http://education.jlab.org/qa/mathatom_04.html

"Ok, here it goes. Hydrogen, oxygen and carbon make up about 99% of the average human. I'm going to cheat a little and leave out the other 1%, which is made up of trace elements (that is, stuff there's only a trace of in the body). Then, let's assume an average adult weighs 70 kilograms. Be sure to keep in mind that the following numbers are based on the number of atoms, not percent of body weight (by weight we are mostly oxygen). A 70 kg body would have approximately 7*1027 atoms. That is, 7 followed by 27 zeros:

7,000,000,000,000,000,000,000,000,000

Of that, 4.7*1027 would be hydrogen atoms, which have one proton and one electron each. Another 1.8*1027 would be oxygen, which has 8 protons, 8 neutrons and 8 electrons. There are 7.0*1026 carbon atoms, which have 6 protons, 6 neutrons and 6 electrons. Now, let's add that all up:"

http://www.foresight.org/Nanomedicine/Ch03_1.html

NOTE: The table doesn't format, there's a table if you follow the link
Table 3-1.
Estimated Atomic Composition

of the Lean 70-kg Male Human Body

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About 62% just from water.

Sounds like we're all very combustible! It's a wonder there are no more instances of spontaneous human combustion than there are.
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