FIGURE 5.

In analogy with reverse‐flow chemical reactors, behavior of a heated mask can be split into different regimes depending on dispersion number Di and reduced volume $\widehat{V}$. (A) The low Di, high $\widehat{V}$ limit (“plastic bag limit”) does not allow virus to reach the mouth but does not provide adequate ventilation. (B) The low Di, low $\widehat{V}$ limit (“straw limit”) provides ventilation but may allow viral inhalation depending on the viral inactivation rate. This is the most promising regime. (C) The large, well‐mixed limit.  Virus and CO₂ are well‐mixed within the mask volume regardless of volume. (D) The small, well‐mixed limit. At high Di, there is no bifurcation at $\widehat{V}=1,$ so the small, well‐mixed limit displays qualitatively similar behavior to the large, well‐mixed limit, with temporal fluctuations in virus and CO₂ concentration during the breath cycle but near‐zero spatial concentration gradients [Color figure can be viewed at wileyonlinelibrary.com]