Which ISO standard defines an atmospheric environment with fewer than 100 particles per cubic foot of air?

ISO 5 is the standard that specifies an atmospheric environment with fewer than 100 particles per cubic foot of air. In the context of cleanrooms and controlled environments, ISO standards define cleanliness levels based on the concentration of airborne particulate contamination in specific sizes.

ISO 5 indicates a certain level of cleanliness achieved through stringent controls on air quality, which is crucial in industries such as pharmaceuticals, biotechnology, and microelectronics, where product integrity can be compromised by airborne particulates. Achieving this standard requires specialized ventilation systems, filtration, and stringent protocols to minimize contamination, ensuring that the environment is suitable for sensitive processes or products.

Other ISO classifications establish different cleanliness levels, with ISO 4 being stricter and allowing even fewer particles, while ISO 6 and ISO 7 allow for higher particle counts. Therefore, ISO 5 is specifically tailored to environments needing high purity and controlled cleanliness levels, making it the correct answer for this question.