First air

Filtered air that has not touched any surface or object before reaching the critical zone, such as open vials or the stopper bowl. In unidirectional airflow, first air sweeps particles away from exposed product. Placing a hand, tool or container between the HEPA filter and the critical site blocks first air and can carry contamination downstream onto the product. Protecting first air is a core principle of aseptic technique and is checked with smoke studies.