8 RADIATION STANDARDS, INCLUDING FALLOUT Dr. Wurierte. Thatis right; yes, sir. Chairman Ho.irtetp. Therefore, when we are talking about 30 roentgens from X-ray tests that Senator Anderson had, weare talking about an exposure in 5 years from chest X-rays which would be about four timesora little over four times as much as you would be exposed to normal backgroundradiation over a period of 70 years. Dr. WurreLe. With the difference that the chest X-rays do not involve the total body. Chairman Ho.irtevp. That is true. But also with the additional difference that the chest X-rays are stronger and are more concentrated than the natural background radiation in point of power and in point of time. Dr. Wuiprte. That is correct. Chairman Houirtevp. For absorption. Dr. Wuteete. That is correct. Chairman Hottrterp. And in point of regeneration of the normal regeneration of body tissue which may or may not be harmed. r. Wuierte. I think we are—— Chairman Hotirreip. This goes to the point that you made, that a dose of radiation received in one jolt, you might say, is more damaging to the body thanif it is received in portions over a numberof years. r. Wurprte. Asa general statement this was true. Whethertaking this half of a roentgen exposure that Mr. Anderson spoke of in 1 minute in a month or spread it uniformly over the month, I don’t believe we have biological data to show that that rate change makes much difference. If it has a difference it is in the direction that I spoke of. Representative Pricr. Proceed, Doctor. Dr. Wurtz. I was speaking of the rem. It is not a precise unit like the kilogram or the rad because the biological response to a given absorbed dose or radiation, like the response to a given dose of some drug, varies with a number of factors, physical, chemical, and biological. Even though it is not precise, the rem is a practical necessity if one is to evaluate the significance of the total radiation exposure received by an individual when this exposure will in general consist of the several types of radiation described at the beginning of this statement. 4. PRINCIPAL SOURCES OF EXPOSURE TO HUMAN POPULATIONS Human populations are exposed to four principal radiation sources: naturally occurring sources, manmade environmental sources, occupational sources, and medical and dental sources. Each of these has quite different scientific, moral, and legal characteristics, as I shall attempt to show. (a) Naturally occurring sources: The naturally occurring radiation sources to which humans are exposed consist of cosmic radiation from outer space and the radiations from natural radioactive materials, such as uranium, radium, and potassium 40, in soil, water, air, building materials, and in the humanbodyitself. On the average, natura] radiation exposure amounts to about 0.1 rem per year, althoughlevels five times as great are not unknown. One can reduce his exposure from natural sources somewhat by living in a tent along the seacoast rather than in a granite house in scstancccenghUnaRRRedeapeegaMaddahaatiog