I was in an agricultural spares emporium a few months ago and, wandering around, as things were being arranged, I realised that it seemed far warmer than might be expected. Eventually, I spotted the heater - a stainless steel, diesel-fired thing, making the slightest hum.
Even coming in from outside an a frosty winter's day, with a 'clean nose', there had been not the slightest detection of "fumes".
This became even more remarkable when I realised that there was no exhaust to the outside world, it just vented straight into the building from the heat exchanger/combustion chamber pipe. The exhaust was at about head height and it was possible to stick my nose into the slight draught of warm "air" that emitted from it - still no nasally-detectable "fumes".
I looked into it afterwards, as it seemed quite magical to me, but an external exhaust was not deemed necessary, if the building was leaky enough. This was in a converted steel farm building, with a bit of additional insulation - the two chaps working in there, for about six hours before we arrived, showed no adverse symptoms whatsoever.