Half-Life Question.?

Question

All of the isotopes of radon have half-lives shorter than four days, yet radon is still found in nature. Explain why all the radon has not yet already decayed.

Answer 1

radon is formed from the decay of other materials see th attached link: * The risk of environmental radon levels is not higher now than in the past, when residential exposures were not considered to be a significant health hazard. It has now been raised from a nominal natural background contributor to human exposures to the prime contributor based on a new way of calculating dose, and not on increased exposures in the home due to tighter more energy efficient houses, or new information on radiation dose and/or biologic risk.
* The great majority of the radiation dose is not from radon itself, but from the short-lived alpha particle-emitting radon daughters, most notably Po-218 (radioactive T1/2=3 minutes), and Po-214 (radioactive T1/2=0.164 milliseconds), along with beta particles from Bi-214 (T1/2=19.7 minutes). [T1/2 is physical half-life].
* Measurement of radon in homes is simple and relatively inexpensive and may be accomplished by various means. Cannisters remain the method of choice for use by the average homeowner.
* There is no way to avoid radiation exposures completely, much of which comes from the natural environment as well as from radioactivity in our own bodies.
* Radon gas can penetrate houses from many sources in many fashions. It is not possible to radon-proof a home, but it is possible to reduce its level. The most important contributor to indoor radon is the soil from which radon can be drawn through large and small subsurface gaps in the house foundation. Houses that are in direct contact with the ground will have higher radon levels than houses with an air space under the dwelling. Radon levels in the upper floors of a multi-story building are lower than on the ground floor. If one lives in a high radon area, it is prudent to determine the level in the home and to take appropriate action if the level is found to be high, i.e. greater than 4-8 pCi/liter (NCRP recommended level is 8 pCi/liter, the EPA recommended level is 4 pCi/liter). [pCi is the activity of the substance and is equal to 10-12 Ci or 2.2 radioacitve emission per minute].
* The shape of the dose response curves for miners exposed to alpha-emitting particles in the work-place is consistent with current biologic knowledge. It is linear in the low dose range and saturates in the high dose range. No detectable increase in lung cancer frequency is seen in the lowest exposed US miners, i.e. those with exposures <120 wlm, the relevant dose interval for most homes. in most cases, however, it is not possible to exclude a positive correlation due to wide uncertainty ranges on both the dose (x) and effect (y) axes. [wlm is a unit of expsoure to radon, working level month and is defined below]
* Evidence for a health effect from radon exposure is based upon data gathered from epidemiologic studies of miners, and animal studies. Extensive radiobiologic data predict a linear dose response curve in the low dose region due to poor biological repair mechanisms for the high density of ionizing events that alpha particles create. If this is the mechanism that underlies lung cancer induction, there is reason to believe that there will be some degree of increased lung cancer risk even from low level environmental radon exposures. However, no compelling evidence for increased lung cancer risk has yet been demonstrated from "acceptable" levels (<4-8 pci/liter). increased lung cancer risk is primarily in cigarette smokers, and abatement of that risk can best be accomplished by changes in smoking status.
* Mitigation of radon in the home can be accomplished by a variety of means. The level to which remediation should be directed is under dispute, to the extent that some groups even suggest the level should be maintained at less than or equal to 0.2 pCi/liter (the average outdoor level). The societal cost of testing and mitigation at the EPA recommended level (4 pCi/liter) is 44.5 billion dollars (1991 dollars), and rises to 101.2 billion dollars if the action level is set at 2 pCi/liter. At the NCRP action level, 8 pCi/liter, the cost is estimated at approx. 15 billion dollars.

Radon, Rn-222 (T 1/2= 3.82 days), is a daughter product of radium, Ra-226, which in turn is derived from the longer-lived antecedent, U-238. Thoron, Rn-220 (T 1/2 = 56 seconds) is a daughter of thorium, Th-232, which is present in larger amount in the earth's crust than radon. Because of thoron's short half-life, it is essentially all gone before it leaves the ground, and is of no significant radiobiologic consequence. These radionuclide series are present in slowly decreasing amounts in the environment (geologic time scale), due to radioactive decay of their parents, which has been known and understood since the end of the last century.

Widely varying radon levels exist in different regions related to geological circumstances. New concern regarding radon exposures is traceable to the discovery that there are more houses with high radon levels than previously realized and to the use of a new method of expressing and summing doses from partial body exposures, such as the lung dose from radon daughters (7-16). This method of expressing dose was promulgated by the ICRP and the NCRP based on defined weighting factors which make it possible to sum partial body doses and thereby estimate a total body dose which would have a quantifiable risk. This quantity is defined as the Effective Dose (ED) (16). Thus, the previously estimated partial body environmental radon dose to the tracheobronchial epithelium (TBE) (2500 mrem/year.) was not included in whole body dose calculations because that exposure was limited to a small fraction of the body.

The new method of calculation multiplies the 2500 mrem/year. dose to the TBE by a weighting factor (WF) which allows the dose to the TBE to be included in the ED from environmental radiation exposure Different WFs have been proposed, including 0.12 (EPA), 0.08 ( NCRP) and (NAS-NRC BEIR V), and 0.06 (ICRP). These WFs raise the radon contribution to the whole body from 0 mrem to 300, 200, and 150 mrem respectively. NCRP quotes an uncertainty of +/- 50% in these numbers. Based on these estimates, radon in equilibrium with its daughters delivers 2 times more dose than previously accepted as the total dose received from all sources of natural background exposure (approx. 100 mrem/year on the average in the United States). Thus, it is not surprising that adoption of the effective dose notion by many radiation protection groups (including the NCRP and the EPA in the United States), has led to increased concern regarding the potential health effects of radon. It should be noted that lung cancer risk coefficients from radon are not increased. There are no new cases of lung cancer that led to the increased dose estimate. In fact, the new estimates of radiation dose, imply a lower risk coefficient. That is, when the same number of lung cancer cases that occur are attributed to the higher doses (ED), the risk per unit exposure is decreased. The effective dose concept is discussed at greater length in NCRP Reports #93 (17) and #100 (18) , and ICRP #60 (7).
TABLE 1

Answer 2

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