A tale of two cows

I hold the view that the cesium isotopes which have been released by the accident in Japan will not show bioaccumulation in the same way as dioxin, DDT or PCBs will. We will consider the organochlorines first using a simple food chain. Plants, herbivores and carnivores. We assume that the animals will keep on growing for ever.

We have a food chain, grass is eaten by a cow, a human then eats the cow.

If we assume that the water insoluble organochlorines can not be metabolised by any of the organisms then the concentration will rise as we go up the food chain. This is because the majority of the food which is eaten by an animal is used to supply fuel for the animal’s energy needs and that only a little (lets assume 10%) is stored or used to make new tissues of the animal.

So for the carnivore to grow by one kilo it must eat about 10 kilos of the herbivore, to grow the ten kilos of the herbivore it must eat 100 kilos of plant matter. If the original plant matter contained 1 mg per kilo of the organo chlorine than the plant eater will contain 10 mg per kilo while the meat eater will contain 100 mg per kilo of body weight, so as a result the poison is being accumulated in the organism at the top of the food chain.

If the carnivore stops growing and reaches adulthood then it can get even worse as the carnivore will continue to absorb the poison, as a result the carnivore will keep on getting more and more of the organochlorine in its body until it finally drops dead (poor thing). As an alternative to dropping dead the carnivore may have difficulty reproducing, for example the shells of birds which have been exposed to some organochlorines as too thin to allow the birds to breed.

OK it is a sad story but now we have to consider radioisotopes, some radioisotopes enter into the metabolism of animals and plants. For example lets consider cesium-137. The cesium will act as a potassium mimic and as a result it will enter many body tissues.

But the good news is that as cesium is water soluble it is possible for the body to excrete it. It is important for use to understand the idea of biological halflife. This is the time required for an organism to lose half of an element.

For radiological protection work it is important to know both the physical (radioactive) half life and the biological half life. By using both we can get the effective half life of the isotope.

1 / t_effective = 1 / t_physical + 1/t_biological

For cesium t_physical = 30 years while t_effective = 70 days (in humans), so by rearranging the equation we get

1 / t_effective – 1 / t_physical = 1/t_biological

Thus we can calculate t_biological to be 70.5 days in humans, if we assume that the biological half life for cesium is the same in cows then we can do some maths to help us understand things.

If the loss of cesium from an animal is a first order process (the rate of loss of cesium is proportional to the amount of cesium in the animal) then

Rate of cesium loss = k . amount of cesium in animal.

K = ln 2 / t_biological

So each day the cow will lose about 1 % of the cesium which is inside it. This is a useful thing to know.

If a cow absorbs 100 Bq of cesium each day then the cesium will at first build up in the first cow (black), but after about a year the activity will stop going up. It will reach a maximum of about 10 kBq. In a second cow (blue) which is being treated with Prussian blue the biological half life for cesium is shorter (thirty days). This second cow reaches equilibrium sooner and only reaches 4.4 kBq of activity.

Two cows being fed radioactive grass

So clearly the use of Prussian blue has lowered the dose to the cow and to anyone who eats the beef from the cow.

An alternative is to feed the cow clean food for some time before slaughter, the idea is that the cesium can be removed from the cow to make it safer to eat. In the following graph two cows have been fed on radioactive food, after 300 days both are given a non-radioactive diet. Cow one is a normal cow while cow two (blue) is given Prussian blue in its diet. Below is the graph of how much radioactivity is inside each of the cows.

Two cows fed on the same diets, one is given prussian blue the other is not.

By the way humans can use Prussian blue to get cesium out of their bodies, but it is important to use the medical grade not paint grade of the pigment.