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Media plates are exposed in the main areas of activity and positioned to capture the maximum amount of particles.

Large particles tend to settle faster on the plates due to gravitational force. Smaller particles, however, take some time in settling due to factors such as air currents which drive them away.

Media plates work best in still-air areas. The microorganisms may settle on the plate alone, or in colonies. While media plates can be exposed in clean rooms for about half an hour, most experts, including the EU guideline to good manufacturing practice, claim that the optimum time for exposure to media plates 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 when a media plate works best without being ruined.

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 many less viable microorganisms than there really are.

Also, media plates start losing their weight as soon as they are put into action. After four hours, they have undergone their maximum weight loss and should not be used for further examinations. Experiments showed that the initial weight loss is very high and then gradually becomes constant until it reaches four hours. After that the loss of weight starts affecting the growth of microorganisms.

Due to these reasons, media plates are exposed for about 30 minutes to four hours and then taken for incubation to avoid misinterpretation of microorganism growth in pharmaceutical labs.