Conditonal formating- leave blank cells blank

Hi,
My name is Devon Linn and I am new to this forum. I have read quite a few threads already and am crazy about this site!

I have a, what I imagine is a very simple thing I am trying to accomplish. I have a spread sheet of data. No inputed functions or anything. Just about 28 rows set up in 5 columns of numbers. What I am trying to accomplish is getting everything under 90 to turn red. Now that is easy enough using to do but when I plug in conditional formatting it also takes all the blank cells and turns them red as well. I just want any cells that are blank to remain blank. How would I go about doing this? Please help.

Devon