Deadly Dad

Actually, the solution to dealing with weeds too tall for a reel mower to deal with is pretty simple: pick up a cheap grass whip, and keep it clipped to your mower.  A quick walk around/knock'em down before you start mowing takes care of the problem right quick.  Of course, you could always DIY a hedge trimmer blade onto the front of it, powered by the mower's gearing, or just find/build one of these.

A bit of a drive-by posting, but you should find it helpful.  The Aprovecho Research Center has a lot of publications available.  (Which, considering they invented the rocket stove concept, is unsurprising.)  Quite a few of them discuss CO emissions, though I didn't look through them enough to see if any other gases have been examined.

If you are interested, a companion technology is 'ceramic water filtration', which can be made out of nothing more than clay and coffee grounds, yet is efficient enough to remove even cholera.