Scientists debate level of threshold tolerance to radiation exposure

The argument that there is no safe level of exposure for nuclear radiation is wrong and is not supported by scientific studies, according to a US researcher who said that cells were very efficient at repairing radiation damage.

"A single fallacy is often more acceptable than a complicated truth," said Dr Otto Raabe of the University of California, Davis. He was addressing a radiation conference in Dublin yesterday organised by the Dublin Institute of Technology during a session on the "linear no-threshold" debate.

The linear no-threshold (LNT) theory assumes that any exposure to radiation carries a risk of developing cancer. It is widely applied by radiological protection agencies and endorsed by the International Commission on Radiological Protection (ICRP).

"The evidence for the threshold has been known for a long time," Dr Raabe said. A new Russian study pointed towards a threshold for radiation, a level that the body could tolerate without subsequent cancers. Breaks in the genetic code inside the cell were commonplace and quickly repaired. On average there are up to 150,000 breaks per cell daily. "We already have a background of DNA breaks," he said, and any contribution to this total by radiation was minor.

"There are still uncertainties that we can't work out because of statistical difficulties," he acknowledged, but there was no connection between threshold and risk. Dr Jack Valentin, scientific secretary of the ICRP, defended the LNT theory. "There is no better hypothesis," he said.

There was no dispute that radiation could cause DNA damage and that such damage was an initiating event in cancer development. Single-strand breaks were easily repaired, but studies had shown it was not so with double-strand breaks.

He referred to a 1996 study which suggested that very low radiation doses could induce cancers in utero and a UN study soon to be published indicated that cancers could be caused at very low radiation doses. Both sides were hampered by a lack of statistical power to determine what happened with low-dose exposures.