One in every five American homes has health-threatening levels of radon in indoor air, the EPA said upon releasing new studies done in nine states during the 1989-1990 winter.

Environmental Protection Agency officials said Wednesday the latest field surveys indicated Nebraska is among the states having a substantial percentage of homes affected by the radioactive carcinogenic gas.Radon is released from uranium deposits in rock and soil as they decay in radioactivity. The gas seeps into homes through basement cracks and pipe openings and can increase to dangerous levels, especially in winter when windows are shut and ventilation is reduced.

Of the nine states tested last November, the EPA found the highest radon risks in Nebraska. Based on measurements taken in 2,027 Nebraska homes, the agency estimates that 54 percent of all homes in that state - or 293,000 households - are likely to have radon levels about 4 picocuries per liter of indoor air, the level at which the EPA recommends remedial action.

Surveys in Idaho, where 1,142 homes were tested, indicate between 15 percent and 20 percent of homes, or 64,000 households, have elevated radon levels.

The other seven states tested last winter were Nevada, where 10 percent of homes, or 35,000, are potentially affected; North Carolina, 7 percent, 146,000 homes; Oklahoma, 3 percent, 36,000 homes; South Carolina, 3 percent, 43,000 homes; California, 2.4 percent, 247,000 homes; Louisiana, 0.8 percent, 12,000 homes; Hawaii, 0.4 percent, 1,700 homes.

Overall, the EPA has now tested 47,598 homes in 34 states and found elevated radon levels in each state. The agency said 21 percent of all homes tested had radon levels above the 4 picocuries per liter "action level." Of that 21 percent, 2 percent had levels above 20 picocuries per liter.