What is radiation?

For most people radiation is an incomprehensible thing. You cannot see or touch it. This is exactly why people are ready to assume the worst when radiation’s impact on human health is concerned.

Meanwhile, radiation is a natural phenomenon. Any resident of the Earth is constantly affected by natural radiation. On the Earth there are areas where the natural background radiation exceeds the planet’s average by several times or even tens of times. The number includes some areas in France, Finland, Sweden, the Altai Territory, coastal territories of India’s southwest, some resorts in Brazil. In the Republic of Altai people get 9.7mSv every year and as much as 7.5mSv in Finland. The human body can handle it even if the background radiation considerably (by tens of times) exceeds the standard value.

How does radiation work?

Affected by radiation, cells of living organisms generate foreign chemical compounds. Radiolysis products “attack” molecular structures of cells, destroying them and disrupting the normal flow of intracellular processes. As a result, the normal operation of cells is disrupted and cells die if exposed to certain dosage. But cells of the human body are capable of “healing” radiation-inflicted damage.

Many serious scientific researches in all the leading countries of the world have been dedicated to studying radiation impact. There are no scientifically proven substantiations of the harmful nature of radiation in small doses (less than 100-200mSv).

According to Japanese scientists, even after the nuclear bombing no increase in cancer rate among people exposed to less than 500mSv was registered in comparison to a control group of people, who had not been affected by radiation. Real consequences for human health have been proven only for high doses – 500mSv and more. The consequences may reveal themselves immediately (radiation injury, disruption of the operation of blood-forming organs, lower immune resistance) or after some time (higher probability of oncological diseases). With doses exceeding 15,000mSv all the irradiated ones die within five days.

Despite all of that, a concept, which says that negative radiation effects can be triggered by any dose of radiation, however small one, is widespread in the world. Therefore, sanitary rules and standards specify the maximum permissible dose from artificial radiation sources – 1mSv.

Although a conservative one, the norm considerably inflates the real risk of getting radiation damage from small doses. Specialists, who work with radiation sources, understand clearly where the border between contrived danger and real danger lies.

How is radiation measured?

In order to characterize the impact of radiation on a substance, the term absorbed dose is used. Absorbed dose stands for radiated energy absorbed by a unit of mass. In the SI system absorbed dose is measured in joule/kg but it has its own name, too – gray(gy). Gray was named after the British physicist Louis Gray, who was one of the early researchers of radiation biology.

The term effective dose is used to describe radiation’s impact on the human body. Effective dose takes into account the fact that different types of radiation (alpha, beta, gamma, proton, neutron radiation) present different biological danger (for instance, 1gy of alpha radiation is 20 times more dangerous than 1gy of beta or gamma radiation).

Different organs and tissues in the human body respond to radiation in different ways: in particular, radiation is way more dangerous for lungs and red bone marrow than for skin and liver.

In order to take into account the biological danger of radiation and the varying susceptibility of human organs and tissues the absorbed dose (gray) is multiplied by relevant coefficients to get effective dose in sieverts (Sv). The unit has been named after the Swedish physicist Rolf Sievert.

All the dosimeters, which are manufactured today, use a gauge marked in sieverts. The term “μSv per hour” stands for “microsieverts per hour”.

Radiation intensity is the dose absorbed over a unit of time. In order to convert μSv/h into mSv/year you have to multiple the former by 8.76.

Let’s use the following examples to show whether the dose of 1Sv is a lot:

- the average effective dose received by a resident of the Earth per year is 2.4millisieverts (mSv);

- the dose received during the annual photofluorographic chest inspection is 0.1-0.9mSv (depending on whether a digital device or an outdated film-based one is used);

- one tomography session guarantees 3-5mSv, full tomography – 15-20mSv (in particular, since 2000 the radiation exposure of the population in the USA has risen from 3mSv to 6mSv per annum exactly due to medical examinations);

- the permissible doze that a human person can get from artificial sources of radiation is 1mSv/year (it does not include the dose generated by medical examinations);

- the permissible emergency dose for professional nuclear industry workers is 250mSv per annum.

Adapted from materials of the Safe Nuclear Industry Development Institute of the Russian Academy of Sciences