LAUSR

The use of enlarged breath shields has been suggested as part of a wide range of infection control measures implemented during the COVID-19 pandemic. Breath shields have long been a standard feature of slit lamps and act as a physical barrier between the examiner and subject but there is an absence of evidence on their effectiveness in reducing droplet transmission and respiratory infections. SARS-CoV-2 shares many of the features of other respiratory viruses including SARS-CoV-1 and is thought to be commonly spread though respiratory droplets (>5 μm) and fomites [1]. Fomites are formed either from droplets settling on surfaces or through direct contamination from touching mucosal surfaces. Smaller aerosolised droplet nuclei (≤5 μm) can travel further and remain in air longer. They have been shown to carry viable virus particles in experimental conditions [2] but are not thought to be a common mode of transmission of COVID-19 [1]. The risk of transmission from tears is also thought to be low [3]. We sought to examine the efficacy of facemasks and standard and augmented slit lamp breath shields using a breathing simulator. These have been described previously and generally comprise of a particle source, commonly a nebuliser attached to a bellows or air tank and a particle detector which can consist of a laser particle counter [4] or an impinger from which viral particles can be sampled from air, cultured in cells and detected as plaques [5]. Direct visual inspection of sprayed dye droplets has also been described as a way to test eye protection [6, 7]. We experimented using nebulised fluorescein 2% but were unable to capture sufficient dye to determine the patterns of droplet distribution. We used a mouthpiece nebuliser (Galemed Corp, Taiwan) containing 5ml of 0.9% saline as our particle source and attached it to a 500ml paediatric bag valve mask that was manually compressed 12 times per minute to simulate normal adult tidal breathing. The device produces a range of particles from 1 to 25 μm with median mass aerodynamic diameter of 3.8 μm. We used a Met One A2400 optical particle counter (Hach Co, Loveland, CO) operating at a flow rate of one cubic foot per minute to detect particles that reached the eyepiece over a 1-min period. This was initially performed without any shielding, and then repeated with the standard (11 × 11 × 0.2 cm) and augmented (45 × 44 × 0.2 cm) acrylic shields attached to the slit lamp objective lens (Fig. 1). We then tested the effect of placing a fluid resistant surgical facemask (BARRIER 4313, Mölnlycke Healthcare, Sweden) over the nebuliser mouthpiece alone and in combination with the large shield. The slit lamp arm was offset to 60° throughout and each barrier was tested five times. Linear regression was used to determine the effect of shield type and particle size on particle count. All analyses were performed using Stata v14. With no shield in place, the mean log particle count was 3.59 (95% CI: 3.48–3.70). There was a significant reduction to 3.01 (95% CI: 2.90–3.13, p < 0.01) with the standard