To the Editor: I would agree with Mr. Luckenbaugh that my paper, "How Much Radon Is Too Much?", does not cover the subject of smokers and nonsmokers and their interactions with radon. At the time I wrote the paper, there was no information about this subject in the literature, and I could not address that issue. I have completed another literature search and found that there is stiil precious little information on this subject, but a t least there is some. The most recent Morbidity and Mortality Weekly Report on Lung Cancer chronicled the 1986 deaths from lung cancer in the United States ( I ) . That year 126,000people died from lung cancer and more than 80% of these deaths are associated with cigarette smoking. Passive smoking was associated with 3800 of these deaths, and asbestos exposure accounted for another 5500. The report estimates that the risk associated with radon and smoking is 6-11 times higher for smokers than nonsmokers. There is also a preliminary estimate that out of the 126,000deaths, home radon exposure contributes 5,000-20,000 deaths. The report notes, "an estimated 85% of these deaths are due to the combined exposure of radon and cigarette smoke." Furthermore, a t the 1991 annual meeting of the Health Physics Society,R. H. Johnson and E. Geiger reported that cigarette smoke increased the concentration of radon progenv in the air of basements and mound level livine areas (2; After Lighting a single cigareke in the basement of a nonsmoking fam~ly'sbasement, the radon daughter levels went up 25% in five hours. A second cigarette lighting, 24 hours later, caused the levels to jump an additional 40%. Twenty cigarettes gradually lit over a 24-hr period, caused the dauehters' levels to double in three hours and triple in 28 houri. Apparently, the fine particulate matter in-cigarette smoke allows the daughters to remain suspended in air rather than depositing on the walls, furniture, or drapery Suspended in air, the radon daughters are much more likely to be inhaled and increase the exposure of the room's occupants. Apparently, the effects of radon and smoking are synergistic and substantially increase the risk of contracting lung cancer. ~ h v i o u s lif~ ,100,800 deaths 1804 of 126,000)are added to the 3800 passive smokinn deaths: 5500 ashestos related deaths; and'20,000 radon related deaths, the numbers exceed 126,000. However, there is considerable uncertainty in the radon estimate. Note that i t was reported as an estimated 5,000-20,000 deaths, showing the uncertainty in the total population exposure and the dose response a t the levels found in homes. If radon related deaths are actually 5.000. then there are still another 5.000 deaths left to be attributed to black lung, silicosis, and so forth. The single most important issue that can be perceived from the morbidity and mortality data is that if radon levels in homes across the US.were dropped to 4 p C f i , the health effect would still be minuscule compared to stopping smoking. While it is true that people living in homes with 200 pCiL Rn concentrations are receiving exposures equivalent to the people evacuated from the Chernobyl vicinity, i t is also not surprising that this is not immediately reflected in their demise. There are the issues of how the dose is delivered, and what organs of the body are effected. Exposure near Chernobyl was through airborne particulate and gaseous material, much of which was in the metallic state. (Many of the Chernobyl plant workers noted a strong metallic taste shortly before experiencing the symptoms of radiation syndrome.) Some of the metals are soluble in body fluids and then are precipitated in blood-producing organs and so forth that are much more radiation sensitive than lung tissue. Furthermore the Ukrainians received their dose in the relatively short span of a few days. Comparing this exposure directly to that of homeowners is fraught
with numerous difficulties. In the paper I did not intend to show an exact correspondence between these exposures but rather to give an idea of the size of the doses involved in home Rn exposure. The highest radiation exposure for the Watras family that I am aware of is 2700 pCiiL which is still quite large. Using the EPA's estimates fromA Citizenk Guide To Radon that corres~ondsto 370 chest X-rays per day (3). I susoect that the E~;Ahas overestimated the risk in ;his instance. I note that anlong other assumptions, the KPA has assumed the house is occupied 75'"rf the time which is prohahly an overestimation. We should also note thnt human belngs can he exposed to large doses of radiation and still live long lives. Marie Curie must have received a radiation exposure in her earlv 30's thnt would rival or exceed the Watras' exposure. ~ h died k in 1934 at the age of 67. I totally agree that the radon levels in houses are greatly affected by rain. Apparently as the water fills up the maces between the mains of sand and dirt. the radon is firced out into the at&osphere. An excellent'correlation of radon levels and rainfall is eiven in reference (4). As our knowledge of the average radon exposure in homes and its effects on the occupants increases, I feel certain that the estimated risk from radon will decrease. As chemists we are aware that the EPA has associated some risk with chemicals, such as acetone and benzene, that we routinelv use in our work. I do not expect laboratories to cease using these chemicals because there is a risk associated with them. However. the knowledne of the risk allows the labs to use these chemicals in a s a f e r fashion. The same can be said about home radon exposure. The knowledge of the risk, which I hope my paper presented forthrightly, gives the readers a chanceto assess their own particular situations and act accordingly.
