According to *Manu Smriti (1/92)* -
*Urdhavam naabhermedhyatrah purushah parikeertitah |
Tasmanedhyatamam tvasya mukhmuktam swayambhuvaa ||*
It means the part above naval is pure but the lower portion is considered to be impure. The lower portion contains filth and is considered impure during disposing the filth. Therefore, during this period, *Yagyopaveet* is hung on the ear.