Pneumatic Impedance of Spit Socks and N95 Masks: The Applicability to Death Investigation

Mark W. Kroll, Michael A. Brave, Stacey L. Hail, Ryan M. Kroll, Howard E. Williams

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish (US)
Pages (from-to)7-10
Number of pages4
JournalAmerican Journal of Forensic Medicine and Pathology
Issue number1
StatePublished - Mar 1 2022
Externally publishedYes


  • arrest
  • mask
  • N95
  • police
  • spit

ASJC Scopus subject areas

  • Pathology and Forensic Medicine


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