Cascade impactors are multistage, multi-jet measurement related devices which classify particles present in a sample of air/ gas into known size ranges. Whole process is based on mechanism of a jet of particle-laden air, impinging on a plate followed by aerodynamic classification of particles.

Aerodynamic diameter of the particle is directly proportional to its inertial energy.
So making it obvious that the larger particles (with higher inertia) will deposit upon the immediate impaction surface, while smaller particles (with less inertia) will remain aboard in the airstream. These smaller particles then flow around the impaction surface and proceed through subsequent stages with smaller jets.
Eventually, all these boarded particles will be deposited upon impaction surfaces in series i.e “cascade” of jet stages and impaction surfaces. Resulting samples are then analysed by suitable method of chemical analysis.

Impactors were first developed in the 1860s for collecting dust and airborne particulates, since then there has been numerous advances in the field with the first cascade impactor (May Cascade Impactor) being developed in 1945 for military applications. In the 1950s the Andersen Cascade Impactor, also known as an ACI, was first developed. The Andersen Cascade Impactor was a breakthrough in impactor evolution as it was the first impactor to grade samples by particle size, known as Aerodynamic Particle Size Distribution. Although this model was initially produced to assess viable particulates it provided the base for the ACIs as they are known today.

The Andersen Cascade Impactor separates particles based on their aerodynamic behaviour hence are widely used for evaluation of inhaled products using Aerodynamic Particle Size Distribution (APSD). ACI are most commonly used for testing drug safety and efficacy, quality control of MDIs, DPIs and other inhalation devices. Even in environmental applications, study of airborne particulates requires ACI as a reliable analysing tool.