Investigation of the Artificial Saliva and Saline Droplet Size Measurement Accuracy for COVID-19 Infection Control

The size of human speech or cough droplets decides their air-borne transport distance, life span and virus infection risk. We have investigated the measurement accuracy of artificial saliva and saline droplet size for more effective COVID-19 infection control. A spray generator was used for polydisperse droplet generation and a special test chamber was designed for droplet measurement. Saline and artificial saliva were gravimetrically prepared and used to generate droplets. The droplet spray generator and the test chamber were circulated among four metrology institutes (NMC, CMS/ITRI, NIM and KRISS) for droplet size measurement and evaluation of deviations. The composition of artificial saliva was determined by measuring the mass fraction of the inorganic ions. The density of dried artificial saliva droplets was estimated using its composition and the density of each non-volatile component. The volume equivalent diameter (VED) of droplets have been measured by aerodynamic particle sizer (APS) and optical particle size spectrometer (OPSS). As a response to the COVID-19 pandemic, this is the first time that a comparative study among four metrology institutes has been conducted to evaluate the accuracy of saliva and saline droplet size measurement. For artificial saliva droplets measured by OPSS, the deviations from the reference VED (~ 4 μm) were below 5.3%. For saline droplets measured by APS, the deviations from the reference VED were below 10.0%. The potential droplet size measurement errors have been discussed. This work underscores the need for new reference size standards to improve the accuracy and establish traceability in saliva and saline droplet size measurement.