The principle of the Fisons Single-Stage Impactor is similar to the Glaxo type impactor in that the unit splits the aerosol cloud into the respirable fraction that effectively passes through to the final filter stage and the non-respirable fraction that is trapped in either the throat or on the sintered glass disc of the stage plate located at the bottom of the jet section.

In contrast to the Glaxo type, the respirable dose is collected on a glass fibre filter located in the Glass Filter Assembly.

The essential metal construction overcomes the problems normally associated with similar glass impactors, namely the difficulties of well defined dimensions and fragility of the glassware employed.