I think the brain is is the most important part of Humans. Any advances we make in science is attributable to the brain. So to answer your question I think the brain is what might limit human life.- If thats makes sense..
I think limits to me means, how little oxygen, carbon dioxide, nitrogen, food and water/minerals and vitamins we can survive on and yet function. I don’t think we can say overall the limits for all these factors combined but the nutritional studies for the 1930s-1960 showed us the limits for the nutrients by themselves. Oxygen, carbon dioxide and nitrogen are quite difficult to study and it appears that different genes allow us to use oxygen more efficiently, so there is a personal genetic component and fitness component too. We know for certain that four days without water is the limit for sustaining human life but after 1.5 days most people are delirious and cannot function. The problem with studying these limits is that humans might die in these studies and therefore it is not ethical/right to do this.