Pneumatic Impedance of Spit Socks and N95 Masks

Abstract

Introduction Restrained subjects often spit on law enforcement and corrections officers and medical responders. Based on the droplet-transmitted risk of COVID-19, such spitting could be considered a potentially life-threatening assault. Officers commonly use “spit socks” over the head and neck of spitting subjects to reduce this risk. The pneumatic impedance of such socks has not been published, so this remains an open issue for arrest-related death investigation. Methods We purchased samples of 3 popular spit sock models, 3 insect-protecting “bug” socks and hats, 3 N95 masks, a standard 3-ply surgical mask, and a common dust mask. We used a BTmeter model BTN8468 digital anemometer, an HTI model HT-1890 digital manometer, and an AC Infinity Cloudline model S6 inline controllable fan to measure air flow versus pressure drop. We compared the curves graphically and also calculated a pneumatic pseudo-impedance by dividing the pressure drop by the air velocity. Results The spit and bug socks allowed nearly maximum airflow with minimal pressure (≤1 mm Hg), whereas none of the masks allowed greater than 2 m/s of airflow at maximum pressure of 3 mm Hg. All of the spit and bug masks were grouped together with the lowest pneumatic impedances, whereas all of the N95 masks were grouped together with the highest values. The dust mask and surgical mask were in between with the dust mask closer to the spit and bug masks, whereas the surgical mask was closer to the N95 masks in impedance. Conclusions Commonly used spit socks offer nearly zero resistance to breathing. The highest resistance spit sock was still 100 times better than the best N95 mask for airflow during inhalation. Our results do not support the occasional hypothesis that spit socks might contribute to an arrest-related death.

Do Saliva-Saturated Spit Hoods Interfere With Ventilation?

“…The MTR fabric achieved 80% of final velocity within 8 seconds, whereas the Stearns hood material required 27 seconds. For similar pressures, a typical dry N95 mask never exceeds 1 m/s of air velocity 8 . All the spit hoods tested exceeded the airflow performance of dry N95 masks within 5 seconds.…”

“…Because of its loose fit and unsealed bottom edge, this drape does not interfere with ventilation. Nevertheless, we tested the dry drape material for airflow and it allowed 1.25 m/s of flow with a pressure of 2.8 mm Hg, giving a pneumatic pseudo-impedance of 2.2 Ω 8 . This compared favorably with N95 masks such as the 3M 9502 (5 Ω) and the SAS 8617 (12.5 Ω) 8 .…”

“…Nevertheless, we tested the dry drape material for airflow and it allowed 1.25 m/s of flow with a pressure of 2.8 mm Hg, giving a pneumatic pseudo-impedance of 2.2 Ω 8 . This compared favorably with N95 masks such as the 3M 9502 (5 Ω) and the SAS 8617 (12.5 Ω) 8 . The drape material was then saturated with artificial saliva whereupon it developed an impedance of 12.5 Ω after 1 second of air pressure, thus exceeding the performance of the dry SAS N95 mask.…”

“…There have been 78 instances of in-custody and arrest-related deaths temporal to use of a spit restriction device based on the following search terms: “spit hood,” “spit mask,” and “spit sock.” 8 Of these, there were 9 mentions of asphyxiation or strangulation related to some restraint tactic as contributing to the cause of death. However, in every case, the officers had also used restraint techniques other than spit guard devices, and autopsy reports did not specify which tactics were seen as causal or contributory.…”

“…However, in every case, the officers had also used restraint techniques other than spit guard devices, and autopsy reports did not specify which tactics were seen as causal or contributory. In no case was the use of a spit guard device directly cited as the cause of death 8 …”