How Much Radon Is Too Much? Charles H. Atwood Mercer University, Macon, GA31207

Over the last 10-15 years scientists and the general pub- lic 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 harm- ful 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 pm- viding 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 min- erals 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 half- life 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 half- lives of 55.6 s or less. These latter half-lives are too short to allow Rn migration in large concentrations into a build- ing. Thus, the primary component of Rn pollution in the home is '"Rn.

"'Rn IS formed by the radloactivc decay cham that starts with '9 and finlshes nith '06Pb The decay ~roccss to Rn is slow. The major decays and half-lives for ihis chain are given below in Table 1, eqs 1-14. As the has slowly

Table 1. Major Radioactive Decays and Half-lives for the 2 3 8 ~ Decay Chain

2 3 8 ~ + 23?h+4~e hn= 4.5 x lo9 yr (1)

23?h + 234~a + p- hn = 24.1 day (2)

2 3 4 ~ a + 2 3 4 ~ + 8 ti@ = 1.2 min (3)

2 3 4 ~ + ' 9 h + 4 ~ e ha=2.5x lo5yr (4)

2 3 0 ~ h + '%a + 4 ~ e hn = 8.0 x lo4 yr (5)

"%a + 2 2 2 ~ n + 4 ~ e t i n=1 .6~10~yr (6)

2 2 ' ~ n 4 2 i 8 ~ o + 4 ~ e tin = 3.825 day (7)

'''PO + '14pb + 4 ~ e ha = 3.05 min (8)

'14pb + 2 1 4 ~ i + p- hn = 26.8 min (9)

'146i + 'l4Po + p- hn = 19.8 min (10)

2 i 4 ~ o + 2 1 0 ~ b + 4 ~ e tin=162p (11)

'lOpb 4 2 1 0 ~ i + p- tin=22 yr (12)

" O B ~ + 210~o + tin = 5.0 day (13)

2 i 0 ~ o + '06Pb + 4 ~ e hn = 138.4 dav 1141

Chemlsby d the Envlmnmsnt

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 222Rn in the air and soil. The major factor that affects this steady state amount is the relative amounts of U8U that was in the rock initially. Rocks that had high initial concentrations of 23TJ will sub- 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 1 m of soil (2). (The Bq and Ci are units of radioactive decay. The Bq is defined as 1 disinte- gration 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 intltra- tion into the home is the soil itself. The typical Rn concen- tration 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 pCf 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 US. 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 en- ergy efficient windows, and increase the amount of iusula- tion in their attics until the avhrage house air turnover rate was decreased some 1030%. These steps achieved an average air turnover rate of 1 to 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 Depart- ment 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 inte- rior and exterior of the house. At first, this idea may not seem sensible. but if vou examine the devices that ooerate in a typical ~merican home therc arc suveral that tkke in- terior 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 heat- ing. We are all aware that "hot air rises" and that is cer- tainly 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 favorable comparisons. First, the model is based on human air from lower to higher floors causes a slight pressure dif- ferential 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 ~ n frbm the soil be- neath a home into the basement or living quarters (3). It is the equilibrium established in a home between the Rn gen- erated 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 av- erage 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 ensu- ing 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- cays represented by eqs 7-14 i ~ a b l e i ) are initiated. I t should be noted that the half-lives for these decavs are rel- atively short. The lon est decay aRer the decay bf " ' ~ n is % . . the decav of 210Pb to BI with a half-life of 22 vears. All of the decays between that of "'Rn and the 210~bdecay have half-lives of less than 30 min. The elements that are cre- ated following the Rn decay are not inert and can be incor- porated into the lung tissue where they will reside for a considerable period of time. If theZZZRn decay occurs while the Rn is in the lungs, the 218Po can 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 ag- gregates 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 210Pb is formed and belns its 22 vear half-hfc. The model- ing of the lung's interakions witLthe '"Rn decay products is a com~licated Drocess that is still being unraveled (2). -

