From BS5499-1:2002 (British Standard for Safety Signs)
12.1 Font
The font used shall be either:
a) Helvetica medium, as shown in Figure 9; or
b) another sans serif font with the following characteristics:
1) widths of strokes throughout the letters constant within 10 %;
2) widths of strokes at least one sixth of the letter height;
3) width of the lower case letter “x” not less than 70 % of the letter height;
4) not condensed, expanded, italic, script, outline or shaded;
5) letters individually formed and not joined together.