I believe diesel and other petroleum products contain benzene and other aromatic compunds like toluene and xylene at very low levels. Most such products are not a single chemical compound but contain a mixture with a variety of compound that distill within a given temperature range. Effort is made to minimise what is in those products, for example, sulphur content in diesel is now much lower than it used to be, terta-ethyl lead is onger in petrol (admittdly this was added rather than being present initially).
The factor that will make most difference to what is produced when burning a fuel, regardless of wlhether it is gas, liquid or solid is how cleanly it is burnt. Burning it slowly in a high oxygen content and at high temperatur will minimise organic compounds produced with most carbon then coming out as CO2, but we have all seen diesel wagons emitting plumes of black smoke, or jet aircraft taking off leaving multiple plumes if smoke. One problem with burning at a high temperature with plenty of air is that whilst most of the carbon will be converted to CO2, the high temperaure will also result in more nitogen oxides being produced which are also not good for health.
To ensure the minmum amount of toxic compounds are emitted, the burner needs to be serviced regulaly and tested for emissions, it also needs an adequate air supply and all exhaust should be to outside. If you are burning anything indoors without adequate ventillation the amount of toxic compound produced will be significantly higher that it should be. You referred to burning natural gas as producing benzene - probably from a US report about a survey of 80+ homes in california, that same survey also highlighted the issue of adequate ventillation. Strangely, that paper indicates induction hobs producing no benzene but what I think they said was electric thermal coil did - not sure what they meant as if they meant normal resstive heating elements where did the caron come from?
You refer to biofuel being better and having read it burns 90% cleaner - can I ask where? It an internal combustion engine it has been shown that biodiesel can produce lower levels of soot and carbon monoxide, but it does result often in higher levels of nitrogen oxides. But burning inside an engine with excess air and high temperature and pressure can have very different results to burning at atmospheric pressure and lowish tempertures as with a hob. A diesel engine relies on the high pressue and temperture to trigger combustion not a spark of naked flame.
If you need to burn something to keep you warm, it would be better to have that unit outside of your living space. Oil boilers, for example, often come as outside models so that risk form fumes indoors is significantly reduced. Wood chip boilers likewise are ofen designed to be in an outbuilding (for multiple reasons though).
Even in vehicles the engine is outside of the passenger space - or at least well separated (ships for example).
I don't think you can work on generalisations about whether the fumes from your leaking heater/hob are worse or better than burning wood would be, there are just too many variables. You would need to measure both approaches and compare the results. I know some results of places with Wood burning found that the rooms with the burners were significantly worse than levels permitted outside, I would expect the same might occur with your heater/hob, but the main problem is ventillation in all these situations - again move the combustion out of the living space makes a huge improvement.
As to your question about mechanics, I dont know the answer, but assuming you are referring to those working on cars, vans etc. most of their work is not done with the engine running, and when the engine is running they should be using an exhaust gas extract system even if it is a simple out the door exhaust hose. Back to ventillation again!
