Create a combobox on your form. The rowsource could be a table that has two fields. One is the label, one is the value.
Open the combobox's property window and set its number of columns to 2. Set its BOUND column to Column 2 (the value). Set the column widths to, say, 1"; 0". This will hide the values and only show the labels.
Because the bound column is the second one, when the user chooses a label, Access will use the value (not the label) in its calculations.
Michigan Dept. of Environmental Quality, Air Quality Division