The effect of respiratory activity, non-invasive respiratory support and facemasks on aerosol generation and its relevance to COVID-19

Abstract

Respirable aerosols (< 5 µm in diameter) present a high risk of SARS-CoV-2 transmission. Guidelines recommend using aerosol precautions during aerosol-generating procedures, and droplet (> 5 µm) precautions at other times. However, emerging evidence indicates respiratory activities may be a more important source of aerosols than clinical procedures such as tracheal intubation. We aimed to measure the size, total number and volume of all human aerosols exhaled during respiratory activities and therapies. We used a novel chamber with an optical particle counter sampling at 100 l.min^-1 to count and size-fractionate close to all exhaled particles (0.5-25 µm). We compared emissions from ten healthy subjects during six respiratory activities (quiet breathing; talking; shouting; forced expiratory manoeuvres; exercise; and coughing) with three respiratory therapies (high-flow nasal oxygen and single or dual circuit non-invasive positive pressure ventilation). Activities were repeated while wearing facemasks. When compared with quiet breathing, exertional respiratory activities increased particle counts 34.6-fold during talking and 370.8-fold during coughing (p < 0.001). High-flow nasal oxygen 60 at l.min^-1 increased particle counts 2.3-fold (p = 0.031) during quiet breathing. Single and dual circuit non-invasive respiratory therapy at 25/10 cm.H₂ O with quiet breathing increased counts by 2.6-fold and 7.8-fold, respectively (both p < 0.001). During exertional activities, respiratory therapies and facemasks reduced emissions compared with activities alone. Respiratory activities (including exertional breathing and coughing) which mimic respiratory patterns during illness generate substantially more aerosols than non-invasive respiratory therapies, which conversely can reduce total emissions. We argue the risk of aerosol exposure is underappreciated and warrants widespread, targeted interventions.

Keywords: aerosol-generating procedure; airborne; nosocomial; particles.

Figure 1

The sampling chamber consists of a rear section containing filters, which supplies clean air through a wall composed of air‐filter media, and a clear‐walled forward section which accommodates the torso of the subject. A flexible non‐porous skirt allows entry of the subject and the tubing of non‐invasive devices. The subject’s head is positioned within a cut‐away section of a large cone, which is attached to an optical particle counter, sampling at 100 l.min^‐1 and mounted outside the chamber. Airflow in the 100‐mm diameter tube at the distal end in the chamber is monitored via an anemometer probe. Humidity and temperature were monitored using a hygrometer and thermometer positioned in front of the subject, on the chamber floor. A moveable pedal exerciser was mounted so the subject could exercise in their seated position.

Figure 2

The total number of exhaled respiratory particles sampled from ten subjects. Samples were measured over a period of 1 min during six respiratory activities and when breathing quietly while three respiratory therapies, designated as aerosol‐generating procedures, were applied. The therapies were high‐flow nasal oxygen (HFNO) and non‐invasive positive pressure ventilation with a single (NIPPV‐S) or dual (NIPPV‐D) circuit. All respiratory therapies shown were recorded at the highest settings used: HFNO at a flow of 60 l.min^‐1 and both NIPPV‐S and NIPPV‐D at inspiratory/expiratory airway pressures of 25/10 cm.H₂O. The geometric mean and upper and lower 95% confidence intervals are shown by the black bars (). The size range in the six particle bins as measured by the optical particle counter are: 10–25 µm (light blue); 5–10 µm (orange); 3–5 µm (grey); 1–3 µm (green); 0.7–1 µm (yellow); and 0.5–0.7 µm (dark blue). A value of 0.3 was added to all counts to facilitate analysis after log transformation, so zero particle counts are shown as 0.3. Overlapping dot points are not shown. Both the forced expiratory volume (FEV) manoeuvre and cough were repeated six times in the sampling min.

Figure 3

A and C show the geometric mean of the total number of particles for 10 subjects, for six respiratory activities (A) and three respiratory therapies (C) by particle size. B and D show the geometric mean of the total estimated respiratory fluid volume for 10 subjects, for six respiratory activities (B) and three respiratory activities (D) by particle size. The therapies were: high flow nasal oxygen (HFNO) (light blue) and single (NIPPV‐S) (green) or dual (NIPPV‐D) (grey) circuit non‐invasive positive pressure ventilation. All respiratory therapies shown were recorded at the highest settings used: HFNO at a flow of 60 l.min^‐1 and both NIPPV‐S and NIPPV‐D at inspiratory/expiratory airway pressures of 25/10 cm.H₂O. Both forced expiratory reserve volume manoeuvres and cough were repeated six times in the sampling min. Particle size bin boundaries are indicated above the x‐axis (orange line). The log centre of each bin is used as the particle size value. The numbers and volumes from the 0.5–0.7 µm and 0.7–1 µm size bins have been combined. The respiratory activities and therapies are: quiet breathing (light blue); exercise (dark red); talking (green); shouting (orange); forced expiratory volumes (light red); and coughing (blue). Particle number is indicated by a solid line, volume by a dashed line.