The major radiation dose comes from the decay of '18Po and 214Po both 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. Fur- thermore, as is the case with all radiation, internal expo- sure 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 Czecho- slovakia, iron miners in Sweden, and some research on ro- dents 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 to the conditions ina house. There are differences in the exertion level of the inhabitants and thus their breathing patterns, the average particle size that is in- haled. 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 (81.1 Ilowevcr, the model does have some

exposure andcorroborated by the rodent tests (4, 6). Sec- ond, 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 inhabi- tants 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 an increased risk of contractine lune cancer from Rn exoosure and that - - there is a relatively good understandingof the dose-re- sDonse factor for Rn exDosure at 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 na- tional survey of Rn exposure has been performed; however, there are some limited survevs that can ~rovide a 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 101 universities in 42 states and the District of Columbia. He chose physics faculty, be- cause 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 concentra- tion was 54 Bq/m3 (1.47 pCfi), the geometric mean was 38 Bq/m3 (1.03 pCfi) and the range of the concentration lev- els 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 re- ported 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 sig- nificant effect on Rn levels. The correlation of what is be- neath 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

352 Journal of Chemical Education

Table 2. Average Radon Levels a s a Function of Various Construction and Environmental

Characteristics of Homes

Age of house (yr) EIq/m3 pCi1L

<I0 48 1.29

3&39 70 1.88

5&69 33 0.88

What is beneath house Win3 oCi/L

basement 59 1.60

ground-no space 48 1.30

crawl space-unpaved and ventilated 46 1.23

Intearitv of barrier beneath house EIdm3 pCi/L

uncracked 57 1.53

slightly cracked 51 1.39

badly cracked 75 2.04

Integrity of pipe seals EIq/rn3 pCiIL

well sealed 61 1.65

poorly sealed 44 1.18

average or unknown 53 1.44

Construction material for house EIqlm3 pCilL exterior

wood 61 1.66

brick 47 1.28

stone 26 0.70

Construction material for house interior 6q/rn3 pCiIL

plaster 43 1.16

dry wall 61 1.64

wood 63 1.71

exterior wind mndiiions EIq/m3 pCiIL

well above U.S. average 65 1.76

about average 52 1.41

well below US. average 53 1.44

with the next two sets of data. Cases where the slab was uncracked had slightly higher levels than homes with slightly cracked slabs and only about 32% lower levels than badly cracked slabs. The pipe entry seals correlation showed that good seals actually increased the Rn levels slightly over poor seals. Construction materials for the homes further add to the confusion. Houses made of brick or stone, both of which should have elements that can pro- duce Rn, had lower levels than wooden exterior homes. In- terior walls made of plaster showed lower levels than those made of dry wall or wood, again contrary to the idea that building materials containing potential Rn sources should raise the levels. And fmally the correlation with exterior wind conditions, which should increase the ventilation rates and lower the Rn levels, is also contrary to expecta-

tions. Cohen concludes that the most obvious explanation for his data is that the single most important determiner of Rn levels is probably the geographic location of the house and construction details play, a t best, a minor role (9).

Cohen Survey Omissions I t should be noted that Cohen did not correlate the age

of houses with construction material and Rn levels. If for instance the newer homes were mostly made of brick then that could possibly account for their low Rn levels. Neither did Cohen attempt to correlate Rn levels with the soil that the house was built on. This could obviously be the most significant correlation.

Nero's Survey Compilation The survey performed by Nero et al. is a compilation of

38 different Rn surveys performed in a variety of different formats and settings throughout the country. The group spent a considerable effort i n normalizing the different ways and formats that the 38 different data sets were ob- tained and treated. (Readers interested in the mathemati- cal and mechanical details of how that was done are di- rected to ref (lo).) The data represents a sample from 22 US. areas of which 99% of the data was taken in single family dwellings. There was a maximum number of 817 homes involved in the surveys in 17 different states with slightly fewer surveys performed in the South and Mid- west. As in the Cohen survey, the data was best repre- sented by a log normal distribution having the following parameters: arithmetic mean = 55 f 7 Bq/m3 (1.5 f 0.2 pCi/L), geometricmean = 34 f 4 Bq/m3 (0.9 f 0.1 pCi/L) and a geometric standard deviation of 2.8. The range of aver- age concentration measurements went from 17 Bq/m3 (0.45 pCilL) to 281 Bq/m3 (7.6 pCilL). (Considering the two totally different methods that were used, i t is both surpris- ing and comforting that the two surveys have essentially the same final results for the arithmetic and geometric means and the form of the data.) The survey by Nem et al. also indicates that somewhere between 1 and 3% of the homes in the United States (roughly 1 million dwellings) have Rn levels exceeding 300 Bq/m3 (8 pCi/L) (10).

