Clean rooms are classified according to the number of particles per cubic meter at a specified particle size. The ISO 14644-1 standard is commonly used to define clean room classes. For example: - ISO Class 1: The cleanest, allowing only 10 particles per cubic meter of 0.1 micrometers or larger. - ISO Class 5: Allows 100,000 particles per cubic meter of 0.1 micrometers or larger. - ISO Class 8: Allows 100,000 particles per cubic meter of 0.5 micrometers or larger.