Low-level radiation has delayed effects.

A brief radiation blast composed of alpha particles can kill a person within months. Even a single alpha particle zinging through the nucleus of a cell can wreak enough genetic havoc to wipe out the cell. But what if the alpha particle passes through the body of a cell, skirting its nucleus? Does it leave in its wake irreparable damage that can lead to disease?

The answer is yes, according to a new study by researchers at the Medical Research Council Radiobiology Unit in Oxfordshire, England. The team, led by Eric G. Wright, found evidence suggesting that low levels of alpha particles cause the chromosomes of laboratory-grown cells to become unstable. This genetic instability shows up as broken, twisted chromosomes in successive generations of the cells, Wright's group reports in the Feb. 20 Nature.

The researchers believe their finding may prompt radiation biologists to change the way they assess the dangers of exposure to alpha particles and other radiation that deposits large amounts of energy, such as the products of radon decay. At present, those assessments assume that if penetrating alpha particles do not directly damage a cell's genes, the cell emerges unscathed.

Wright and his team found evidence to the contrary after exposing bone marrow cells taken from mice to one of three dosages of alpha particles emitted from plutonium-238. To set up a control group, they exposed similar cells to a single, massive X-ray dose.

The researchers discovered that 18 percent of the bone marrow cells that received the highest dose of alpha radiation survived and went on to divide, even though the team's calculations indicated that most of those cells had been penetrated by an alpha particle. But nearly two-thirds of the survivors' "daughter cells" suffered varying chromosomal abnormalities, they found. In contrast, most of the control cells died; if they did live to produce progeny, all of their daughter cells bore the same abnormality.

Wright's group concludes that cells irradiated with alpha particles "transmit to their daughter cells some chromosomal instability that may result in one or more visible cytogenetic aberrations many cell cycles later." But they caution that they have not yet figured out how an alpha particle's glancing blow could brint about such delayed effects.

In an editorial accompanying the report, H. John Evans of the Medical Research Council Human Genetics Unit in Edinburgh, Scotland, says the new indications of risk "are likely to provoke something of a stir."