This prediction of excessively high Rn levels i n some homes in the United States has been confirmed in several cases around the country but none quite as remarkable as the case of Stanley Watras and his family. The Watras' home was situated in Colebmokdale Township in eastern Pennsylvania on a geologic formation called the Reading Prong that is rich in U-bearing bedrock. Watras worked for the Limerick Nuclear Generating Station, and in Decem- ber 1985 he set off the entrance d w r alarms a t the plant as he was coming to work (12). Further investigation showed that the Watras' home had Rn levels of 100,000 Bq/m3 (2.700 ~CiiLl. nearlv 2.000 times the national averaee de- . . - terrninrd in the twd surveys discussed above (2, . A survey of the homes in Colebrookdalc Township was performed bv the Pennsylvania Department of ~nv i ronmen ta l RL- sources. The survey showed that there were many homes in the area that had elevated Rn levels including some with levels in excess of 7,400 Bq/m3 (200 pCi/L). Fortu- nately, no other home had levels as high as those in the Watras' home (12).

Knowing that the US . national average Rn level is -50 Bq/m3 (1.35 pCi/L), i t is possible to begin assessing the overall effects of the radiation dose on the general popula- tion. The annual average radiation dose in the United States is 3.6 mSv (360 mrem) with about 56% of that dose, 2.0 mSv (200 mrem), coming from Rn exposure. (The rem and Sv are units of radiation dose. The Sv is defmed as a radiation dose that delivers the equivalent of 1 J k g of ex-

Volume 69 Number 5 May 1992 353

posed tissue. There are 100remin 1 Sv.)The remaining 1.6 mSv comes primarily from exposure to terrestrial prays, 0.28 mSv, ingested natural radionuclides, 0.39 mSv, medi- cal exoosures. 0.53 mSv. and cosmic ravs. 0.27 mSv (4. 13). Obvidusly, ~h exposure is the single"&eatest radiation source in the typical American's life. Each year the average citizen of the United States receives an exposure from Rn exceedine the averaee lifetime dose to residents of Europe and Asia-from the fklout produced by the Chernobyl nu- clear reactor accident (2).

Dangers from Radon Exposure - Radiation exposure to Rn a t 50 Balm3 (1.35 DCVL) in- -~~~~~~~ ~~~~~ ~ .

creases thechake ofcontractinglungcancer by -0.3-0&. Those individuals livinein homes with Rn levels above 300 Bq/m3 (8 pCi/L) have an increased lung cancer risk of -2% and the risk eoes up meatlv for those in high exposure homes such a i the ~HtFas (2,3,10). People living in homes with high exposures, more than 7,400 Bq/m3 (200 pCi/L), are receiving annual exposures that are essentially equiv- alent to those received by the people evacuated from the vicinity of the ~hernob~lkucle& Gador in 1986 after the accident (3).

Statistical Perspective Radon Risks Compared to Other General Hazards

If the averaee indoor Rn levels are inteerated over the U.S. populati~u, the result is that there are about 10,000 lung cancer deaths per year that can be attributed to Rn exposure (2,3). To put these numbers in some perspective, there are rouehlv 130.000 lung cancer deaths in the United States per year and 85% i f those are attributed to smoking (14). The chance of dying in a fire or from a seri- ous fall at home is nearly the same as dying from lung can- cer caused bv the U.S. average Rn concentration. The risk of dying in acar accident is -2%, the same risk as the lung cancer threat from 300 Bq/m3 Rn levels. Smoking in- creases the risk of prematu;e death from lung cancer by nearly 30% (2, 3).

For people who smoke, or live with smokers, and live in homes that have high Rn concentrations, the risk is espe- cially high. The recent BEIR N Report indicates that the effects of smoking are synergistic with those of Rn expo- sure (5). The report determined that the effects of smoking and Rn exposure were somewhere between additive and multiplicative. If this effect is correct then perhaps 85% of the Rn-related lung cancers will also be smoking related (16).

