Room air is 20%. 0ne Liter increases the FIO2 4%. So 1L =24%, 2L=28% and so on to 6L=44%, do not use a cannula above 6 lmp. Becomes uncomfortable and dries out the nares and can cause bloody noses...
You should wash your cannula with soap and water at least twice a week, and replace it at least once a month. If you are sick with a cold or similar infection, you should replace your cannula...
You could unless maxfax trauma is present. You wouldn't use nasal cannulae in trauma patients as well, you would probably give 15L/min O2 via a non-rebreather mask until stabilised, sats aim for...