
As you can see here, the helmet is visibly under the suit. On the right the suit levels are obviously combining all of them.
The level/heal increases as you add more, while fire and radiation decrease.
Original levels:
Hazmat: Lvl 32.4 Hl 20 Fr 4 Rad 48
Boots: Lvl 13.5 Hl 12 Fr 0 Rad 0
Chestplate: Lvl 18 Hl 12 Fr 0 Rad 0
Helmate: LvL 13.5 Hl 12 Fr 0 Rad 0
Notice I excluded the leggings, because anything after the suit doesn't get added, and putting another piece before the suit removes the suit.
Logging in and out doesn't change anything.
To reproduce this, just put any 3 pieces of armor in the first 3 slots and put the hazmat suit in the 4th (or any of the remaining) slot.