How Much Radon Is Too Much? Chemlsby d the Envlmnmsnt
Charles H. Atwood Mercer University, Macon, GA31207 Over the last 10-15 years scientists and the general public have become aware of the risk associated with indoor air pollution and particularly the risk from exposure to '"Rn in OUT homes. As more and more data is accumulated about this problem, the general public hears an assorted array of statements concerning this hazard and how harmful it is. Now is a good time to look a t the problem of Rn infiltration and see what the current understanding of Rn pollution in our homes is. Thus, this paper is aimed at pmviding teachers with the most recent data on Rn so that they can communicate that information to their students and to the general public. The Source of Radon in the Home Radon is one of the intermediate decay products from the natural radioactive decay of U and Th. In the soil, U is present in granite, shale, phosphate, and pitchblende minerals while Th is found in certain phosphates, granite, and gneiss ( I ) . Both of these elements radioactively decay through a series of a-and P-decays until one of the stable isotopes of Pb is reached. These decay processes must form one of the isotopes of Rn in their transformation from U or Th to Pb. The longest-lived isotope of Rn is "%n which has a halflife of 3.825 days. The other isotopes of Rn have much shorter half-lives and, in particular, the other Rn isotopes generated by natural radioactive decay chains have halflives of 55.6 s or less. These latter half-lives are too short to allow Rn migration in large concentrations into a building. Thus, the primary component of Rn pollution in the home is '"Rn. "'Rn IS formed by the radloactivc decay cham that starts 9 and finlshes nith '06Pb The decay ~roccssto Rn with ' is slow. The major decays and half-lives for ihis chain are has slowly given below in Table 1, eqs 1-14. As the Table 1. Major Radioactive Decays and Half-lives for the 2 3 8 Decay ~ Chain
+
23?h+4~e
hn= 4.5 x lo9yr
(1)
+ 2 3 4 ~+ a
2 3 4 ~+ap-
hn = 24.1 day
(2)
ti@= 1.2 min
(3)
+ 230~h +
' 9 h + 4 ~ e
ha=2.5x lo5yr
(4)
+4 ~ e
hn = 8.0 x lo4yr
(5)
"%a
+
2 2 2 ~ n+ 4 ~ e
tin=1.6~10~yr
(6)
22'~n
4
2 i 8 ~+o4 ~ e
tin = 3.825 day
(7)
''PO
+
238~
23?h
234~
'14pb + '146i
+
2 3 4 ~+
'%a
8
'14pb + 4 ~ e
ha = 3.05 min
(8)
2 1 4 ~ i + p-
hn = 26.8 min
(9)
'l4Po + p-
hn = 19.8 min
(10)
2 i 4 ~ o+
2 1 0 ~ b + 4 ~ e tin=162p
'lOpb
210~i
4
+ 2 i 0 ~+ o "OB~
+ p-
(11)
tin=22 yr
(12)
2 1 0 ~+o
tin = 5.0 day
(13)
'06Pb + 4 ~ e
hn = 138.4 dav
1141
progressed through the radioactive decay chain during the earth's lifetime, an essentially constant amount of "%n has been generated from the preceding decays. This decay provides a steady state amount of 222Rnin the air and soil. The major factor that affects this steady state amount is the relative amounts of U8Uthat was in the rock initially. will subRocks that had high initial concentrations of 23TJ sequently have high '"Rn levels. All of the decay products of natural radioactive families, except Rn, are solids and, therefore, are trapped in the soil with little or no mobility. However, Rn-the heaviest inert gas on the periodic chart-can migrate into the surrounding air when it is generated. Evidence for this is given by the average '"Rn concentration in outdoor air, 5-10 Bq/m3 (0.135-0.270 pCfi), which is generated by the seepage of Rn from the uppermost 1m of soil (2). (The Bq and Ci are units of radioactive decay. The Bq is defined as 1 disintegration per second and the Ci is 3.7 x 10'' disintegrations per second. One pCiiL is equivalent to 37 Bq/m3.) Interestingly, one of the best barriers to stop Rn intltration into the home is the soil itself. The typical Rn concentration between grains of soil is tens of thousands of Bqlm3. yet the average air concentration is thousands of times less and the average home Rn concentration is -50 Bq/m3(1.35 p C f i ) (2). There is a large range of permeabilities for Rn in soil. In fine clay the Rn permeability rate is nearly a million times less than that in a coarse sand (2).Thus. one of the determining factors for home Kn concentrations is the soil or ruck that is heneath the slab ofthe house. The recent interest in Rn pollution has its genesis in the energy crunch that hit the U S . in the mid 1970's. Prior to that time, most homes in the United States were relatively energy inefficient and ventilation rates in American homes were relatively high. After the crunch, the American public began to caulk their windows and doors, install more energy efficient windows, and increase the amount of iusulation in their attics until the avhrage house air turnover rate was decreased some 1030%. These steps achieved an average air turnover rate of 1to 2 times per hour, in other words the air in our homes is exchanged with the outside air about once or twice an hour In effect the air that is in our homes stays there for a longer time. As the Department of Energy (DOE) began to study the effects