| MadSci Network: Chemistry |
Lili: You have asked a very good question. I am going to try to make it a little clearer before I try to answer it. When you talk about "preventing" radon in basements, the answer is "No." Radon is a noble gas that is constantly being "created" in the earth's soils due to the radioactive transformation of radium, which is present in varying concentrations everywhere. Some of this radon is continually escaping from the soil and these atoms enter the air near the surface of the earth. At times, the radon concentration near the surface of the earth can approach 1 picocurie per liter of air when the air is very still. Usually, however, the concentration is about one fourth of this value when the wind is blowing and the air is mixing. This amount of radon is always in the air we breathe, and no matter what we do to our basements, this "outdoor background" concentration of radon will always be there. This background concentration of radon amounts to approximately 4 to about 17 atoms of the gas in every cubic centimeter of air. Since there are no studies that have ever shown any adverse health effects of breathing these "outdoor background" concentrations of the gas, I will assume from here on that you meant concentrations above those naturally occurring outdoors. Some houses, depending on the soil they are built on and the "tightness" of the weatherproofing among other things, can effectively trap the atoms of radon from soil gases in contact with the home's foundation. These atoms of gas contained in underground soils are usually relatively "rich" in radon, so when just a little bit of soil gas leaks into a house, it can carry thousands of atoms of radon in every cubic centimeter. When these atoms get into a home that is extremely well "weatherproofed," they don't get diluted with outside air very much because the windows and doors are so well sealed. For that reason, the radon concentration can build up to relatively high numbers. In a very small percentage of homes, concentrations (in basements, in particular) have sometimes approached the concentrations measured in underground uranium mines where serious health effects have been observed. That is why some government agencies have concerns about high levels of this radioactive noble gas. Now, if your question really means, "Can these high levels of radon be prevented?", I think the answer is "Yes, to a large degree." The way to prevent these atoms of radon from entering a home is not so much with a "substance" as with a technique. A sealant such as silicone caulk can help the technique to work better, so I guess I should have said that the answer lies in a combination of sealing any openings in the home's foundation below ground, along with (and this is the most important part) reversing the pressure gradient in the basement. What do I mean by "the pressure gradient?" All gases have a pressure. The outdoor air pressure is usually reported as the "barometric pressure" on the daily weather report. The soil gases (even though they are under the ground) are also at approximately this same pressure. BUT (especially in well- sealed homes and especially in winter), the pressure of the air indoors is usually just a little bit less than the pressure outdoors. What that means is when an atom of radon in the soil at the outer foundation wall encounters a microscopic crack in the concrete, it tends to get pushed into the basement by other soil gas atoms moving naturally toward areas of lower pressure. To prevent this from happening, either the pressure inside the basement must be raised a little, or the pressure in the soil near the foundation must be lowered a little. Either of these actions would "reverse the pressure gradient" across the basement wall. The result of this would cause that same atom of radon at the outer surface of a tiny crack to encounter atoms of basement air moving OUT of the basement toward the lower pressure of the soil gas. Therefore, these outside atoms would never enter the basement. Typically, this technique of "reversing the pressure gradient" is able to lower basement concentrations of radon by about 90%. To briefly add some specifics to how this pressure gradient reversal is usually accomplished... One way would be to seal the basement off from the rest of the house and then use a fan to blow air from the upper levels of the house into the basement to increase its pressure slightly. This has some difficulties because the air being pumped into the basement has usually been heated and it will cost money to heat the air that replaces it from outside. The other way (more popular, I think) is to drill a few strategically placed holes in the foundation floor and/or walls and fit them with, say, 4-inch plastic PVC pipes, that all connect to one continuously operating exhaust fan. This fan will then take the soil gases from just outside the foundation and pull them into the PVC pipe network and then push them outside above soil level. This will result in lowering the pressure of the soil gases in just the critical area near the foundation walls. Removing some of the radon from the outside of the foundation is not what makes this technique so effective. What makes it so effective is causing the pressure on the outside of the walls to be just a little LESS, rather than a little MORE, than the pressure inside. Lili, the U.S. Environmental Protection Agency has a number of publications on residential radon programs and techniques. You can probably get these for free if you contact the local EPA Public Information Officer. Also, there is a nationally recognized radon expert at the University of Iowa. His name is Dr. William Field, and I think he would be able to give you direction and guidance also. I am also willing to answer any follow-up questions you might have, or to clarify anything I've said that you might not understand. Good luck with your science fair, and please feel free to keep in touch with me on any questions you have. Jerry Gerald L. Gels gelsg@aol.com
Try the links in the MadSci Library for more information on Chemistry.