By Thomas B. Cross Executive Director Pandemic Tech Alliance
In the Proceedings from the National Academy of Sciences called The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission (May 13). Please read the entire article for the complete analysis, however here are the major findings:
- At an average viral load of 7 × 106 per milliliter (7), we estimate that 1 min of loud speaking (85db) generates at least 1,000 virion-containing droplet nuclei that remain airborne for more than 8 min.
generates at least 1,000 virion-containing droplet nuclei that remain airborne for more than 8 min.
- Some patients have viral titers that exceed the average titer of Wölfel et al by more than two orders of magnitude (7, 18), thereby increasing the number of virions in the emitted droplets to well over 100,000 per minute of speaking.
- However, we also note that, even while the smallest droplet nuclei effectively remain airborne indefinitely and have half-lives that are dominated by the ventilation rate.
While cannonballs and pumpkins have their own trajectory models, the study also noted:
- According to Stokes’ law, the terminal velocity of a falling droplet scales as the square of its diameter. The distance over which droplets travel laterally from the speaker’s mouth during their downward trajectory is dominated by the total volume and flow velocity of exhaled air (8).
- The flow velocity varies with phonation (word choice) (14), while the total volume and droplet count increase with loudness (9).
- We also note that the saliva viral load shows large patient-to-patient variation.
Bottom-line – Teaching, loud speaking, singing, exercising are all forms of large exhale of droplets and aerosol The length of virus decay which can be hours or days (see other sources for virus decay on surfaces) and depends on air flow, environmental conditions, evaporate, temperature and other factors. This supports recommendations for significant downward (negative air pressure see below) air flow in buildings or what is called built-environments (BE) in other studies.
Elevators present another level of concern along with the elevator shaft should be disinfected possibly hourly.