The Industrial Design consists of the features of shape, pattern, ornament and configuration of respiratory mask shown in the drawings. Figures 1 to 8 show the present design. Figures 9 to 16 show a first variant. Figures 18 to 25 show a second variant. Figures 26 to 33 show a third variant. The portions shown in stippled lines are for illustrative purposes only and do not form part of the design. Figure 1 is a top perspective view of a respiratory mask illustrating the present design; Figure 2 is a bottom perspective view of a respiratory mask as shown in Figure 1; Figure 3 is a top plan view of a respiratory mask as shown in Figure 1; Figure 4 is a bottom plan view of a respiratory mask as shown in Figure 1; Figure 5 is a front elevation view of a respiratory mask as shown in Figure 1; Figure 6 is a rear elevation view of a respiratory mask as shown in Figure 1; Figure 7 is a left elevation view of a respiratory mask as shown in Figure 1; Figure 8 is a right elevation view of a respiratory mask as shown in Figure 1; Figure 9 is a top perspective view of a respiratory mask showing a first variant of the present design; Figure 10 is a bottom perspective view of a respiratory mask as shown in Figure 9; Figure 11 is a top plan view of a respiratory mask as shown in Figure 9; Figure 12 is a bottom plan view of a respiratory mask as shown in Figure 9; Figure 13 is a front elevation view of a respiratory mask as shown in Figure 9; Figure 14 is a rear elevation view of a respiratory mask as shown in Figure 9; Figure 15 is a left elevation view of a respiratory mask as shown in Figure 9; Figure 16 is a right elevation view of a respiratory mask as shown in Figure 9; Figure 17 is a top perspective view of a respiratory mask as shown in Figure 9, shown with a headgear attachment; Figure 18 is a top perspective view of a respiratory mask showing a second variant of the present design; Figure 19 is a bottom perspective view of a respiratory mask as shown in Figure 18; Figure 20 is a top plan view of a respiratory mask as shown in Figure 18; Figure 21 is a bottom plan view of a respiratory mask as shown in Figure 18; Figure 22 is a front elevation view of a respiratory mask as shown in Figure 18; Figure 23 is a rear elevation view of a respiratory mask as shown in Figure 18; Figure 24 is a left elevation view of a respiratory mask as shown in Figure 18; Figure 25 is a right elevation view of a respiratory mask as shown in Figure 18; Figure 26 is a top perspective view of a respiratory mask showing a third variant of the present design; Figure 27 is a bottom perspective view of a respiratory mask as shown in Figure 26; Figure 28 is a top plan view of a respiratory mask as shown in Figure 26; Figure 29 is a bottom plan view of a respiratory mask as shown in Figure 26; Figure 30 is a front elevation view of a respiratory mask as shown in Figure 26; Figure 31 is a rear elevation view of a respiratory mask as shown in Figure 26; Figure 32 is a left elevation view of a respiratory mask as shown in Figure 26; and Figure 33 is a right elevation view of a respiratory mask as shown in Figure 26.