Table 1.
Methods and instrumentation adopted to investigate particles emitted from human respiratory activities, and the main findings from the studies
| Year | Methods and instrumentation | Participants | Quantity measured | Particle diameter measurement range (µm) | Main findings | Ref. |
|---|---|---|---|---|---|---|
| 1945 | Bacteria applied to mucous membranes of the throat and nose; emitted particles deposited either on a bacterial growth medium or a glass slide, counted by microscope | 5 | Number of exhaled particles | >20 | 0 particles found from normal mouth breathing; counting loudly resulted in 4–14 times higher particle counts than softly counting; cough results depended on cough performance | 163 |
| 1967 | Mouth swabbed with dye (thus the origin of counted particles was the mouth). Particles settled on paper slips in a box over 30 min were counted | 3 | Size distribution | >1 | Number of particles emitted during coughing is highly variable; particle generation and emission depends on several factors including the amount of secretion; movement of lips, tongue and teeth | 60 |
| 1997 | Several respiratory activities were studied (nose breathing, mouth breathing, coughing and speaking) using real- time analysis by OPC and analysis of dried droplet residues by electron microscopy | 5 | Particle number concentrations | <1 and >1 | Results according to the OPC method showed a prevalent number of particles in the submicrometre range both for mouth breathing and coughing. Conversely, from electron microscopy the size distribution was more heavily weighted towards larger particles. According to the authors, the evaporation and/or losses of large particles in the experimental apparatus may have produced an underestimation in the measure of the original droplet size through the OPC method | 54 |
| 2009 | Participants placed heads in wind tunnel, particles measured using APS | 15 healthy volunteers, age <35 y | Particle number concentration | 0.5–20 | Mouth breathing: 98 particles l–1; unmodulated whisper (speaking): 672 particles l–1; unmodulated vocalization (loudly speaking): 1,088 particles l–1; whispered counting: 100 particles l–1; voiced counting: 130 particles l–1; coughing: 678 particles l–1. Error bars range from 15% to 60% | 56 |
| 2009 | Particle size measured with IMI; air velocity measured by PIV close to mouth during coughing and speaking (loudly counting) | 11 healthy volunteers, age <30 y | Particle size; air velocity | >1 | Measurement of wide size range (2–2,000 μm) with the same measuring system near the point of emission, when the effect of evaporation/condensation was still negligible. Size measurements at 10 mm from the mouth negligibly influenced by evaporation and condensation and can be considered as representative of the ‘original’ emitted size profile | 58 |
| 2009 | Number and size of respiratory droplets produced from the mouth of healthy individuals during talking and coughing, with and without a food dye, were measured using glass slides and a microscope, and an aerosol spectrometer | 25 healthy volunteers | Size distribution and particle number concentration | >1 | Mean size of droplets captured using glass slides and microscope was ~50–100 µm | 164 |
| 2011 | Results from APS and DDA were integrated into a single composite size distribution | 15 healthy volunteers, age <35 y | Size distribution | 0.7–1,000 | The most prominent modes in particle number distribution were identified and linked to distinct sites of origin and mechanisms of generation: one deep in the lower respiratory tract, another in the region of the larynx and a third in the upper respiratory tract including oral cavity | 5 |
| 2012 | Laser diffraction system; participants asked to give best effort to reproduce a ‘real cough’ | 45 healthy non- smokers | Size distribution and particle number concentration | 0.5–20 | Emitted particles 0.1–900 μm. 97% of total number of measured particles had diameter <1 μm. The particle number distribution was not statistically influenced by age, gender, weight, height or corporal mass | 21 |
| 2019 | Emission measured using APS during speaking and breathing | 48 healthy volunteers | Rate of particle emission | 0.5–20 | The rate of particle emission during normal human speech is positively correlated with the loudness (amplitude) of vocalization | 44 |
| 2020 | Real-time visualization of particle emissions speech was conducted with laser light scattering method | – | Airborne lifetime | – | At least 1,000 droplet nuclei that remain airborne for >8 min were estimated for 1 min of loud speaking | 17 |
APS, aerodynamic particle sizer; DDA, droplet deposition analysis; IMI, interferometric Mie imaging; OPC, optical particle counter; PIV, particle image velocimetry.