Reply: We thank Leung and Liu for their interest in our study. However, the points raised by them are misinformed and not relevant to our study. They refer to contribution of respiratory aerosols from patients/staff and reiterate evidence from an incomparable study by Liu et al., which found SARS-CoV-2 particles in the protective apparel doffing rooms to be predominantly 0.25 to 1.0 μm.1 This quoted study acquired aerosol samples through a filter and analyzed them using 3 different methods for a variety of environments.1
We would like to emphasize that our study aimed to investigate real-time aerosol generation during phacoemulsification only and not at gathering cumulative aerosol generation in theater through theater personnel/patient or in different parts of operating areas. To avoid any unnecessary noise in the data, we standardized the acquisition process and included a minimal number of investigators who all wore fit-tested FFP3 respirators to reduce respiratory aerosol contribution. Furthermore, these personnel remained in the same position throughout the experiment to reduce aerosol turbulence due to movement.
With the routine time for phacoemulsification procedure being a few minutes, sampling times inclusive of additional 8-hour sampling with personal monitoring as suggested by Leung and Liu are impractical to answer our specific study question. On the contrary, longer duration sampling might produce unreliable data because the temperature and humidity changes with longer duration of breathing, talking, and movement. To reduce the impact of dynamic changes in humidity and temperature in theater, we conducted the experiment as a sequential, 1 day ex vivo study in a positive pressure ventilation system setting.
We would like to stress that our experiment was not designed to innovate a new optical particle counter (OPC), and therefore, the concepts of limits of detection and quantification do not apply. Our study used the TROTEC PC200 OPC, which is certified and adheres to the ISO 21501-4 standards for light scattering airborne particle counters (Table 1).2–4 This instrument was selected because of its portability and accuracy and because it was nondisruptive to the theater setup. Similar OPCs have also been used in relevant respiratory studies published in high impact journals such as the study by Doggett et al., which investigated aerosols produced during intubation and bronchoscopy.4 Because this was the first phacoemulsification experiment with an OPC with no previous standardization published, we performed robust standardization experiments in controlled and real-world environments prior to conducting the study and found coaxial placement of OPC optimally captured aerosols. Moreover, to reduce any risk of spurious single measurements skewing the data and to capture accurate and best representative data during real-time phacoemulsification, we repeated OPC measurements 5 times over 2 minutes (total sampling volume of 5 L), resulting in a total of 75 measurements.
The real-time data from our study, captured in a real-world theater setting, shows no significant difference in aerosols measuring 10 μm or less during phacoemulsification compared with baseline. These results add to the existing evidence base to support personal protective equipment protocols, such as the Public Health England guidance for aerosol-generating procedures, and help safely resume cataract surgery services.5
1. Liu Y, Ning Z, Chen Y, Guo M, Liu Y, Gali NK, Sun L, Duan Y, Cai J, Westerdahl D, Liu X, Xu K, Ho KF, Kan H, Fu Q, Lan K. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 2020;582:557–560
3. Particle measuring systems: understanding ISO 21501-4. Available at: https://www.pmeasuring.com/PMS/files/87/87e95f59-14f7-4b77-acb7-b0341a5aad97.pdf
. Accessed August 2, 2020
4. Doggett N, Chow CW, Mubareka S. Characterization of experimental and clinical bioaerosol generation during potential aerosol-generating procedures. Chest [Epub ahead of print July 21, 2020.]; S0012-3692(20)31955-3. doi: 10.1016/j.chest.2020.07.026