Exactly what A4BG says.
Ballistic Mask: this armor is worn on the face and held
by comfortable straps, like an old-fashioned hockey mask. It is
most often worn by combat participants in the Desert Wars for a
number of reasons: they are cooler in the desert heat than helmets
while offering comparable protection, they look great on camera,
and they help protect the face, which is the most marketable
part of a successful soldier’s anatomy. They come in a variety of
custom moldings and paint jobs. Ballistic masks can take vision
enhancements as well as any modification a helmet can take, and
like other helmets, their armor bonus stacks with the rating of
other worn armor.
Now, if you want to argue that the Ballistic Mask's description of "held on by straps" precludes it being part of a "full body suit", that's fine; doing so would be debating the finer points of the rules. But don't bloody accuse me of cheating just for having a different interpretation of the "full body" requirement (specifically, I'm of the opinion that the chameleon suit, since it works WITHOUT a helmet, must include a hood, making up for what coverage the mask lacks).
...
Oh, and also? I never even mentioned Chemical Seal myself, at all, in the first place. I was only talking about armor ratings, and pointing out that any mods the OP wanted for their helmet (vision, respirator, etc) would also be possible in the mask.