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Literature Cited 1. J.h e r Med. Assoe. 1989.262191.117&1172
2. R ~ I OJ.sei. ~ , N#WS 1991. i40,79.' 3. A Citum'a Guide lb Rodon: 1986; United States En+mnrnental Rotedim Agency Officeoffir and Rad,stion.U.S. Department of Health sndHvrnan Services. Cenfenfor Dinease Contmi. U.S. Government PRntlng Office: Washington, DC,l986:
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OPA.RE.nnd
4.Radonond asDemy Pmducfs in IndmrAicNazaroff, W. W.; Nem. A. V. Eds.:Wiley: New York, 1988.
Charles H. Awood Mercer University 1400 Coleman Avenue Macon, Georgia 31207 Soda Water, Supercooling or Freezing Point Depression? To the Editor: Soda water contains water, carbon dioxide, and trace amounts of sodium bicarbonate (maybe a ppm fluoride) and nothing else! The concentration of COP will vary slightly from bottle to bottle'but should never exceed 0.17 m (5 atm COz pressure) at 25 "C. The Henry's Law constant is 0.0345 mol COz per kg Hz0 a t 25 "C, which means 'A 0.1 m solution of C0, is only 0.2%dissociated (Ka=4.5x lo-' at
25 "C) which means that 99.8% of the C02 remains as molecular
C02(aq).However the 0.2% dissociation is sufficient to give the solution a OH of about 3.7. Commercial soda is made bv dissolvina. under pressure.one to fwe voLmes of CO, gas per vo ;me of waier The pressure in tne oonles sno-Id never exreea 5 atm even ar 50 C b-t the rated press-re of tne bonle s 20 am D Herent flavorsmay nave slightly different pressures of CO,. The manufacturers of soda are ve& proud of their safe, new plastic bottles which are refillableunder certain conditions. In Canada the makers of Big 8 soft drinks have soda fountains in certain grocery storesfor custohers to refill their 2-L bottles. The oolvethvlene tereohthalate (PET) , , , . . bottles have been a great sLccess an0 gone are tne aays ofexplod ng glass bon es Nevenneless r is srlll a p ry thar soda IS ess expens ve and O J ~se s m k Volume 71 Number 10 October 1994
903
50-
P
10-
O Temp PC]
P atrn F g.re 1 Plot of me mo a so ~b l y of C02 gas n water as a IJnnlon of the eq~ll.ont.mpressJre 01 gaseoLs COL Tne henrys law constants at 0 and 25 'C are 0 077 an0 0 0345 mol per kg *arer per am of COe [Ref. 1, q. L?. liauid 1. H?O dissolved in
CO?
