitreya Upanishad. II-7. It always naturally exudes at the appropriate time impure secretions through the nine apertures (eyes, ears, etc.,). Having impure matter it smells foul. Touching this, bathing (as aforesaid) is ordained.
So, when we get cuts and blood oozes out and touches the skin, then that is considered as the impurity and not when the same blood is flowing inside us. And, the same logic applies to urine, faeces etc.
Our mind might easily get distracted by seeing these things like Human Hair , some material things like food items e.g. grains etc. . Also we possibly feel no freshness of mind in dusty or unclean place. (It can also cause us to sneeze).
So we consider some places as holy , sacred , or clean for doing prayers , meditation (dhyana , Yoga) . Which are free from any foreign particles , Noise , smells , crowd etc. by seeing /sensing which our mind will loose it's ability to concentrate. So it's easily understandable that the bad/unwanted smell arose from these impurities /matter can cause distrubance in the ability of mind to concentrate.
The Svetasvatara Upanishad Chapter 2 -Verse 10 gives us the guidance about a place ideal for meditation.
समे शुचौ शर्करावन्हिवालुका विवर्जिते शब्दाजलाश्रयादिभि:।
मनोSनुकूले न तु चक्षुपीडने गुहनिवाताश्रयणे प्रयोजयेत् ।।2.10।।
Choosing a place that is plain and clean, free from pebbles, fire and gravel, with soothing sounds of flowing water coming from near by, and with features that are pleasing to the mind and the eyes, in a secret cave, protected from the disturbance of the wind, let him practice his meditation.
Now the in the commentary of above verse Shree Shankaracharya tells us that a yogi should choose the place which should be free from the items which are used in human habitations.Like the things we use in our everyday life. And the place should be free from the things which can produce distress in our mind .e.g. dust ,dirt , hair etc.
See what Shree Krishna says about such pure & impure substances.In Bhagvat Purana.
śuddhy-aśuddhī vidhīyete samāneṣv api vastuṣu dravyasya vicikitsārthaḿ guṇa-doṣau śubhāśubhau
O sinless Uddhava, in order to understand what is proper in life one must evaluate a given object within its particular category. Thus, in analyzing religious principles one must consider purity and impurity. Similarly, in one’s ordinary dealings one must distinguish between good and bad, and to insure one’s physical survival one must recognize that which is auspicious and inauspicious.
Here Shree Krishna telling us that even though all the things are similar they differenncieated like (pure /impure) just to deside what's wrong and write.
So although such substances are not considered (in spiritual path) different than other things. But inorder to decide what's helpful /right and to adopt them ,these substances are considered as impure in path of yoga.