NAC is N Acetyl Cysteine -
Here is what Mount Sinai Hospital in New York has to say:
Acetylcysteine; L-cysteine; N-acetylcysteine; NAC. Cysteine is an amino acid, a building block of proteins that are used throughout the body. When taken as a supplement, it is usually in the form of N-acetyl-L-cysteine (NAC). The body makes this into cysteine and then into glutathione, a powerful antioxidant.
There is, confusingly, another N-Acetyl L-Carnitine, sometimes marketed as NAC, but more properly called L-carnitine. You can read about it on Web MD:
https://www.webmd.com/vitamins/ai/ingredientmono-834/acetyl-l-carnitine
So NAC is an anti-oxidant, and a few studies indicate it also helps to thin mucus. There is no RDA established for it, but many people take one 600mg capsule morning and night.
Sue