Safety threshold for radiation

[Shukie]

I've heard talk of this safety threshold for ionizing radiation, whereby if the dose is sufficiently low enough, it would produce no harmful side-effects. Can someone explain to me the reasoning behind this?

The way I see it, every single ionizing particle that passes through a cell has a chance of damaging it. The cell-repair mechanism is imperfect, so even a single particle can damage a cell beyond repair, causing cancer. It's not as if at low radiation doses the particles won't have enough energy to penetrate a cell or something right? So no matter how low the dose, there will always be a risk.

 

[mathman]

Part of the reasoning has to do with the fact that on a day to day basis we are subject to natural radiation from cosmic ray secondaries as well as natural radioactivity in the soil, concrete, etc. Also physical exams usually involve exposure to X-rays.

 

[Hologram0110]

Sufficiently low energy radiation is non ionizing because it doesn't have enough energy to cause remove an electron. Some radiation which has enough energy to ionize just doesn't have enough penetration to reach far enough into the body to cause damage (visible light for example).

Your reasoning is correct with regards to ionizing radiation causing damage in a probabilistic manner. However, the body's response ionizing radiation is less understood. People must remember, ionizing radiation has existed for as long as the universe, people and animals evolved with the natural radiation fields of the earth.

In fact it seems mostly from the fact that when you extrapolate the dose/survival curves it doesn't intersect at (0,0) dose risk.

In fact, some evidence shows that small amounts of radiation might actually be beneficial.
-The theory is that the body's response to radiation might be a bit like training a muscle, if you work it out it becomes stronger.
-Sick or already damaged cells might be at greater risk of dying due to radiation damage preventing future problems
-Trigger some sort of defensive mechanism in the body leading to better long term survival.

Although since there isn't a strong theoretical basis for this, and data in the super low regime is pretty sketchy since 20, 30, 40 and 50 year studies take along time to preform, most people work with lowest risk is due to zero dose.

Most dose limits are set with a theory of a linear relationship between dose & risk for the low dose regime using natural background radiation to set the scale. If for example you keep occupational exposure to 1/10 natural background radiation, it is assumed that this represent a marginal increase in risk. For the public the limit could be set lower at say 1/100.

The situation becomes more complicated when you start considering which organs are most sensitive and what types of radiation are most likely to lead to unrepaired DNA damage. In this case you start talking about 'effective doses' and 'equivalent doses'.

 

[Xnn]

It's not as if at low radiation doses the particles won't have enough energy to penetrate a cell or something right?

Depends on the type of radiation.

There are low energy beta and alpha's that can not penetrate the skin or clothing.

Gamma radiation can, but when the dose is low enough, then the consequence are so low that they are no longer measurable.

 

[HotCells]

but what about the therapeutic effects of radiation?

here is a quote from the textbook Radioactivity, Radionuclides and Radiation (available online at...http://www.springer.com/new+&+forthcoming+titles+(default)/book/978-3-540-21116-7)

"Observations of the beneficial effects of radon on human health date back to prehistoric times... The ancient Romans used radon spas and in Ischia, Italy, the therapeutic baths have been in use for over 2000 years... Today there are many therapeutic radon centres in use... Currently some 75000 patients are treated annually in Germany and Austrian radon spas (e.g. Bad Gastein) mostly for painful inflamatory joint diseases such as rheumatism, arthritic problems, Morbus Bechterew, psoriasis, gout, chronic bronchitis, asthma, etc.

Following a three week treatment period, beneficial effects are claimed to last for periods of six months or more. Treatment involves inhaling radon in high concentrations. In Bad Gastein's Heilstollen the radon concentration is 170,000 Bq/m³ (alomst a thousand times higher than current legislation) or by drinking or bathing in radon water. .

It is surprising to note that the costs for the radon therapy are partly covered by medical health insurance schemes."

 

[Morbius]

The way I see it, every single ionizing particle that passes through a cell has a chance of damaging it. The cell-repair mechanism is imperfect, so even a single particle can damage a cell beyond repair, causing cancer. It's not as if at low radiation doses the particles won't have enough energy to penetrate a cell or something right? So no matter how low the dose, there will always be a risk.

Shukie,

You could make exactly that same argument for your immune system. The immune system is imperfect, and each germ or pathogen has a chance at making you sick. Therefore, you need to avoid germs and pathogens at all cost...

That's also one of the arguments that people used to make to justify avoiding casual contact with people who are HIV positive.

The fact is that, as has been pointed out, we EVOLVED in a field of radiation. The people
who live in Denver get more radiation from the sky because they have less air shielding them.
So, I guess in your way of "thinking"; you should also avoid Denver at all costs..

Your attitude toward radiation is similar to the attitude that a person with OCD -
Obsessive Compulsive Disorder - has with germs. [ Remember Howard Hughes? ]
We now recognize such preoccupations as a mental illness.

Scientists have also shown that exposure to radiation can have a beneficial effect - just
like having an exposure to a germ in a vaccine helps to prepare the immune system and
makes it stronger.

From scientists at Lawrence Livermore National Laboratory:

https://www.llnl.gov/str/JulAug03/Wyrobek.html

"The team also discovered that the human lymphoblastoid cells exhibit what is called an adaptive response to ionizing radiation. An extremely low dose (also called a priming dose) appears to offer protection to the cell from a subsequent high dose (2 grays) of ionizing radiation. The degree of protection was measured by the amount of reduced chromosomal damage. A priming dose of 0.05 gray, administered about 6 hours before the high dose, can reduce chromosomal damage by 20 to 50 percent, compared with damage to cells that were not exposed to the priming dose."

The field of radiation damage is more complex than your analysis indicates.

Dr. Gregory Greenman
Physicist