RH has large impacts on the viability of viruses in droplets, larger than the effect of chemical composition in some cases. We observed U-shaped patterns in the viability of MS2 against RH, and monotonically decreasing relationships between the viability of Φ6 and RH, respectively, in droplets of different compositions. We reported previously that the viability of MS2 and Φ6 in droplets composed of culture medium follows U-shaped patterns, in which the lowest viability occurs at 55% and 85% RH, respectively [13]. Many other studies have reported a similar pattern with greater decay at intermediate RHs than at lower or higher RHs [17, 18, 25, 27, 72]. The viability patterns observed in this study for Φ6, decreasing with RH rather than U-shaped, seem to conflict with results in the literature. However, we examined the viability of Φ6 between 20% and 80% RH in the current study. Over this range, the viability of viruses also decreased monotonically in previous studies; we have shown that the minimum viability of Φ6 in droplets occurs around 85% RH, beyond the range examined in the present study [13].