The SMTL are able to test the performance characteristis of nebulisers for both of these critical factors in accordance with the following standards:

  • BS EN 13544-1 Respiratory therapy equipment — Part 1: Nebulizing systems and their components  — Annex CC
  • ISO 27427 Anaesthetic and respiratory equipment — Nebulizing systems and components  —Annex C & D

Aerosol output and aerosol output rate

The nebuliser is filled with the required volume of a drug tracer test solution (sodium fluoride) and connected to a breathing simulator (a sine-wave pump) to simulate respiratory flow. During operation of the nebuliser, the aerosol emitted at the patient interface is collected on a filter. This filter can be extracted to analyse the mass of the collected test substance using an appropriately validated analytical method.

During the experiment, the nebulisation is conducted for a known time (e.g. 1 min), after which the filter is exchanged and analysed.  Dividing the mass of test substance by this time gives the aerosol output rate.  The experiment is then continued to collect the total mass of test substance emitted at the aerosol.

Particle sizing

The nebulising system is tested for particle size by capturing the aerosol output in a low flow cascade impactor (calibrated at flows leess than 15l/m).  Cascade impactors operate on the principle of inertial impaction. Each stage of the impactor comprises a series of nozzles or jets through which the tracer drug laden air is drawn, thus directing airborne drug towards the surface of the collection plate for that particular stage.

cascade impactorExample Cascade Impactor


Whether a particular particle impacts on that stage is dependent on its aerodynamic diameter. Particles having sufficient inertia will impact on that particular stage collection plate, whilst smaller particles will remain entrained in the air stream and pass to the next stage where the process is repeated.

The stages are normally assembled in a stack or row in order of decreasing particle size. As the jets get smaller, the air velocity increases such that smaller particles are collected. At the end of the test, the particle mass relating to each stage is recovered and then analysed usually using a validated analytical method to determine the amount of drug present.  The measurement of aerosol particle size distribution where the mass median aerodynamic diameter (MMAD), percentage mean aerosol size distribution and percentage aerosol mass under 5 microns are reported.


sizing log cut graph resize crop


Nebulisers are widely used to deliver drugs and vaccines in an aerosol form to humans through the respiratory system.  To achieve the intended treatment, aerosol particles have to be deposited in specific parts of the respiratory tract. Different size particles tend to deposit in different parts of the respiratory system;

In general, it is considered that aerosol particles with an aerodynamic diameter of:

—     >5 μm results in deposition in the upper airways;

—     2 μm to 5 μm results in deposition in the lower airways;

—     <2 μm may result in deposition in the alveoli, and;

—     <1 μm may remain entrained in the airstream and be exhaled during the next breathing cycle:


Therefore rapid and efficient delivery of the drug to the correct part of the respiratory tract is the defining performance characteristic, and the rate of nebulisation and droplet size within the aerosol are critical factors in achieving this. 

If you wish to commission testing of your nebuliser to BS EN 13544-1 Annex CC or ISO 27427 Annex C & D then please contact the SMTL Physical Testing Department on +44 (0)1656-752820 or

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