Which body part catches fire the easiest?
Hair is keratin which is composed of approximately 51% carbon, 21% oxygen, 17% nitrogen, 6% hydrogen, and 5% sulfur.
When some heat is applied then the chemical bonds which hold those elements in their place in complex molecules, start to break down, and new,
simpler rearrangements are made.
The carbon combines with oxygen to produce CO2, The oxygen in the hair assists the burning, the nitrogen gets converted to Nitrogen oxides and the sulfur combines with oxygen to produce sulfur dioxide and to a small degree with hydrogen to produce hydrogen sulfide - which gives burning hair its distinctive smell.
Hair. However, if you want a self-sustaining fire, the area needs to be fatty tissue. In my case, that is my belly. Once fat starts burning, it is self-sustaining. It is likely ignition of fat (by things like cigarettes) that cause "spontaneous human combustion". (Not a real thing in my opinion.)