The Petkau Effect
The Petkau effect is an early counterexample to linear-effect assumptions usually made about radiation exposure. It was found by Dr. Abram Petkau at the Atomic Energy of Canada Whiteshell Nuclear Research Establishment, Manitoba and published in Health Physics March 1972.
Petkau had been measuring, in the usual way, the dose that would rupture a particular cell membrane. He found that 3500 rads delivered in 2¼ hours (26 rad/min) would do it. Then, almost by chance, he tried again with much weaker radiation and found that 0.7 rads delivered in 11½ hours (1 millirad/min) would also destroy the membrane. This was counter to the prevailing assumption of a linear relationship between total dose or dose rate and the consequences.
The problem with the Petkau
The problem with the Petkau effect is that it doesn't really explain why aren't we all dying of cancer as a result of background radiation.
Huh? You tell me, has the
Huh? You tell me, has the cancer rate (especially among children) stayed the same over the last 100 years?
The background radiation is killing some of us.
The most important variable
The most important variable when it comes to cancer is age. We live longer, we get more cancer. Also, 100 years ago there were large portions of the population that lived and died without even getting a cancer screening or an autopsy.
But anyway, if the Tepkau effect were true, or at least the effects of low dose radiation that some are deducing from it, we would be all dying from cancer, since we are all surrounded by radiation from day one, our body contains a significant amount of beta emitters like Potassium-40 or Carbon-14 and we are all exposed to radon (some of us much more than others, depending on the area, and no link to increased cases of cancer has been detected as far as I know).
Abram Petkau, head of the
Abram Petkau, head of the Medical Biophysics Branch of the Canadian Atomic Energy research laboratory in Manitoba, announced a discovery that should have shut down the nuclear industry as a matter of human survival. But he was not heeded and nuclear testing, nuclear power plant operation, and nuclear waste disposal continue 26 years later.
Dr. Petkau discovered that chronic low-level nuclear radiation exposure produced far worse damage to living tissues than high-dose, short-term exposure.
Good stuff guys! I wonder
Good stuff guys! I wonder why the Petkau effect hasn't been discussed here more. Here's some good reading.
http://doctorapsley.com/RadiationTherapy.aspx
All of the research and
All of the research and experience of the scientific and medical community since the discovery of X-rays and radioactivity just before the turn of the century did not warn us of the seriousness of low doses of radiation from internally deposited fission products. As explained in The Petkau Effect, by Ralph Graub, the successful experience with the medical uses of X-rays without side effects convinced the nuclear scientists who developed the atomic bomb that its principal effects would be produced by blast and fire. Fallout from drifting radioactive fallout was believed to produce very low doses, far below the levels of background radiation from cosmic rays, natural sources and diagnostic X-rays, and therefore was considered to present no significant danger.
Petkau's discovery was first published in the March 1972 issue of Health Physics under the innocuous title, "Effect of Na-22 on Phospholipid Lipid Membranes." In it he describes how he found that cell membranes immersed in water which had withstood X-ray doses as large as hundreds to thousands of rads without breaking, ruptured at less than one rad when subjected to low intensity, protracted radiation such as that produced by radioactive salts immersed in water.
This finding was completely contrary to all previous observations of biological damage by radiation such as genetic effects, and cancer induction in laboratory animals or humans, which had shown almost no dependence on the rate at which radiation is delivered to tissue.
As Petkau and his associates discovered, the cell membrane damage due to low level radiation was the result of a completely different biological mechanism than the direct hit on the DNA molecules in the nucleus of cells exposed to high doses of radiation. They found that the cell membranes were destroyed by the action of negatively charged oxygen molecules or "free-radicals," produced by the absorbed radiation from the life-giving oxygen dissolved in the surrounding fluid. This highly toxic form of oxygen diffused to the outer surface of the membrane, where it initiated a chain reaction that dissolved the membrane in a matter of minutes to hours, causing the cell to leak and die.
It became clear that a single free-radical molecule was sufficient to destroy an entire cell, so that only a handful was needed to be produced per cell-volume at very low dose rates. But at high dose rates, many millions would be formed in the same volume in the lifetime of the molecule. This results in a form of "overkill," much like the case of a balloon, where a single dart is enough to destroy it, and throwing millions of darts is wasteful. In fact the more free-radicals are created in a given volume (as from higher levels of radiation intensity), the more they tend to collide with each other, causing them to become deposited in living tissue; high doses given at the rate of 10,000 rads per minute were found to be 100 billion times less efficient in destroying a cell than at one ten-millionth of a rad per minute, the rate at which we experience background radiation.
The consequence of the enormously greater efficiency of internal radiation at low dose rates is that the dose response curve rises very rapidly at the small doses and dose rates near background radiation, and flattens out at high doses and dose rates, so that the risk per unit dose declines with increasing dose rates.
Thus we have what appears to be a perverse situation in which low doses protracted over periods of days, months or years are far more dangerous, per unit of absorbed radiation, than high doses from external sources. Mathematically, this turns out to be of the form of a concave downward or logarithmic relation between dose and the biological response for individuals exposed to different amounts of radiation during a given time period, as in the case of releases into the environment that enter the diet and concentrate in critical organs such as the bone marrow, the thyroid and the pituitary gland.
Thus the so-called "Petkau Effect" explains why man-made fission products such as strontium-90 concentrating in the bone, introduced into a pristine biosphere in the earliest years of the nuclear age did so much more damage to the immune system cells than had been believed possible based on our experience with medical X-rays or the study of animals exposed to high radiation doses.