We tested the changes in viability of IAV spiked into human mucus at RH levels ranging from ∼26 to ∼100%. As shown in Figure 1C, at ∼100% RH, the viability of IAV was only 1.7±0.26%, an order of magnitude lower than in model media. The finding that viability was highest at this extremely high RH was consistent with results in other media. At RH between ∼50% and 84%, viabilities increased dramatically with decreasing RH. Only 0.0027±0.0023% was recovered at 84% RH, compared to 1.5±1.4% recovered at 48% RH, a decrease of over two orders of magnitude. At RH below 48%, viabilities leveled off and remained around 1%. We repeated the experiment at four (27%, 60%, 84%, and 99%) of the six RHs, and the resulting viabilities followed the same trend as shown in Figure 1C. With spiking titers of 5.1±2.6×104 TCID50, no viable IAV was detected in 5 out of 9 samples at ∼84% RH.