Hi Betty - I'm not sure about brain fog, but NAC is a pretty good anti-inflammatory, and it has protective effects on the lungs, liver and a few other organs.
It is generally regarded as safe in doses up to 1500 mg per day. I use it for mucus thinning and lung inflammation and it has worked well for me. I found dividing the dose 650 mg in morning and evening, works better than once a day.
My pulmonologist and primary have agreed it is okay for continuous use.
Sue