When there is an accident at a nuclear power plant — if there's an explosion or a leak or it's damaged in some way in war — radioactive iodine is one of the first substances that's released into the atmosphere.
If that radioactive iodine gets into the body, it can damage cells in the thyroid and result in cancer.
You can inhale radiation, or it can get into your body via the skin. But you can't see, smell or taste it in the air. It's an invisible threat.
Some of the worst effects of an overexposure to radiation are thyroid cancer, tumors, acute leukemia, eye diseases and psychological or mental disorders. Radiation can even damage your genes for generations to come.
In the most extreme cases, a high dose of radiation over a short period of time will cause death within days or even hours.
Is it worth taking iodine against radiation?
Our bodies do not produce iodine themselves. But we do need it, so we consume iodine through food or supplements.
You can purchase iodine in the form of a tablet. When consumed, the iodine is collected or stored in the thyroid gland, where it is used to produce hormones. They help various bodily functions and even support the development of the brain.
The thyroid can, however, become saturated with iodine. And when that happens it can't store anymore.
So, the theory is that if you take enough "good" iodine, there will be no room left in the thyroid for any "bad" or radioactive iodine. That radioactive iodine should then simply pass through the body and get excreted via the kidneys.
There is no point in taking iodine as a precautionary measure to prevent against radiation exposure after a leak or attack on a nuclear power plant.
The thyroid only stores iodine for a limited amount of time.
And taking too much iodine — even the good stuff — can be dangerous.
Many people in Germany, for instance, suffer from an overactive thyroid. And health experts advise against taking any iodine supplements unless there is an acute medical reason to do so.
Germany's Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV) says iodine supplements can help after a nuclear power plant accident in a radius of up to 100 kilometers (62 miles).
But the iodine is still only effective if taken when it is needed. Experts say an iodine "block" only has a chance of helping if the good iodine is taken just before or during contact with radioactive iodine.
Cesium, strontium absorbed by the body
The radioactive isotopes iodine 131 and iodine 133 cause thyroid cancer. They are also the isotopes most associated with radiation exposure caused by a leak or explosion at a nuclear power plant.
The radioactive isotopes strontium 90 and cesium 137 are also part of the mix. They settle in bone tissue and likewise increase the risk of cancer.
Our body mistakes these isotopes for calcium. It can absorb and use them in the physiological processes of our muscles and bones. If that happens, the bone marrow can spin out of control.
Bone marrow is responsible for producing new blood cells. And when it fails, it can lead to a blood cancer known as leukemia, which is often fatal.
Radioactive exposure can also damage genetic material in the body.
That is known to have happened after atomic bombs were dropped on the Japanese cities of Nagasaki and Hiroshima at the end of World War II — children were born with deformities after the war.
Long-term effects were also observed after an accident at the Chernobyl nuclear facility in Ukraine in April 1986.
Twenty years after the catastrophe, cancer rates in most of the affected regions had risen by 40%. An estimated 25,000 people in Russia died as a result of having helped clean up the reactor site.
Almost no treatment for radiation exposure
There is hardly any treatment for radiation exposure. What's decisive is whether a person has been "contaminated" or whether the radiation has been "incorporated" into the body.
In the case of a contamination, radioactive waste settles on the surface of the body.
It may sound ridiculous, but the first thing people should do in those cases is wash off the radioactive waste with normal soap and water.
A "radioactive incorporation" is far more dangerous. Once radioactive waste has made its way into the body, it's almost impossible to flush it out again.
Intensity and time
Radioactivity is measured in millisieverts.
Exposure with 250 millisieverts (or 0.25 sievert), over a short period of time, is enough to cause radiation sickness.
To put that in context, Germany's Federal Office for Radiation Protection (BfS) tends to measure an average of 2.1 millisievert in the environment. That's over a whole year.
At a measure of 4,000 millisievert (or 4 sievert), acute radiation sickness starts quickly. The risk of death increases significantly. At 6 sievert, the risk of death is 100% — there is no chance of survival. Death is almost immediate.