Understanding Radioactivity Harm Thresholds (in plain English)

Do you understand the nuclear radioactivity levels resulting from the Japanese nuclear plant crisis?

Radioactive Warning

Radioactive Warning

Can you figure out how much radioactivity is dangerous?

This article explains the three major ways radioactivity is measured, to help you compare the numbers reported by the media – so you can make up your own mind whether the amount of radioactive material leakage is dangerous.

Measuring Radioactivity

Radioactivity is measured in three fundamentally different ways: Energy emitted, energy absorbed (by a human body), and a calculation of the harm due to the different kinds of energy absorbed.

1. Emitted Energy (Becquerel): The first way is to measure the emitted energy of the particles or the radiation. The international standard for radioactive emitted energy rate is called the Becquerel.
One Becquerel means one nuclear decay per second.
One Curie = 37 Billion Becquerels.

2. Received/Absorbed Dose (Gray): The international standard unit for absorbed radioactivity dose is the Gray. One Gray = one Joule of ionizing energy absorbed by one kilogram of water (~body tissue). Death can occur with a radioactive dose of as low as 1 Gray. (One Gray ~ 100 Rads.) Hair loss typically occurs at 1/10th of a Gray.

    The Dose Rate is measured using Gray-seconds, or Gray-minutes etc.

3. Body Harm (Sievert): Because the different kinds of radioactivity (Alpha, Beta, Gamma) do different amounts of damage to different parts of our bodies (bone, brain, skin), we need another measure. A third way is to estimate the harm done to a human body by the different kinds of radioactive energy that a body absorbs. The international standard unit for the harm from the different kinds of radioactivity is the Sievert. About 5 Sieverts is typically fatal. (1 Sievert = 100 Rem).

    Sievert Warning: This third measure, even though an international standard unit, is made using a table of estimates. While I believe this is a useful measure using educated guesses, it uses fuzzy data (technical term). The Sievert is not a defensible physics measurement like the Becquerel or the Gray.

Averages Misleading
The problem with all these units of measure is that they are averages. This means a measurement may seem low enough to seem safe, yet a hot spot in water or air can have a colossally higher radiation value that could harm or even kill you.

For example, a few tiny bits of plutonium in the air could have a very low number of Becquerels per cubic meter, yet if just one of those specs of plutonium got into your lungs – it could cause lung cancer.

Harm Thresholds, Acute/Immediate
Nausea, vomiting, headaches, fatigue, fever skin reddening symptoms may occur with short term radioactive doses as low as 35 rad or 0.35 Gray.

Long Term Low Level Radioactivity Harm (Generally Cancer)

There are so many variables involved (e.g. external vs internal radioactivity) it is difficult at best to make predictions of harm from low level radioactivity.

What is clear is that the more and longer the dose – the greater the probability of cancer.

Linear No-Threshold Established

The scientific dispute is settled as to whether there is a threshold, a lower limit, below which there is no increase in cancer. The US National Academy of Sciences, the US Congress’ National Council on Radiation Protection and Measurements and the United Nations Scientific Committee on the effects of atomic radiation (UNSCEAR) all agree that “linear no threshold is the appropriate dose response model.”

While DNA has mechanisms to easily repair single strand breaks, Radioactivity typically breaks both DNA strands making the repair processes guess at what the original DNA was. Unless it guesses right, this causes mutations.

With all that in mind, the maximum dose you should get in a year is 20 milliSieverts. Since 350 millisieverts was the amount triggering Chernobyl relocations – I suggest a maximum of 200 millisieverts in a lifetime. Click here to see some comparative examples. (See Sievert warning above.)

Compared to Legal Limits:

Ocean water has a normal radioactivity of about 12 becquerels/liter (emitted). The Japanese legal limit for infants is 100 becquerels/liter of drinking water; for adults and other children, the legal limit is 300 becquerels/liter. Europe set a limit of 500 becquerels/liter of drinking water.

In Tokyo on March 23 2011, drinking water was measured at 210 becquerels/liter from Iodine 131 – above their legal limit for infants.

Water in a trench outside Fukushima Nuclear Plant # 2 had radioactivity readings exceeding 1 Sievert. (See Sievert warning above.)

Eating a typical lunch would provide you with at least 100-200 becquerels.

Land/Soil Hot Spots:
Cesium 137 (very long half -life), was measured in one Japanese village near Fukoshima at at a thousandth of a Curie / square meter (3.7 million becquerels/m^2).

    Chernobyl ordered mandatory evacuation when the Cesium levels reached about half that amount: 1.5 million becquerels / square meter.

Iodine-131 in soils 25 kilometers (15 miles) from the plant showed “25 million becquerels per square meter.”

Another Iodine-131 hot spot measured 57 million becquerels per square meter. (While alarming, remember Iodine-131 has a half-life of only 8 days. So in about 6 months its radioactivity is trivial. But keep your eye on the Cesium readings.)

Radioactivity close to the Fukushima reactors was measured at 400 milliSieverts / hour. (See Sievert warning above.)

PS There is one more useful unit that describes the amount of radioactive that will kill you in one day. While roughly 5 Sieverts or 400 rem, One “Cheney” is the amount of radioactivity which will kill a healthy adult human in 24 hours or less.

(Just kidding . . . about the Cheney)


Radioactivity Primer by the Center for Disease Control and Prevention (or CDC), a United States federal agency under the Department of Health and Human Services

Are nuclear reactions restarting at Fukushima?







Beware of the myopically misleading claims by Wade Allison in his article “We should stop running away from radiation.” While the article makes a few good points, it fails to recognize several logical fallacies including the use of average measurements.



This high quality news and analysis is free of charge to the public even though it costs considerable time and money to produce it. We would like to continue to provide this service for free that brings you the best available science, news and the highest quality research and analysis.

You can help right now by making a donation.

Thank you,
-David Dilworth


This entry was posted in Physics, Radioactivity and tagged , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *