Media plates are used in critical activity areas, strategically placed to capture the maximum number of airborne particles. Large particles settle quickly on the plates due to gravity, while ones take longer to settle, often influenced by air currents. These plates are most effective in areas with minimal airflow.
While media plates can be exposed in clean rooms for about half an hour too, most experts, including the EU guideline to good manufacturing practice, claim that the optimum time for media plate exposure is four hours.
This time has been determined as the best time according to mathematical calculations of the rate of settling of organisms per the exposed hours.
After four hours, the rate starts decreasing and experts say that this is the time at which a media plate works best without being ruined itself.
According to studies, it has been noted that after a four hour period, the agar forms a skin layer on it which reduces the access of water to the microorganisms, thus reducing their growth and hindering the test by leading us to believe that there are much less viable microorganisms than there really are.