Just to clarify a few points, tracers do not contain WP, but most use a Strontium compound (no, I don‘t know what the **** is in Strontium either), although there are others (like magnesium).
As Jungle said, willie pete ignites on contact with oxygen, and there is of course air in the cartridge itself.
Trace compound is ignited by the powder, and is contained in the rear of the bullet, not the tip. Tracer bullets are long for calibre, so that they weigh the same as ball rounds (the light tracer material reduces density and the BC).
Unless ignited by fire, tracers are stable, enough so that they can be pulled from mil ammo and are sold as seperate components (try ammoman.com)
The odds of tracer inflicting burns or damage above and beyond that of ball rounds is minimal, given the very small amount of compound used and the speed of bullet travel. If one impacted nearby and broke apart and the tracer pellet flew out and landed on you, you may get a burn like if someone put a cigarette out on you, but that‘s about it. Even shooting propane tanks with trace won‘t reliably set them off (don‘t try this at home).
. 
. 
. 