of this decreased air turnover rate on indoor air pollution, the DOE also noted the relatively high Rn concentrations in some homes and alerted the public to this potential health hazard (2,3). However, the entrapment of Rn in our homes is not the major reason for elevated Rn levels, the rate of seepage of Rn into our homes is. The primary reason that Rn infiltrates homes is that there is a slight air pressure difference between the interior and exterior of the house. At first, this idea may not seem sensible. but if vou examine the devices that ooerate in a typical ~ m e r i c a nhome therc arc suveral that tkke interior air and exhaust it to the outside. Vents in bathrooms and kitchens are obviously such devices and others such as clothes dryers and garbage disposals are less obvious air removers. The primary pressure differential in American homes comes from an even less obvious source, home heating. We are all aware that "hot air rises" and that is certainly true in our homes where higher floors are always Volume 69 Number 5 May 1992
351
warmer than the lower ones. The movement of the warmed air from lower to higher floors causes a slight pressure differential that is referred to as the "stack effect", because it is the driving force behind air movement in smokestacks. This pressure differential is small, perhaps only 0.0001 atm, but significant enough to d r a w ~ nfrbm the soil beneath a home into the basement or living quarters (3).It is the equilibrium established in a home between the Rn generated in the soil, the pressure differential between the house and the ground, and the ventilation rate between the house and outdoors that ultimately determines the average Rn concentration in a building (2). Effects of Radon on Home Inhabitants Once Rn has entered a home, the residents obviously will breathe it in their normal respiration. Since Rn is an inert gas it would seem that we wodd just exhale it in the ensuing breaths and there would be no problem. But in this case both nuclear and chemical phenomena conspire to cause some harm. If a "2Rn atom that is in the lungs or in a home's air should radioactivelv decav. then the set of de) initiated. I t cays represented by eqs 7-14 i ~ a b l e i are should be noted that the half-lives for these decavs are relatively short. The lon est decay aRer the decay bf " ' ~ n is . a half-life of 22 vears. All of the decav of 210Pbto %BI.with the decays between that of "'Rn and the 210~bdecay have half-lives of less than 30 min. The elements that are created following the Rn decay are not inert and can be incorporated into the lung tissue where they will reside for a decay occurs while considerable period of time. If theZZZRn the Rn is in the lungs, the 218Pocan attachitself directly to the bronchial walls or form a molecular aggregate with other molecules in the lung and attach to the lung. Even decays that occur outside the lungs can bring the decay products into the lung via the formation of molecular aggregates in surrounding air that can then be inhaled (2). Eventually some of the molecular aggregates are expelled from the lunes via the normal lune ..Drocesses. hut some of the material adheres to the lungs even up to the stage that 210Pbis formed and b e l n s its 22 vear half-hfc. The modeling of the lung's interakions witLthe '"Rn decay products is a com~licatedDrocess that is still being - unraveled (2). The major radiation dose comes from the decay of '18Po and 214Poboth of which have short half-lives and decay via a-particle decay. Alpha emitters deliver a large radiation dose because they emit large mass, highly charged particles that interact with tissue exceptionally well. Furthermore, as is the case with all radiation, internal exposure is much more detrimental than external exposure and Rn brings this problem home. Now the difficult problem of assessing the extent of Rn exposure in a typical home and the interpretation of that exposure to the increased risk of contracting lung cancer must be addressed. The model for assessing the increased risk of contracting lung cancer from exposure to Rn levels that are typical in the home is provided by the uranium mine workers in the United States, Canada, and Czechoslovakia, iron miners in Sweden, and some research on rodents exposed to excess "'Rn levels (2, 4-71,All of these groups have exhibited an excess level of lung cancers that is attributed to their Rn exposure. Obviouslv. the environment in a mine does not exactlv correlate the conditions i n a house. There are differences in the exertion level of the inhabitants and thus their breathing patterns, the average particle size that is inhaled. and the fact that the workers were exposed to more Rn than is typical in an American home. (in fact, a May 1991 National Research Council remrt asserts that the dosage has heen ovrrrdtimated by -jooi due to the factors mentioned above (81.1Ilowevcr, the model does have some
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.
