Low Radiation Doses in Nature May Pose Risk

A study has found that radiation can trigger widespread mutations in living cells at much lower doses than previously thought possible. Conducted by researchers at Columbia University College of Physicians and Surgeons (New York, NY, USA), the study was published in the December 4, 2001, issue of the Proceedings of the National Academy of Sciences.

The study found that a dose that strikes as few as one in 10 cells has nearly the same mutagenic effect as a dose that strikes every cell. The added damage occurs because of what is called the "bystander effect,” in which injured cells send aberrant signals to neighboring cells. This effect occurs with alpha particles, which come from such sources as nuclear explosions, plutonium, and radon, a naturally occurring gas that seeps into many homes.

The risks of exposure to lower radiation levels are poorly understood and have been the subject of controversy for decades, say the researchers. In the study, they found that when they beamed alpha particles at only one in 10 cells, the results were almost the same as if they had beamed them at all cells. Nearly all the cells showed mutations in their DNA. However, the researchers found they could prevent widespread damage by blocking the signaling network that exists between cells. By using chemicals that obstruct key cell-to-cell pathways, the researchers could restrict the damage to the 10% of cells that had directly received alpha particles. This suggests that signaling networks are responsible for spreading the damage, but little is known about the signaling mechanism at this time.

Previously, public health officials have assumed the risk of radiation is proportional to the dose. Lower doses carry lower risk. The new findings suggest such a linear relationship might not be operating. Instead, the study suggests that at low doses, the risk rises steeply in proportion to the dose, while at higher doses, the risk rises more gently in relation to the dose.

According to Dr. Howard L. Liber, associate professor of radiobiology at Harvard School of Public Health, the study is "another piece of hard evidence to suggest we may need to reassess acceptable radiation levels.”