TABLE I.
Key parameter values for masks including air, water, and mucus at 20 aC and atmospheric pressure 105 Pa. Note that small droplets dry rapidly and this will cause their viscosity to increase. Flow rates are determined from the volume typically exhaled during one minute. Moderate exertion is defined as that readily able to be sustained daily during 8 h of work, whereas maximal exertion is the upper limit of what can be sustained for short periods of time (e.g., during competitive sports). Flow speeds are calculated for the stated mask area and flow rates assuming perfect face seal; in practice, leakage would reduce flow through the mask.
| Quantity | Value | Reference |
|---|---|---|
| Air | ||
| Mass density | 1.2 kg m−3 | 68 |
| Dynamic viscosity μ | 1.8 × 10−5 Pa s | 68 |
| Kinematic viscosity ν | 1.5 × 10−5 m2 s−1 | 68 |
| Water/mucus | ||
| Mass density ρp (water) | 998 kg m−1 | 68 |
| Dynamic viscosity (mucus) | 0.1 Pa s | 55 |
| Mucus/air surface tension γ | 55 | |
| Typical breathing flow rates | ||
| Tidal breathing at rest | 6 l min−1 | 69 |
| During mild exertion | 20 l min−1 | 69 |
| During moderate exertion | 30 l min−1 | 69 |
| During maximal exertion | 85 l min−1 | 69 |
| Average flow speeds | ||
| Effective mask area | 190 cm2 | 70 |
| Flow speed (rest) | 0.5 cm s−1 | |
| Flow speed (mild) | 1.8 cm s−1 | |
| Flow speed (moderate) | 2.7 cm s−1 | |
| Flow speed (maximal) | 7.5 cm s−1 |