Radon: some concrete issues.

For years, environmental engineers have recommended that homeowners seal cracks in the concrete of basement or slab floors to limit the infiltration of radon, a radioactive gas emitted by radium in the soil. However, while cracks do provide an easy entry, radon also can pass through intact concrete.

In fact, because intact floors and below-grade walls usually cover a dramatically greater surface area than cracks do, diffusion through solid concrete may actually prove a home's dominant portal for radon, observe Vern C. Rogers, a Salt Lake City-based engineering consultant, and his colleagues in the September Health Physics.

Rogers' team analyzed radon's diffusion through 25 samples of residential concrete, 16 of them cored from Florida homes. They found that gas molecules passed through the 2-to 4-inch-thick samples at rates of about a ten-millionth to a hundred-millionth of a meter squared per second -- or up to five times faster than reported previously.

Where the soil's radium content is relatively high and the ground fairly impermeable to air flow (such as in clays), there will be relatively little air-driven seepage of radon through concrete cracks, says coauthor Kirk K. Nielson. Diffusion, however, is fairly insensitive to soil permeability. As such, he notes, where high-radium, low-permeability soils occur, diffusion can predominate -- sometimes accounting for up to 90 percent of the radon entering a home from soil.

The new data also indicate that the amount of water used to mix the cement affects its porosity and therefore radon seepage. The wetter the mix, the faster radon diffuses through hardened concrete, explains Nielson.

But concrete can itself be a source of radon, points out a pair of researchers from the Netherlands in the same journal. The two scientists, at N.V. KEMA in Arnhem, recorded radon emissions that continued for at least 8 years after a concrete was poured. The good news: The release rate fell with time -- particularly after the first 2 years.

These investigators focused on the role of fly ash -- the fine ash produced by incinerators and power plants -- which the Dutch have begun adding to much of their concrete. Because this material tends to contain more radium than cement, concern has been building that ash-based concrete may become an increasingly important source of indoor radon.

But in the Dutch study, adding fly ash to Portland cement reduced a concrete's radon-emission rate "in all cases." For concretes made with "blast-furnace cement," however, adding fly ash increased the amount of radon seepage - though not beyond that seen in normal, ashfree concretes.

The Arnhem researchers also observed that for the concretes they studied, radon emissions rose as the humidity in the environment increased -- to a maximum of 75 percent relative humidity. This provides yet another reason to keep those basements dry.