not immune to oxygen deprivation

Accelerator gritted his teeth, but then he noticed that he was short of breath. At first he
thought he had used up too much oxygen by talking while running around, but it was
too odd for that. A sharp stench sent off alarm bells in his head.
“Tonight is a windless night.” Misaka Imouto’s voice reverberated throughout the still
air of the switchyard. “As such, Misaka may have a chance of winning, calls out Misaka.”
Accelerator checked on his surroundings again. Misaka Imouto continued to run away,
she was firing electrical attacks around him, he was oddly out of breath, and he could
reflect any direct attack.
(Ohhh, I see. Ozone, huh?)
The oxygen in the air could be broken apart with electricity. Oxygen molecules were
normally formed from two oxygen atoms, but once the two oxygen atoms broke apart,
they had a disposition toward connecting together in threes as ozone.
Oxygen and ozone were two different things. Breathing it in would not satisfy one’s
lungs.
And as was obvious from its use in sterilization, it was toxic.
No attack would reach Accelerator, but that did not change the fact that he was a human
that breathed in oxygen and breathed out carbon dioxide. If all the oxygen were
removed from his surroundings, he would suffer from oxygen deprivation.

- Toaru Majutsu no Index Volume 3, Chapter 4 Part 5