Radon Risks Compared to Other Hazardous Chemicals

Essentially all of us are willing to accept the risk of driv- ing or living in a home, so that these Rn risks do not sound too horrendous. But if they are looked at in another context they take on a slightly different significance. The Environ- mental Protection Agency (EPA) normally targets chemi- cals and other hazards that ~ re sen t a risk to the general population of 0.001%. The ri$k to the general population from Rn is 100 to 1000 times greater than that presented by ethylene dibromide, chloroform, or benzene and about 10 times greater than that presented by asbestos (2,3). We chemists are aware of the headaches that EPA warnings for these chemicals have presented us. One can only imag- ine the difficulties that R; must be giving the EPA:

-

There certainly will be adjustmentsto these estimates as time goes on and more data on Rn exposure are accumu- lated. In fact. recent news reports on both the W a n d print ~ ~

media have shown that this topic is a current news event. Naomi H. Harlev announced ut an American Cancer Soci-

30% of that measured by Rn monitors placed in basements (15). This particular point has been made on several occa- sions and especially forcefully by Cohen and Nero (11). However. i t is fair to sav that there is a certain level of risk associated with high R; exposures in the home and that it would be prudent to reduce that exposure to some accept- able level.

Detection and Reduction of Radon in the Home It would be advantageous if i t were possible to look a t a

house and determine from site characteristics if it had un- acceptable levels of Rn. Unfortunatelv our abilitv to ore- dict k n levels is not quite at that level of sophi&ica<iou. Certainlv if the house is situated on a eeolopical formation that is &own to contain large amounts of ij-bearing rock, such as the Reading Prong, one might suspect that the Rn levels in the houseiouldbe high. However, note from the discussion above that only a few of the houses in Col- ebrookdale Township had excess Rn levels. Such things as the soil barrier between the house and ground can play an important role in the interior Ru levels. The only sure way to determine the Rn levels in a house is to measure them.

Radon Detection Methods

The two most common and acceptable ways to determine the Rn levels in a house are alpha track detectors or the absorption of Rn on activated charcoal and the subsequent c o u n t k of the r-ravs from the Rn decav products. Alpha track dkectors'are generally pieces OF &astic that *are placed in the home for a period of time up to a year. (If left for a year, the home owner has an annual Rn exposure measurement that is verv valuable in determinine the overall risk. If only done over the winter months, the'esti- mated annual levels wilI almost certainly be high because home heating will raise the average Rn levels.) The plastic is then mailed back to a laboratory that examines micro- scopically the plastick surface. ~ h k a-decay of Rn and its products is a very energetic process that literally makes an impression in the plastic. As the a-particles pass through the plastic, a trail is left that can be seen in a microscope after the plastic has been etched in acid. The laboratories count these trails and can relate that to the number of a- decays that must have occurred in the house. The cost for the a-track detector and associated lab work is around $25, and they can be bought in hardware stores.

The second method is to place an open canister of acti- vated charcoal somewhere the living space of the house. Some fraction of the Rn will be adsorbed by the charcoal and effectively trapped in the canister. Aft& a period of time, usuallv 3-7 davs, the top is replaced on the canister and the c a ~ i t e r is sent t the iestinglaboratory There the canister is opened and placcd beneath a NaI y-ray detector. The detector determines the actinty of the charcoal sam- ple by measuring they-rays that are emitted by the daugh- ters of Kn us thev decav. This activitv can be related t u the average Rn concentration in the house. Again the cost for this test is reasonable, 510425, and the charcoal canisters can be purchased at hardware and grocery stores.

EPA Recommendations

If either of these two tests determines that there are ex- cessive Rn levels. more testine is usuallv done to set an absolute Rn concentration and'possible exposure le&s for the residents. lt is im~ortant that the second set of tests be performed in living areas of the home, not crawl spaces, so that an accurate exposure for the occupants can be deter- mined. At the present lime the EPA is recummending that anion be taken to reduce the Rn in a home if the Hn con-

ety meeting in Phoenix that the human exposure was only

354 Journal of Chemical Education

centration is 150 Bq/m3 (4 pCi/L) or higher (14). Nero et al.

estimate that around 7% of the homes in the United States have levels of 150 Bq/m3 or higher (10).

