Inhalation dominates exposure of expired particles in close contact with head movement

Face-to-face communication is a frequent daily social interaction. Proximal inhalation and facial touch are considered potential respiratory disease transmission routes. This paper compares the particle inhalation number with facial attachment number during conversation to quantify exposure by two routes. Effects of target’s inhalation type (oral-nasal, oral-only, nasal-only), target’s inhalation flow rate (no, normal, high), and source’s head movements (motionless, nodding, shaking) on the particle inhalation number and facial attachment number were investigated. To obtain high spatial resolution information on facial attachment number, the face was divided into 2 key zones (noses, lips), 3 mucosal zones (nares, oral, ocular) and 10 other non-mucosal zones. Results revealed that particle inhalation number was the highest for nasal-only cases, followed by oral-nasal cases, and the lowest for oral-only cases. The forehead had the highest particle attachment number, especially for upper facial zones. Under normal inhalation scenarios, particle inhalation number (23–1046#) is significantly greater than mucosal (3–52#) and key facial zone (7–84#) attachment number, and is lower than total facial attachment number (51–1456#). For high-inhalation scenarios, particle inhalation number increases by an average of 26 % to 37 %. Nodding can increase particle inhalation number, which is 2.65 times greater than motionless and 28.5 times greater than shaking. Moreover, target’s inhalation rhythm affects particle inhalation number, with the maximum particle inhalation number being approximately 1.8 times the minimum inhalation number. Although masks can reduce facial attachment and particle inhalation number, the potential attachment exposure from frontal hair and forehead should not be ignored.