Toxic gases can penetrate concrete blocks.
For years, experts have advised people whose homes harbor worrisome radon levels to seal basement cracks as a first line of defense against further entry of the gas from soil. Now a study suggests that for homeowners whose basement walls are constructed of hollow-core concrete blocks -- as most in the northern and eastern United States are -- cracks may represent only a small part of the gas infiltration problem. Even uncracked block walls can let in radon and other toxic gases, transmitting up to 10 times more pollution from soil than do major cracks, the new study shows.
These "astounding" findings suggest the conventional focus on cracks "is in error," says John S. Ruppersberger, a radon-mitigation engineer at the Environmental Protection Agency in Research Triangle Park, N.C. If verified, the findings could reorient priorities for treating residential radon problems, he says.
Karina Garbesi and Richard G. Sextro of the Lawrence Berkeley (Calif.) Laboratory decided to investigate the transmission of gaseous soil pollutants through hollow-core concrete blocks after another study by their lab showed that sealing cracks in such a foundation didn't cure indoor radon problems. Indeed, Sextro notes, that study showed that sealing cracks achieved only a 10 to 20 percent reduction in this radioactive carcinogen.
Soil gases tend to enter structures only when driven by higher air pressures in the soil. On a calm, warm spring day, a home's air pressure generally matches the soil's. But a number of factors can depressurize a home, including wind blowing against the side of a house, use of large air-moving appliances (such as attic fans and clothes driers) and a large difference between indoor and outdoor temperatures.
Sextro's team studied an uninhabited house in central California, depressurizing its apparently crack-free basement with a large indoor fan. After flushing toxic gases from the inddor air, they monitored the levels of two tracers -- Freon-12, apparently from a landfill 300 feet away, and sulfur hexafluoride injected into the home's yard -- as these pollutants passed through the soil and into the basement.
The indoor depressurization exerted a pulling influence on soil gases to a depth of at least 9 feet below the surface, through an area skirting out from the house by as much as 42 feet, they report in the December ENVIRONMENTAL SCIENCE AND TECHNOLOGY. A computer model they adapted to analyze the data indicates that a natural, highly compacted, 18-inch-thick soil layer more than 6 feet underground helped extend the perimeter of the gas-pulling influence by reducing the home's ability to draw in diluting, near-surface air through the lower parts of its foundation.
The new data highlight the role basement walls can play in transmitting a host of dangerous compounds, including pesticides, says chemist Philip Hopke of Clarkson University in Potsdam, N.Y. In fact, he says, this underappreciated portal may explain how toxic levels of chemicals -- such as the chlordane formerly used in termite control -- pollute indoor environments even when applied correctly around a home's exterior.
The solution? Hopke suggests builders may need to look to other foundation materials such as solid concrete blocks or poured-concrete walls. But according to Ruppersberger, that may not be necessary. He has found that the gas permeability of hollow concrete blocks can vary by as much as a factor of 10 depending on how they were made.
Unfortunately, he says, the blocks favored by builders today are those that are more gas permeable, not less. Ruppersberger adds, however, that his research suggests homeowners can cut gas transmission through even the most permeable of these blocks by as much as 99 percent by liberally coating the indoor surface with latex paint or a paintable concrete topcoat.
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|Date:||Dec 16, 1989|
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