We assume that when you type that you "reviewed grants for the NIH [National Institutes of Health]" that this is indeed true . . . rather than that you "received" grants from the NIH, which is something entirely different.
The former would be taxable as true self-employment income; and the latter would be taxable as grant, fellowship, and scholarship income (which is reported differently and isn't subject to self-employment taxation).
That much made clear, although it won't alter any dollar amounts on your tax return, and
thus won't change the net amount of your refund or taxes owed, it's
important to at least make an attempt to enter an accurate (or as
accurate as possible) business code for Form 1040, Schedule C activities.
Aside from government statisticians who track such things, it's an
indication to the IRS that you've completed your tax return
conscientiously.
That said, one reasonable "fit" for this activity would seem to be to categorize the review of NIH grant proposals as Code 541990 (Other Professional, Scientific, & Technical Services). Please see the screen-capture image below this
text for the IRS source material where I found this code; simply click
to open the picture.
If you'd like to look through the complete list of
available business codes yourself, you can do so by reviewing the table
found on Pages C-17 and C-18 of the Form 1040 Schedule C instructions. A
courtesy link to that document is provided for you below:
https://www.irs.gov/pub/irs-pdf/i1040sc.pdf
However, if you are comfortable with Code 541990, then you may safely use that.
Thank you for asking this question.