that 0.0345 mol of COZgas will dissolve in one kg of Hz0 if the equilibrium pressure of CO2 above the solution is 1.0 atmosphere ( I , 2). The solubility of COz in water a t 25 "C and a t 0 "C is shown in Figure 1.Gases are less soluble in warm water. Naturally effervescent soda water can occur if cold (0 "C) spring water, nearly saturated in COz, is bottled cold before the gas escapes (Henry's Law constant is 0.077 a t 0 "C). Commercial soda water is prepared by dissolving COz in water under pressure. Usually1 commercial soda is about 0.10 m in COz from which the freezing point depression can be estimated to be about 0.186 "C (from AT = m krwhere k r = 1.86 Kmol-I). On the basis of a colligative properties argument, the equilibrium freezing point for soda water should not fall below -0.2 "C. The explanation of Bare (3) cannot be correct. Furthermore, the applied .. pressure is in;tufficicnt to account for the melting of ice in the manner that ice malts undcr the iceskate blade. W h y does the soda often freeze a t -8"C? Colligative properties of the COz solute cannot be the explanation for two reasons. There is simply not enough Con in solution and even if there were enough COz in solution, the solution would still freeze close to 0 "C because there IS a clathrate hydrate (C02.6Hz0)formed with a freezing point of -1.4 "C under a pressure of 10.3 atm. The phase diagram (Fig. 2) indicates that the COz.6Hz0 solid may be present up to 10 "C under 44.3 atm of COz (1,4).Clathrate hydrates are interesting compounds (P6)and it is quite likely that when the soda water does freeze, there is a t least some CO2.6H20solid (C) together with the ice (I). I t is clear from the phase diagram (Fig. 2) that the solid clathrate hydrate, fC),has a n existence range between the two quadruple points A (-1.4 "C, 10.3 atm) and B (+lo "C, 44.3 atrn). Soda water may even freeze a t a temperature higher than the freezing point of pure water provided there is sufficient COz in the system. The solubility of COz in ice is much less than in water and provided the expan-
904
Journal of Chemical Education
10
Figure 2. Projection on the pressure-temperature plane of the phase diagram for CO, and H,O. Adapted from References one and four. Notice that at high pressures and temperatures the two liquids are immiscible. A and Bare quadruple points. sion of liquid water to ice does not break the bottle, the pressure of COz may just about reach 10 atm. Water readily supercools. Pure water may be supercooled a s the metastable liquid to 4 0 "C (7).Liquid water in emulsions and capillaries may supercool to -80 'C (8). Small amounts (less than 0.1 m) of solutes such a s C09 1; and HCl actually decrease the rate of crystallization (9). the great polywater debacle, the fact that normal water supercooled caused many workers to believe that they had uolvwater (10). The test for the true eauilibrium freezing point in the soda experiment is togentlyshake the soda taD the bottle, while it cools. A few bubbles to act a s nucleating centers will initiate solidification. The whole concept of the freezing uoint deuression measurement is based on reversible eq;iiihrium i h e r m ~ d ~ n a m i c Supercooled s. liquids and metastable states provide interesting problems for the experimentalist. One problem with this experiment even as a n experiment to illustrate supercooling is this: how does one monitor the temperature of the liquid m the bottle? Perhaps a thermocouple attached to the metallic cap and insulated from the cooling bath solution? Soda water is interesting stuff. The water-carbon dioxide system can he used to illustrate Henry's law, the phase rule, and clathrate formation. I t is another example of the way that mother nature adds interesting twists to what should be simple systems. Acknowledgement The author thanks Frank Zwicker of Big 8 beverages for his information on the bottling process. Literature Cited 1. Gmelin Hondbuck dm onorgonischm Chmzlo. Kohlenstoff, MI C3. Nummer 14;
Krista V.Baerko. Ed.: GMBH: Weinheim. 1973: oo 37-60.
WE
1951,19, 1425.28.
5 . Claussen, J ~ h m zPhvr, . 6. Stackelberg, M.u and H.R.; J Chom Phys, 1951,19, 1319-20. 7. Ma8on.B. J.Adu Phvs.. 1858. 7, 13641. Bsbin. L. Campl. R& h o d . Scl. Paris. 11
Muller
8. 1965,256,36099. Walfon, J.H.:Bran", A. J. Am. Chem. Soe. 1918.38, 317-10. 10.Allen. L . M m Scknfisl. 1973,(Aug 161. pp 37680.
Murray H. Brooker Memorial University of Newfoundland St. John's, Newfoundland Canada A1 B 3x7
