In hearings set to establish compensation for exposure to radiation when Pacific nuclear test fallout drifted over Rongelap Island, their lawyer responded to the measured radiation level: "500 millirems? 500 of anything is an awful lot, isn’t it."
Today’s widespread confusion over radiation amounts reflects a lack of understanding about how radiation is measured and what are normal levels. But it may take several hundred million nanoseconds to explain it.
Since alpha, beta and gamma rays are not felt by our senses, workers who may be exposed to higher-than-background radiation wear radiation "dosimeters." Such workers include nuclear power plant workers, doctors using radiotherapy, researchers in labs using radionuclides, HAZMAT teams working with radioactive materials, and astronauts. Damage to the body can involve a quick acute dose, but is usually related to the total dose received over time.
The "rem" (Roentgen Equivalent Man) is a unit of measurement of ionizing radiation that produces the same effect on humans as one roentgen of high-voltage x-rays. It takes a thousand millirems to make one rem. To echo the simple-minded statement about a half-rem (500 millirems) made at Rongelap: "Half of anything isn’t much, is it?"
A publication from the Massachusetts Institute of Technology (MIT Tech Talk Vol.38 No. 18) "Radiation, how much is considered safe for humans?" describes the various dose levels.
With little experience with radiation, the federal occupational limit during WWII was 25,000 millirems per year. This was lowered to 15,000 millirems per year in 1950, and 5,000 millirems in 1957.
Additional occupational limitations were established for minors of 500 millirems per year.
Our average natural background radiation is 300 millirems per year. In Denver, the mile-high city, natural background is 400 millirems. An airline pilot who spends much time at 35,000 feet altitude gets even more.
And "...it is recommended that the exposure of a fetus be limited to no more than about 50 millirems above background levels per month." Radiation has greater potential to damage rapidly growing cells where the DNA is a condensed target. This is the reason radiation poses more danger to rapidly growing cells, and those mutations can result in cancer.
But since cancer cells are more rapidly growing, radiation can be used as a treatment to kill cancerous cells. And it can be used to reduce the number of normal cells when tissue is too active, as in hyperthyroidism. A radioactive drink is used to deliver radioactive iodine, up to 10,000,000 millirems, to the thyroid but the dose to the rest of the body is very low. Radiation beamed at a cancerous tumor can deliver six million millirems to the tumor, but again the whole body radiation exposure is kept very low.
From research in the aftermath of the atomic bombing at Hiroshima and Nagasaki in 1945, "...half of the people died whose entire bodies were exposed to 450,000 millirems of radiation from the atomic bomb. All persons died whose bodies were exposed to 600,000 millirems of radiation." These are acute doses over a short time. Obviously, today’s safety levels for radiation have been set dramatically lower than levels that cause radiation sickness. Thus when the press proclaims levels slightly above the safety threshold be "dangerous levels," they broadcast their utter lack of understanding of radiation. And they spread unnecessary fear.
A final factor is distance. If you move twice as far from a radiation source, you receive one-fourth the dose. If you move five times further away, you receive one-25th the dosage. Exposure drops dramatically with distance from a source.
If humans are ever to journey to Mars, the public must have some commonsense about radiation levels. We cannot shield astronauts from cosmic rays. NASA's Mars Odyssey spacecraft carried the Mars Radiation Environment Experiment. Data indicate a Mars crew will receive 120 millirems of cosmic radiation each day of their voyage to Mars. That is far below lethal levels but well above our conservative "safety levels." Can we stand a dose of commonsense?