What ISO class is considered the dirtiest?

The correct answer is that ISO class 9 is considered the dirtiest among the options provided. ISO classifications are part of a system that categorizes the cleanliness of an environment based on the number of airborne particles present. The lower the ISO class number, the cleaner the environment.

ISO class 9 has a significantly higher allowable particle count compared to the other classes listed. This means that the air in an ISO 9 environment contains more particulates, making it less suitable for processes that require a sterile environment, such as sterile compounding. In practices where sterility is paramount, such as in the preparation of injectable medications, environments classified as ISO class 5 or lower (with class 5 being much cleaner) are preferred to minimize the risk of contamination.