SMC Backgrounder on radiation

This is a backgrounder preared by the Science Media Centre of Canada in response to the unfolding situation at the Fukushima plant in Japan. We hope it will be useful when dealing with some of the technicalities of radiation.

Because the situation continues to unfold, we will update the backgrounder periodically if needed.

How do scientists measure radiation?

The biological effect of radiation in the human body, called the dose equivalent, is measured in an SI unit called the Sievert (Sv). The basic idea of this unit is to approximate the degree of damage to the cells of our bodies, proportional to the number of ionizing particles that pass through them.

The nuclear incidents in Japan are reporting dosages using Sieverts as the unit. Most doses that humans are exposed to are smaller than one Sievert – they are in the range of millisieverts (thousandths (0.001) of a Sievert, or mSv), or even microsieverts (thousandths of a millisievert, or 0.000001 Sv).

Often, exposures are described by millisieverts per year, as that’s the scale of background exposures. However, when reading about doses, check if the dose is given as a dose rate (microsieverts or millisieverts per year, millisieverts per hour), or a single dose (microsieverts or millisieverts).

The energy absorption can also use the old unit Roentgens equivalent man (rem). 1 rem = 0.01 Sv = 10 mSv)

What do the numbers mean?

According to the UN Scientific Committee on the Effects of Atomic Radiation, the average person receives about 2.4 mSv (0.0024 Sv) of radiation a year through background sources like cosmic rays, soils, and food. In addition, the average person receives about 1 mSv/year from medical X-rays. This can vary widely depending on where you live. In North America, we receive about 3.5 mSv a year.

One Sievert, in a single dose, would cause radiation sickness and nausea in an unprotected person. If people are exposed to 1-2 Sieverts, roughly 1 in 20 people would die within 30 days if they did not receive medical treatment. In fact, radiation sickness can occur at doses as low as 100-500 mSv.

According to the US Nuclear Regulatory Commission, if each person were exposed to 3500 to 5000 mSv (or 3.5 to 5 Sieverts), we would expect 50 per cent of a population would die within thirty days after a dose of this magnitude to the body.

Links to radiation dose charts:

So how much radiation is being released from the Fukushima plant?

Up to now, most radiation released has been in the form of radioactive gasses that can spread through the air, driven primarily by winds. The latest reports show levels harmful to human health at the site itself, though such levels drop off rapidly as distance increases – Japanese officials reported maximum levels of 0.33 millisieverts per hour 20 km from the plant on Wednesday, March 16. The levels will drop off rapidly with time after each explosion, as well. The situation is unstable, and we’ll continue to monitor.

What kind of radiation is being released?

Different elements have different half-lives. A half-life is the time it takes for half of a sample of radioactive material to decay to daughter isotopes. The shorter the half-life, the more intense the radiation, as particles are being emitted at a greater rate.

Radioactive gasses such as iodine, cesium, xenon and krypton could be released in the steam and emissions from the Fukushima plant. All of these undergo beta decay, where a positron is emitted from the atom as it decays. These particles being absorbed by the body are where the energy comes from. Other forms of decay include gamma decay, in which a gamma ray is emitted, and alpha decay, where an alpha particle combination of 2 neutrons and 2 protons is emitted (essentially an ionized helium atom).

Isotopes could include:

Iodine-131 – has a half-life of about 8 days. Half-life indicates that half the amount of Iodine 131 will disappear in 8 days, and half of that amount will disappear in another 8 days, and so on.

This isotope can be taken easily up by the thyroid gland, and those at risk of exposure often take potassium iodide pills to saturate the thyroid with non-radioactive iodine. This prevents the gland from taking up the radioactive isotope when and if the person is exposed. Iodine-131 is sometimes used clinically for medical imaging scans. Potassium iodide should only be taken upon the advice of a physician, when there is potential exposure to radioiodine, as it can have some health effects if abused.

Cesium-137 – has a half-life of about 30 years. This isotope can be taken up by the body and increase cancer risk.

Xenon-135 – has a half-life of about 9 hours. This isotope is relatively harmless to humans.

Xenon-133 – has a half-life of about 5 days. This isotope is relatively harmless to humans.

Krypton-85 – has a half-life of about 10 years. This isotope is relatively harmless to humans.

In the event of fire or damage to the rods themselves, or a full meltdown, solids like uranium and plutonium could also be ejected into the air from any explosion if the containment structures were breached. These substances, though solid, undergo alpha decay. This kind of radiation is stopped by clothing or skin if it is kept outside the body. On the other hand, alpha decay is more damaging than beta decay when inside the body. These metals also persist for a long time in the environment, and may be stored for years in human bone if absorbed into the body, so exposing a person for years to come. It would take a huge explosion, beyond what we have seen, to eject these heavy metals, and they would quickly drop to the ground very near the plant.

How do they measure radiation in the environment?

Monitoring will include air samples and swab tests to detect the concentration of radioactive particles, at the plant and throughout the surrounding area. International agreements have developed a worldwide network of radiation monitors and detection equipment as part of the Comprehensive Test Ban Treaty and non-proliferation initiatives. As a result, very little goes on that is not carefully monitored and tracked in terms of airborne radioactivity. It would not be possible for a major release in Japan to go undetected by international scientists. When the Chernobyl accident released radioisotopes into the environment, Swedish scientists detected the event very quickly, and brought it to the attention of the world.

What are the chances of BC receiving dangerous doses of radiation?

If radioactive gases are released into the atmosphere in Japan, they could blow across the Pacific. In Japan, the prevailing winds blow west-to-east, carrying emissions out over the ocean. As this happens, it will disperse and become quite diffuse.

According to Health Canada, “Given current wind patterns, it would take several days for any radioactive material to reach Canada. Based on the information available, it is anticipated that the amount of radiation reaching Canada, if any, would be negligible and not pose a health risk to Canadians.”

Do we need to worry in Canada?

At current levels, any radiation released from the plant would be negligible if it reached Canada.


Journalists should take note that news reports, medical literature, and background information can describe radiation and its effects in different ways. The nuclear activity given off by decaying radioactive particles are measured in becquerels (1 decay / second). The radiation absorbed dose, or rad, is defined as a unit of energy per mass: The gray is defined as one Joule per kilogram of absorbing mass (usually a human body), and replaces the old rad.

One Sievert is defined as the gray, multiplied by a Quality Factor, a number that’s specific to the type of radiation to account for the different cell damage potential of alpha, beta, and gamma decay. (Alpha particles ionize more densely than beta particles and this causes more damage to a cell in the same volume).

**The SMCC will update this backgrounder as the situation unfolds and reliable information becomes available.


Dr. Tim Meyer, TRIUMF

Dr. Adriaan Buijs, McMaster University

Dr. John Luxat, McMaster University

Dr. Dan Meneley, UOIT

Dr. Fiona McNeill, McMaster University

Report of the United Nations Scientific Committee on the Effects of Atomic Radiation to the General Assembly (

US Environmental Protection Agency (

Health Canada