Awareness of the need for good air quality with regard to health protection has increased enormously in recent years. For various reasons, studies are being conducted to investigate the influence of air quality on the respective field of research. As a result of the COVID-19 pandemic, in addition to the investigation of outdoor air, there has been an increased focus on indoor air quality and aerosol filtration in particular. However, the SARS-CoV-2 virus, with a size of approx. 0.06 to 0.14 µm,1 is usually not transported individually, but via larger solid or liquid particles (droplets). In this context, the term aerosol has become more familiar, but unfortunately, the term is often wrongly defined and thus erroneously interpreted as a mixture of particles. On the other hand, if we consider that an adult human being inhales on average about one hundred billion particles per day,2 it is worth taking a closer look at the terms aerosol and particle instead of interpreting these terms identically in one breath.
Fig.1: Representation of an aerosol and the solid and/or liquid particles contained therein Source: Own representation based on GAeF (2020): p. 9
The term aerosol is composed of the ancient Greek word ἀήρ (aēr) for air and the Latin word for solution solutio. According to DIN EN ISO 29464:2020 - Cleaning of air and other gases - an aerosol is a system of solid or liquid particles suspended in gas.3
An aerosol is therefore a system of substances. Our ambient air, for example, is an aerosol. In turn, millions of particles can be present in one litre of air. The particles can remain in the air for hours or even days and are present in both solid and liquid form.4 The particle distribution in the aerosol is extremely agile. New particles are constantly being formed, and their aggregate state, size and sinking behaviour can in turn change continuously.
1Cf. Drewnick, F. (2020): p. 6.
2Cf. GAeF (2020): p. 8.
3Cf. DIN EN ISO 29464: p. 10.
4Cf. McNeill, V. (2017): p. 428.
The size distribution of particles distributed in an aerosol can be specified in the micrometre and nanometre range.5 However, this particle distribution behaves extremely dynamically within an aerosol. A continuous change in the size distribution in turn influences the movement of the particles and thus their separation, which is based on various filtration effects. But what and how is the environment in an aerosol influenced?
For example, liquid particles, like droplets, spread orally by humans can shrink due to evaporation. When we exhale, speak, cough or sneeze, aerosols are formed that contain many droplets in the range of 0.5 µm to 100 µm.6 80 - 90 % of these particles have a diameter of about 1 µm and evaporate in milliseconds, droplets of 10 µm evaporate in less than one second. Droplets > 100 µm, on the other hand, can persist for over a minute and sink to the ground before evaporating.7
Regarding the sinking behaviour, it can be stated that in low-motion air an aerosol particle of the size 100 µm and the density of water [1 g/cm³] sinks by approx. 25 cm/s at a drop height of 2m. Assuming identical conditions, the sinking rate is reduced for smaller particles, so that particles with a diameter of 1 µm would take about 16 hours to sink to the ground.8
However, particles can remain in the air much longer due to the air flow. For example, indoor particles typically have a flow velocity of 0.1 m/s,9 which can lead to a longer residence time of particles than in the above example. And as described earlier, the size of liquid particles in an aerosol can decrease rapidly due to evaporation. For example, solid particles of size 50 µm take about 30 seconds to reach the ground from a height of 2 m in calm conditions.10 In contrast, a water droplet of identical diameter evaporates in less than 3 seconds at a relative humidity of 50%.11 Hence, a water droplet would evaporate before a solid particle of identical initial diameter reaches the ground.
9GAeF (2020): p. 11.
10Drewnick, F. (2020): p. 7.
11Cf. Vuorinen, Ville et al. (2020): p. 6.
In some publications, the terms aerosol and particle are erroneously defined identically. However, an aerosol is a system of solid or liquid particles suspended in gas. Our ambient air, for example, is such a heterogeneous mixture in that particles can exist in both solid and liquid form. So talking about aerosol filtration and aerosol filters, we do not mean `filtering an aerosol out of air´, but filtering particles out of an aerosol.
Moreover, particles and droplets are often mistakenly said to have different movement behaviour. However, liquid parts of a particle evaporate very quickly at low humidity, which changes the sinking behaviour. At high humidity, particles can in turn absorb moisture, thus increasing in size and sinking faster. In this respect, the size distribution, among other things, is decisive for analysing the movement behaviour of particles. This is because changes in particle size alter both the movement properties of the particles and the filtration effects that take effect, which must be considered when selecting suitable air filters for particle separation. The EMW® team will be happy to assist you in selecting suitable air filters for your area of application and any general conditions.
Ripperger, Siegfried (2020): Luftreinigung in Räumen während der Corona-Pandemie, F&S Filtrieren und Separieren, Jahrgang 34, Nr. 6, Essen, p. 330 - 333.