I have a low level of confidence on this one because of the lack of context. However, I am always telling my staff to put some patient in such and such room as soon as it is clear, meaning unoccupied.

In some hospital situations, patients can find themselves in areas with less privacy.

Particles here refer to biological particles (bacteria, etc.) as well physical particles (dust, smoke, etc.). That's what I understand from these two refs. Ref 2 also contains image of a clear room.

Ref 1:
http://www.ijo.in/article.asp?issn=0301-4738;year=2003;volum...
A clear room is defined as one constructed and used in a manner to minimise introduction, generation and retention of particles inside the room and in which other comfort parameters such as temperature, humidity, and pressure are controlled.

http://www.md.chula.ac.th/radiology/division/nucmed.php
Fourth floor located a clear room, one of our facilitise, mainly for radiopharmaceutical - in-hourse kit preparations, such as cold kits ... This biological clean room technology is intended to control contamination by particulate matter in air (dust and bacteria).