Pesachim 92B

שבודקין לעושי פסח, ואין בודקין לאוכלי תרומה.

that we examine a beit haperas that lies in the path to Jerusalem for those offering the Paschal lamb to determine whether there is actually any ritual impurity present, in order to enable those who pass through it to know whether they are still ritually pure and able to offer the Paschal lamb. But we do not examine a beit haperas for the sake of those eating teruma in order that they should be able to eat teruma in purity. The Talmud asks:

מאי בודקין (לעושי פסח)? אמר רב יהודה אמר שמואל: מנפח בית הפרס והולך. רב יהודה בר אביי משמיה דרב יהודה אמר: בית הפרס שנידש טהור.

What is meant when we say that we examine for those offering the Paschal lamb? Practically, how is it examined? Rav Yehuda said that Shmuel said: One must blow the dust on the path before taking each step in the beit haperas as he walks through it, in order to see if there is a hidden bone there. Rav Yehuda bar Abaye in the name of Rav Yehuda said: A beit haperas that has been trodden underfoot is considered ritually pure, since it is assumed that any bones that were there have been removed or broken. The impurity of a beit haperas is due to a rabbinic decree. The rabbis waived this decree in a case where the necessary examination is made, in order to allow people to be able to offer their Paschal lambs. However, this leniency is limited to the case of the Paschal lamb, since its neglect carries the prohibition of karet. It is not extended to other cases, such as the prohibition to eat impure teruma.