Table 1.

Probable aerosolization mechanisms involved in different breathing activities and the measured initial droplet sizes and concentration.

		Respiratory activities
		Breathing	Speaking	Coughing	Sneezing
Aerosolization mechanism (Fig. 2)	(a) Film burst and filament in the mouth	−	+	+	+
	(b) Laryngeal generation due to vocal fold vibration	−	+	+	+
	(c) Film burst by shear stress in large bronchi	−	−	+	+
	(d) BFFB in terminal bronchioles	+	+	+	+
Initial droplet size and concentration distribution expelled at mouth/nose opening	Droplet size (μm)	• Mode: 0.8 (Morawska et al., 2009)	• Peak size: 0.8-0.9 (Morawska et al., 2009)	• Peak size: 0.8-0.9 (Morawska et al., 2009)	• Range: 1-1000 (Duguid, 1946)
		• 0.3-0.4 (Almstrand et al., 2010)	• Geometric mean: 16 (Chao et al., 2009)	• Geometric mean: 13.5 (Chao et al., 2009)	• Range: 0.1-1000; 360.1 μm (geometric mean of
		• 0.05-5 (Milton et al., 2013)	• 1.6, 2.5 and 145 μm (count median) were detected. (Johnson and Morawska, 2009)	• Range: 1-2000, 95% between 2 and 100 μm (Duguid, 1946)	unimodal distribution, SD = 1.5) and 74.4 μm (geometric mean of bimodal distribution, SD = 1.7) (Han et al., 2013)
	Droplet concentration (cm−3)	• 0.05-0.092 (Morawska et al., 2009)	• 0.307 (Morawska et al., 2009)	• 0.67 (Morawska et al., 2009)	• An average number of a sneeze releases is 1 × 106, 200-fold more
			• 0.004-0.223 (Chao et al., 2009)	• 2.4-5.2 (Chao et al., 2009)	particles than a cough. Fluid volume for a sneeze is 1.2 ×
				• 5 × 103(average number emitted per cough) (Duguid, 1946)	10−5 mL. (Duguid, 1946)

*”+” means possible and “−” means largely impossible to occur.