Heating

Wood Stoves
I really like the idea of getting off the grid as much as possible, and so I'm attracted to wood-burning stoves for that reason. But, because of the poor design of just about all stoves, the post-combustion wood smoke is very toxic. The particulate matter has been linked to brain impairment, cancers, lung diseases, etc. See Families for Clean Air. For a safer wood-burning stove, consider the addition of an "afterburner," which is an inexpensive contraption in the flu that burns the residue and black carbon making the fire about 99% clean. Pellet stoves and "rocket stoves" are also supposed to burn very clean. I have no experience with either.

Propane
Propane gas is extremely toxic. A lot of MCS/ EI people can't tolerate any gas or propane. But, if the heating device is airtight inside the home, it won't be a problem unless the exhaust fumes find their way back into the house. Inevitably, a small percentage probably will, but how much depends on your particular set-up and environment conditions.

Electric
Although usually the most expensive option in terms of monthly cost, electric is the most tolerated heating source by people with MCS/ EI. A ceramic heating element is preferred as well as a metal casing. Heaters that contain fans which push out the heat often do not work because it pushes out dust along with it. Try to find an older model that has been used for some years, since these have already done their out-gassing.