Nov 102013

Radiation is a danger to life, but this is a radioactive planet and we have quite a few radioactive elements in our bodies. Let us look at just one, the best known one: uranium. How many atoms of uranium in the human body undergo radioactive decay per minute?

The amount of uranium in the average person is 0.1mg although some may have only 0.01mg while others can have up to 0.4mg.

The chemical unit of 1 mole is the atomic weight in grams and for uranium this is 238 grams, and 1 mole of any element contains 6.02 x 1023 atoms. This means that 0.1 mg of uranium, which is 0.0001 gram, contains 6.02 x 1023 x 10-4 atoms which is 6.20 x 1019 atoms. We have more than a billion billion uranium atoms in our body.

Uranium consists of two isotopes; 99.3% is U-238 (half-life = 4.5 x 109 years) and 0.7% U-235 (half-life 700 x 106 years). Therefore, of the 6.20 x 1019 atoms uranium in the human body, 5.98 x 1019 atoms are U-238 and 0.04 x 1019 atoms are U-235.

Consider U-238

The 5.89 x 1019 atoms will reduce to 2.94 x 1019 atoms in 4.5 x 109 years.

Atoms per year undergoing radioactive decay = 2.94 x 1019/4.5 x 10-9 = 0.65 x 1010.

Minutes in a year = 365 x 24 x 60 = 525,600

Atoms per minute undergoing radioactive decay = 0.65 x 1010 / 525,600 = 12,300

Consider U235

Its 0.04 x 1019 atoms will reduce to 0.02 x 1019 atoms in 700 x 106 years.

Atoms per year undergoing radioactive decay = 0.02 x 1019 / 700 x 106 = 0.286 x 109.

Atoms per minute undergoing radioactive decay = 0.286 x 109/525,600 = 544

Total = 12,800 atoms per minute undergoing radioactive decay.

Which sounds a lot, but is a real threat? However, it is rarely mentioned when others protest about our exposure to other body ‘insults’ from traces of ‘harmful chemicals’ in our food, the implication often being that these are the cause cancer. However, radiation is definitely a cause of cancer but uranium is not the only radioactive element in our body. The others are potassium, carbon-14, thorium, etc.

Does the effect of this natural radioactivity outweigh any impact on the body from chemicals? The body’s repair mechanism obviously evolved to cope with this threat from radiation, as it had to do when dealing with natural materials for which it had no use. The uranium is mainly deposited in the skeleton as uranium phosphate, so it probably does little harm in any case.

Have I got these numbers right? If not then please let me know.