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Uranium in cells

Dear Reader,

I was reading recently about the biochemistry of uranium inside living cells, I found the work of Diane M. Stearns, she works on the effects of uranium and other metals on the DNA inside living cells. Now before I get going I would like to warn you that a lot of odd things have been written about uranium.

1. Uranium is not man made, it is in almost all soils as a result of the actions of nature and not the “hand of man” (or even the master/slave grabber thing of man). Thus as a result it could be very hard to prove if man has (or has not) added uranium to the environment through his actions.

2. Uranium is not intensely radioactive, I always use the following test in my mind for “intense radioactivity”. The test is to ask myself the question of “would I be willing to pick up a pencil sized rod of the material ?” with only a thin surgical glove on my hand. In the case of natural uranium I would say yes, the radiation dose to my hand would be very low. In the case of used uranium dioxide fuel from a nuclear reactor the answer is a big no. But the reason why I would never dream of going near a pencil length of used fuel is not the uranium content. The reason is that the high concentration of fission products such as Cs-137 would make the radiation level near the fuel very high. So we can rest assured that U-234, U-235 and U-238 are not gram for gram such potent radioactive menaces as Ir-192, Ra-226, Sr-90, P-32, Co-60 or Cs-137.

3. The long half life of uranium does not make it dangerous, a long half life means that the chance of a given atom decaying during a second is low. Both things with supershort and superlong half lives tend to pose lower threats to the public than things with medium term half lives. For example the nitrogen-16 formed in a water cooled nuclear reactor by the np reaction from oxygen-16 is not a credible threat to the public as it has a half life of 7.13 seconds. While a BWR or PWR might have nitrogen-16 activity inside it during normal operation this activity is not a threat to the general public as within two minutes the activity has dropped by a factor of 118451. So as a result if the reactor shuts down during a loss of cooling accident then the nitrogen-16 will rapidly vanish from the coolant.

On the other hand bismuth which is used for making Pepto-Bismol which is a bright pink over the counter medicine, I think it is about the same colour as a breast cancer ribbon. It has been found that bismuth-209 (natural bismuth) undergoes alpha decay with a half life of years 2 x 1019 years. This compared with uranium-238 which has a half life of 4.5 x 109 years. This does not mean that your jar of Pepto-Bismol is a radioactive monster which will endanger your health. According to the Pepto-Bismol site a 15 ml tea spoon of the medicine contains 262 mg (0.262 grams) of bismuth subsalicylate. A bottle of the pink medicine contains 354 ml (12 fluid ounces). Lets make an estimate of the bismuth radioactivity in the bottle.

The formula weight of bismuth subsalicylate is 361 grams per mole, so the 15 ml tea spoon contained 726 μmol of bismuth. This means that the whole bottle contains 17.13 mmol of bismuth. This is 1.03145 x 1022atoms of bismuth, for those of you without a science background who were not paying attention in their maths lessons this is 10314468254847600000000 atoms of bismuth. Now I want to tell you do not ever let anyone try to frighten you with a large number of digits.

Now we have a nice and useful equation for predicting the number of radioactive decays per time period it is

A (activity) = N (number of atoms) x λ (decay constant)

Now lets work out the value for the decay constant (λ),

λ = ln (2) / half life

As the half life for the bismuth is 6,3072 x 1026seconds, the decay constant will be 1,09898 x 1026 s-1. Which means that the number of radioactive decays per second in our medicine bottle will be 0.00001134 decays per second (or 11.36 μBq). This means that if we had a perfect ability to detect the alpha emission of the bismuth in the bottle we would have to wait on average 24.5 hours between the radioactive decay events. This is a far lower rate of events as you will get in a typical place on earth due to the background of cosmic rays and other events. To observe this low level of radioactivity would require a super clean lab with very low background detectors.

So as a result do not get nervous just when you see a long half life, what you should ask yourself is how much activity is present. The half life will tell you how long the radioactivity will last but a long half life does not make a radioisotope dangerous to your health.

Now back to uranium, it is important to understand that some shorter lived isotopes of uranium such as uranium-232 (half life 69 years) do emit so many alpha particles per second per gram that they may well pose a threat on the same scale as Pu-238 to your health, but for normal uranium we need to think about the chemical effects.

It has been shown that uranium increases the oxidative stress in living things, oxidative stress has been blamed for inducing cancer and causing the body to age. I always thought that Micheal Jackson’s reported habit of sleeping in an oxygen tent might have increased the rate at which his body aged. I think that prolonged exposure to high pressure oxygen for a prolonged time is likely to speed up aging or make something else go wrong inside a human.

It has been shown that uranium and other metals are able to react with things like hydrogen peroxide to create free radicals such as the hydroxyl radical. The hydroxyl radical is like a molecular scale drunken yob, it has little respect for its surroundings and it will attack anything which has the bad luck to cross its path. In the same way as the drunken yob is a threat to nuns, old ladies, policemen and bar men the hydroxyl radical will attack close to any organic molecule. It can attack fats, proteins and DNA with equal wild abandon. The good news is that after attacking a molecule the hydroxyl radical is normally converted into a molecule of ordinary simple harmless water, so unlike the yob it will not beat up a series of molecules in your body.

One classic way to make hydroxyl radicals is to react iron(II) sulphate with hydrogen peroxide, I used to use this reaction to generate hydroxyl radicals and use them to make chemical reactions happen for me. In the same way it is possible to generate free radicals using uranium(VI) inside cells. The free radicals will then damage the molecules inside the cell which include the DNA. A paper by A.C. Miller, M. Stewart, K. Brooks, L. Shi and N. Page, Journal of Inorganic Biochemistry, 2002, 91, 246-252 states that the majority of the carcinogenic activity of depleted uranium is due to the chemical effects of the metal. The paper points out that the concentration of 8-oxo-7-hydro-2-deoxyguanosine (a marker for oxidative stress related damage to DNA) is increased when either nickel, iron or uranium is added to the cells. This is good evidence that the uranium is able to damage the DNA.

One last point, it is important that when considering the question of “is uranium bad for your health” to be very careful about using health / disease data from humans. Most uranium miners from the bad olde daze have been exposed to radon, radon daughters and dust/smoke. While V.H. Coryell and D.M. Stearns include in the introduction of one paper on chemically induced mutations caused by uranium a mention of uranium miners, they do quite rightly include a caveat that these miners were exposed to higher than normal levels of radon.

My own view of radon is that it is a much nastier threat to the lungs when it is combined with smoke from either diesel engines or cigarettes. I was taught years ago that the most important health protection action which relates to lung cancer in a uranium mine is to enforce a smoking ban inside the mine. A strong synagism exists between radon and smoking, either is bad for your health but when taken together the effect is very dire.

Becuase of the radon exposure to uranium miners I think it is important to be careful when trying to apply the lessons from uranium mining to people who have been exposed to uranium only.

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