Lists of companies and testing laboratories that have oroven their nroficiencv in accuratelv measurine Rn levels .~~ ~ ~~~ ~ ~~

can he obtln'cd from t;e EI'Aby calling the ~ar ional Tech- nical Information Service at 703-487-4650 and askine for the Cumulative Proficiency Report for National ido on Measurement Proficiencv Promam. The cost for this r e ~ o r t " - is $15 for a microfiche copy and $39 for a printed copy. A new conv of this rewrt is slated for release in the summer of 199i..~imilar liHts that are specific to the area of resi- dence can be obtained from State Environmental Agencies.

For people who have lived in their homes for longer peri- ods of time, determining their absolute radiation expo- sures is a rather tricky task. However, recently Samuels- son has determined that the absolute e m s u r e levels can be determined from the glass taken from a house's win- dows and pictures (17). A section of the glass is cut out and taken to a laboratory. The long-lived isotope '"Pb will plate out on the glass and slowly decay to 'lOBi and Z1oPo, eqs 12-13. The 210Po a-decay to '06Pb, eq 14, is detected in a pulse ionization detector and the number of decays can be related to the total amount of210Pb present in the house. The total exposure in the house can be determined from the amount of 210Pb detected. Interested readers are di- rected to ref (17) for the full details of this procedure.

Radon Reduction Methods

Homes that are determined to have 'excess Rn levels need to have these levels reduced by some mechanical means. The exact method chosen depends a great deal on the construction of the individual home. Basically the methods rely on exhausting the Rn gas from beneath the home's foundation and exhaustine it outside. This can be done by drilling a hole through the slab, or using natural holes in the house sub-space such as drain tiles or the spaces in between concrete blocks, inserting a pipe into the hole and having a vent system attached to the exit end of the pipe outside the house. Another possible method for homes with crawl spaces is to put a forced air fan into a wall surrounding the crawl space and remove the Rn with

the fan. Most of these reduction procedures are relatively efficient and low in cost, running from the $100 range up to about $2000. Even houses with very high levels of Rn can have their levels reduced to the EPA limits or below. The Watras' house now has a Rn level of 150 Bq/m3 (4 pCiiL) but the cost was high, $32,000, to achieve those lev- els (16). Agood review of the various Rn reduction methods and their viability in differently constructed homes can be obtained from the EPA (18).

Conclusion Clearly there is a potential hazard from Rn exposure in

the United States. As teachers, it is our duty to present to our students and other interested parties the most recent information regarding this problem. I t is hoped that this paper has provided sufficient information on Rn for teach- ers to address the needs of their constituencies in both the classroom and the general public.

Literature Cited - ~~ ~ ~~ ~ - ~~ ~

1. Choppin. G. R.: R y d k g , J . Nuelwr Cbmbtry Theory ondApplieofiom; Pergamon: New York. 1980: Chanter 12.

. . NstionalAcademy~&: Washington, OC, 1988.

6. Cmss,F.T. InRodanonditsfiay Pdwts i i IndmndaAir;N~~mR,W. W.; Nem,A. V, Eds.; Wile?: New York. 1988.

7. Sci. Am. 1986,255 (31. -6. 8. C b m Em. Nemr 1991.a 161. 12. ~ ~ ~ - ~ .~ 9. Cohen.8. L.HmlthPhys. 1986.51, 17Frl83

10. Nem, A.V.; Sehwehr, M. B.: NaaamE, W. W.: Rewan, K L. S c i e m 1988,234,992 00"

12. G A ~ , T. M. ~ ~ ~ i ~ ~ ~ & . t iwr.29 (11. I W ~ . 13, Ionking Radiotion Elposur. of f b Popvlofhn o f f b United Stofe~: 1987; National

Coundl on Radiation Rotedon and Measurementp. U S . Government Pnnting Ofice: Washington, DC, 1987: Repott No. 93.

14. A Citizen% Guide lo Rodan: 1986; United States Env"mmerhl Rotedon Agency Office ofAk andhdiation. U S . Department ofHealth and HvmaoSenicea. Cen- tern f a Dieease Contml. U.S. Gwernment Plinting mice : Washmgton, DC, 1986:

17. sam&rnn, C. Noturn b n d m l 1988,334, 33W40. 18. Radon Reduction Methods, A H o m o m r h Gu& (Third Editiml: 1989: United

States Environmental Rotedon Ageney Oflice of Research and Development US. Government Rinting Office: Washington, DC, 1989: RD-681.

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