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Keeping an overview in the world of aerosols and particles

In this online glossary we provide interested readers with an overview of technical terms and foreign words from our area of expertise, particle and aerosol technology.

All terms mentioned here are formulated in accordance with the standards listed.


An aerosol is a “[metastable, disperse] system of solid and/or liquid particles [dispersed] in gas”. In simple words, an aerosol is a mixture of droplets and/or solid particles, which are spatial distributed in gas.

ISO 15900:2009   ISO/TS 80004 6:2021   21683:2019

Synonyms: fog, smoke, smog

As defined by Topas GmbH, an aerosol substance is the solid, liquid or gaseous primary material for the generation of aerosols.

Depending on the application, aerosol substances can be pure liquids, solutions, suspensions or specific gases.

An aggregate is a particulate/granular entity composed of individual solid particles, which are so strongly bounded or fused together that a further dispersion is not possible.

Note that the term aggregate is defined differently depending on the discipline or specific application.

ISO 8780-1:1990   ISO 19430:2016   ISO 23900-1:2015   ISO 26824:2022   ISO 29464:2017

Synonyms: smallest dispersible unit


clean room

A cleanroom is a room or microenvironment in which the number of airborne or deposited particles is controlled.

Cleanrooms are operated i.a. for the preparation of medical drugs, for the manufacturing of electronic components, for the production of food or within healthcare.

ISO 14644-1:2015   ISO14698-2:2003   ISO 15378:2017   ISO 15388:2022   ISO 16232:2018

Synonyms: clean room, cleanroom installation, safety cabinet, laminar flow bench, operating theatre, isolator
cleanroom installation



Dispersing is a process where an exterior energy supply serves for the generation of a disperse system.

Examples for dispersing are: spraying of liquids, swirl desert dust or supply of air into water (whirlpool).

Synonyms: disperse, mixing, distributing


A filter resp. filter element serves for the removal of particulate and/or gaseous contaminations from fluids and is typically composed of one or more layers of filter medium (partly formed) as well as an outer frame. For stabilisation, an inner supporting structure is often present.

Nowadays, filters are categorised i.a. by their separation mechanism (e.g. electro filter, electret filter), their structure/design (e.g. bag filter, filter cartridge), their application (e.g. diesel particle filter, ambient air filter) or their separation efficiency (e.g. coarse dust filter, fine particle filter, HEPA).

ISO 5011:2020   ISO 9912-1:2004   ISO 11841-1:2000   ISO 16890 Normreihe   ISO 29461-2:2022   ISO 29463-1:2017   ISO 29464:2017

Synonyms: Filterelement
filter assembly
filter efficiency
filter element
filter media

A filter medium is the primary material of filters that serves for removal of particulate and/or gaseous contaminations from fluids.

These are often flat, porous or fibrous materials. Filter media also include granular materials (e.g. activated carbon), solid foams (e.g. ceramic filters) or specific coatings.

ISO 9912-1:2004   ISO 11057:2011   ISO 11933-4:2001   ISO 16170:2016   ISO 16891:2016   ISO 21018-1:2008  ISO 29463-1:2017   ISO 29464:2017

Synonyms: filter media, filtering material
filtering material

The filtration efficiency is the ability of a filter or filter medium to retain specific contaminants (particles and/or gases) under defined conditions (ISO 3857-4:2012, ISO 4548-12:2017, ISO 19438:2023). Separation efficiencies are used as descriptive parameter, which can be determined by evaluating the quantity of the contaminant before, on and/or after the filter/filter element.

Separation efficiencies can be determined over the entire test specimen (separation efficiency) or partial areas (local separation efficiency). For particulate contaminants, a differentiation between particle-size-resolved (fractional separation efficiency) and non-particle-size-resolved (integral separation efficiency) separation is used. The types and terms of specific separation efficiency used depend fundamentally on the field of application.

ISO 3857-4:2012   ISO 4548-12:2017   ISO 19438:2023

Synonyms: separation efficiency, filter efficiency
fractional filtration efficiency

The fractional separation efficiency is a descriptive parameter for filter efficiency/separator efficiency of particulate contaminants. For this, the quantity of retained particles is determined size-resolved (at least two size fractions.

Fractionated (monodisperse, quasi-monodisperse) or non-fractionated (polydisperse) particle systems (aerosols, suspensions, emulsions) can be used to determine the fractional separation efficiency. In the case of fractionated particle systems only concentration analyses are necessary, whereas non-fractionated particle systems require in addition the determination of the particle size distribution.

ISO 11841-2:2000   ISO 16890-1:2016   ISO 16890-2:2016   ISO 17536-4:2019   ISO 17536-5:2019   ISO 29463-1:2017   ISO 29464:2017

Synonyms: particle size efficiency, fractional filtration efficiency, size-specific separation efficiency






A monomodal particle size distribution shows in the density function only one maximum and in the cumulative function only one point of inflection.

Please note that the terms monomodal and monodisperse have different meanings.

VDI 3491-1:2005   ISO/TS 4807:2022

Synonyms: simple modal

A multimodal particle size distribution shows in the density function more than one maximum and in the cumulative function several points of inflection.

VDI 3491-1:2005



A particle is defined as a “minute piece of matter minute piece of matter with defined physical boundaries” or “discrete element of […] material regardless of size”.

Particles are solid, liquid or gaseous objects, which are spatial distributed within solid materials, liquids or gases. This includes cereals in milk (suspension), dust in the air (aerosol), oily droplets in milk (emulsion), airborne water droplets (fog, aerosol) or wholes in cheese (solid foam).

ISO 14644-1:2015   ISO 14644-13:2017   ISO/TS 19807-1:2019   ISO/TS 80004-2:2015

ISO 2395:1990   ISO 21501-1:2009

Synonyms: granules, piece, particulate matter
particle size efficiency
particulate matter


The separation efficiency is a descriptive parameter for the filtration efficiency against gaseous and/or particulate contaminants under defined conditions. It describes the average separation of the contaminant either over the entire test specimen (integral separation efficiency) or over a limited area (local separation efficiency).

In the case of particulate contaminants, the separation efficiency is considered only for a single size range (i.e., non-particle-size-resolved). The separation efficiency depends on the state of dispersion of the applied particle system and on the measuring principle as well as the measuring range of the analytical instruments. This leads to a dependency on the type of quantity and can thus cause significant differences in determined values. Accordingly, a separation efficiency determined by weighing is referred to as gravimetric separation efficiency (also denoted as arrestance).

ISO 16890-1:2016   ISO 21904-1:2020   ISO 29042-1:2010   ISO 29042-5:2010   ISO 29042-6:2010  ISO 29464:2017

Synonyms: total separation efficiency, total separation efficiency
size-specific separation efficiency
smallest dispersible unit

A solution is a homogeneous, single-phase mixture composed of a solvent and at least one dissolved substance (solute).

Typical examples of liquid solutions are sugar water, isotonic sodium chloride solutions and spiritous beverages.

ISO 13457:2021

A suspension is defined as a “two-phase system in which one phase, the [solid] disperse phase, is distributed throughout the other, known as the [liquid] continuous phase”.

In simple words, a suspension describes a mixture of spatial distributed solid particles in a surrounding liquid.

Combination of:

ISO 29464:2017   ISO 472:2013   ISO/TS 80004-6:2021   ISO 18473-4:2022   ISO4618:2023   ISO/TS 19807-1:2019   ISO/TS 19808:2020

Synonyms: dispersion