Table 1.

Thickness of various types of normal human mucus.

Type of Mucus	Average Thickness (μm)	Reported Thicknesses (μm)
Respiratory
Airway	15	7 [64, 66]; 30 [65]
Bronchial	55	55 ± 5 [67]
Gastrointestinal*
Gastric	170	144 ± 52 [208]; 192 ± 7 [209]
Ileal	10	10 [74]
Cecal	37	36.7 ± 7.2 [72]
Colonic†	100	39.1 ± 9.9 (A), 57.5 ± 14.5 (T), and 69.6 ± 32.1 (D) [72]; 79 ± 40 [74]; 100–150 [68]; 107 ± 48 (A-T) and 134 ± 68 (T-D) [73]; 110–160 [69]
Rectal	125	101.5 ± 80.3 [72]; 155 ± 54 [73]
Ocular
Mucus Layer (only)	0.035	0.02–0.05 [77]
Tear Film	5	3 [77]; 6–7 [76]
	40	34–45 [78]

^*Gastrointestinal mucus generally exists as two layers, a basal “unstirred” or “firmly adherent” layer and a luminal “stirred” or “sloppy” layer [210–212]; however, most of the cited references report only one value for mucus layer thickness. Matsuo et al. [72] report the thickness of both an “inner layer” and “outer layer” of mucus; in the cecum, the inner layer was measured to be 5.6 +/− 0.2 μm; in the colon, 4.7 +/− 1.4 μm for the ascending colon, 7.0 +/− 3.7 μm for the transverse colon, and 7.6 +/− 3.4 μm for the descending colon; and in the rectum, 12.7 +/− 6.0 μm.

^†A = ascending colon; T = transverse colon; D = descending colon.