You should not be breathing anything at all. I have an electronic CO detector and it reads zero during normal ops. Sometimes, in slow flight, you get 10-30ppm and it goes off, and sometimes it picks up the exhaust from the plane that landed before me. But during flight, the reading is zero.
turbines start to make sense at 400BHP.
That's perhaps true for the 1960s Allison heli engines whose business Rolls Royce bought, and are trying to flog a 450HP one with a prop shaft on the end. There are some interesting developments now -
example. I have no financial interest in that venture but know the people behind it well.
But no turbine will match a piston engine for SFC. Even the huge and state of the art engines on a 787 only just about match a piston engine for efficiency.