Level 19 is the nineteenth level of the Main Tunnel. This level is one of the easiest to gain 'A Breath of Fresh Nothing', where you leave the tunnel, float up towards empty space, and somehow return to the tunnel, and land safely.
Using the Skater, you can constantly hit the crumbling tiles and reach the end of the level. With the Runner, just follow the level.
Or you could the Runner and just play normally, or float out of the tunnel and return. You will have skipped quite a bit of the level by then.
'A Breath of Fresh Nothing' can be done very easily on this level. It awards 100 power cells. I recommend choosing the Runner, floating out to the right at the first gap on the right in the picture, and floating in when you have floated upside-down.