Foods highest in sulfur: Garlic, onions, and all the cruciferous vegetables (broccoli, kale, cauliflower, cabbage, collards, watercress, etc) Foods highest in protein with extra cysteine: Eggs, meat (need variety) red peppers, garlic, broccoli, brussel sprouts, oats, wheat germ, etc
This makes it easier for the body to clear mucus from the respiratory tract, which is why NAC has been used in clinical settings for respiratory conditions
You can consider it like vitamins or supplements you take for immune function or energy
(4) M Graf, H Christen, H J Tobler, P F Maier, and G A Schoenenberger (1981) Effects of repeated DSIP and DSIP-P administration on the circadian locomotor activity of rats Pharmacology, Biochemistry and Behaviour, 1981 Nov, Volume 15 (Issue 5), Pages 717-21