That boy could alter all vectors be they motion, heat, or electricity, so he would not be injured even if he were directly hit by that last resort of a nuclear missile. He would just reflect the shockwave that would blow everything way, the heat that would scorch everything, and the neutrons and radiation that would kill everything.

Toaru Majutsu no Index vol.3 ch.2 pt.5

What the heck, a lovers' squabble? Accelerator thus redirected that excessive noise, the air vibrations away from his ears. If he had done that a few seconds later, he would have heard the familiar painful cry of that Level 0.
The reflection ability could be done unconsciously through some simple calculations. He just needed to calculate the minimum required forces (gravity, air pressure, light, oxygen, heat, sound wavelength, etc.) and redirect everything else. If he really redirected all forces, he would be tossed out of the atmosphere because gravity was redirected.

Toaru Majutsu no Index vol.5 ch.1 pt.1