Minuscule levels of radiation from Japan's Fukushima nuclear plant incident have been detected in a widening number of U.S. states, but the Environmental Protection Agency reaffirmed this week that the levels represent no threat to public health.

At least 15 states reported detecting radioisotopes in air or water or both. No states have recommended that residents take potassium iodide, a salt that protects the thyroid gland from radioactive iodine.

Progress Energy reported over the weekend that iodine-131 was detected in the air near its nuclear power plants near Hartsville, South Carolina, and Crystal River, Florida.

EPA is using the nationwide radiation monitoring system, RadNet, to monitor the nation's air and drinking water, milk and precipitation. An analysis from 12 monitors nationwide found "slightly higher" levels of radioactive isotopes in Alaska, Alabama, California, Guam, Hawaii, Idaho, Nevada, Saipan, the Northern Mariana Islands, and Washington state over the past week, the agency said.

"Some of the filter results show levels slightly higher than those found by EPA monitors last week and a Department of Energy monitor the week before," the agency said. "These types of findings are to be expected in the coming days and are still far below levels of public health concern,", CNN reported.