How many box fans are required under AHERA to conduct clearance?

• A. 1 fan per 10,000 ft
• B. 2 fans per 10,000 ft
• C. 1 fan per 1,000 ft
• D. 1 fan per 100,000 ft

Answer: (A)

In AHERA clearance, controlling how air moves inside the containment is important to avoid stirring up fibers while ensuring airflow is representative of what occupants would experience. Box fans are used to provide a mild, uniform air movement during final cleaning and sampling, and the standard guideline ties the number of fans to the size of the area being cleared. The best approach is one box fan for every 10,000 square feet of area. This ratio keeps airflow steady enough to help distribute dust without creating strong drafts that could resuspend fibers or bias the air samples.

Using more fans (for example, one per 1,000 ft) would create stronger air currents that might stir up fibers and skew results, while far fewer (such as one per 100,000 ft) could leave areas with stagnant air and inconsistent clearance readings. The one-per-10,000-ft rule provides a balanced, practical standard across typical spaces.