to
352
Journal of Chemical Education
favorable comparisons. First, the model is based on human exposure andcorroborated by the rodent tests (4, 6). Second, a comparison of the exposure differences between the miners and home inhabitants is not as many orders of magnitude different, as say a comparison of the victims of the Hiroshima nuclear bomb explosion and home inhabitants would be. Thus, the extrapolation from miners' to home residents' exposure is a much smaller expansion of the data scale and fraught with fewer difficulties (2). The overall conclusion that can be made about the exposure risk of Rn in the home is that there is indeed a n increased risk of contractine lune cancer from Rn exoosure and that there is a relatively good understandingof the dose-resDonse factor for Rn exDosure a t the levels observed in tw-. i k l American homes (i, 4). Assessing the amount of Rn exposure in the average American home also has proven to be a quite difficult and tricky task. As of this writing (April 1991), no truly national survey of Rn exposure has been performed; however, there are some limited survevs that can ~rovidea measure of the extent of the problem."F'erhaps the two best surveys were performed by Cohen (9)and Nero, Scbwehr,Nazaroff, and Revzan (10). (Intentionally, the recent survey from the Environmental Protection Agency and Nuclear Regulatory Commission (5) is being ignored, because the survey was designed as a screening device and attempted "to obtain measurements of the highest detectable radon levels!' These levels are not truly indicative of the problem in the average house (ll).)
- -
Cohen's Survey Cohen's survey was performed on a total of 453 houses of physics faculty members from 101universities in 42 states and the District of Columbia. He chose physics faculty, because as a part of the survey the participants were asked some detailed questions about the construction of their homes.. orevailine winds. and concrete slab conditions that Cohen felt wuld he answered best hy people with their Irwl oftrainine. This survev not onlv attemots to establish an average Rn level hut also to correlate the Rn levels with the construction characteristics of the houses. The Cohen survey determined that the arithmetic average concentration was 54 Bq/m3(1.47 pCfi), the geometric mean was 38 Bq/m3 (1.03 p C f i ) and the range of the concentration levels ran from 3.7 Bq/m3 (0.1 pCilL) to 559 Bq/m3 (15.1 pCiIL). The data from this survey is represented best by a log normal distribution with a median of 39 Bq/m3 (1.05 pCi1L). The correlation with the construction characteristics of the homes was rather surprising because there was not as strong a correlation as was expected. In Table 2 several of the more interesting correlations from Cohen's survey are listed. (Onlv a small ort ti on of the entire data set is shown. For the entire set see ref (9).)For each set in Table 2 the highest and lowest average Rn levels in that set of data are listed plus in some cases other interesting values are also listed. As can be seen in the first set of data, the correlation of house age and Rn levels shows that the 50-69 year-old homes had the lowest Rn levels with the highest levels reported in 3 0 3 9 year old homes. The youngest homes, and presumably most energy efficient, had levels less than that of the 30-39 year old set. This, of course, is the opposite of what was expected if the "tightness" of a house had a significant effect on Rn levels. The correlation of what is beneath a house shows the expected low level for a ventilated crawl space but that is only slightly lower than the case for a house built on a slab and only 28% lower than the case for basements, which had the highest levels. This latter result is even more surprising when taken in conjunction
.
-
Table 2. Average Radon Levels a s a Function of Various Construction and Environmental Characteristics of Homes Age of house (yr)
EIq/m3
pCi1L
