( CNN ) -- 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 . `` To date , data from EPA 's real-time radiation air monitoring networks continue to show typical fluctuations in background radiation levels , '' Jonathan Edwards , director of the EPA 's Radiation Protection Division , said in a statement Monday . `` The levels we are seeing are far below any levels of concern . '' 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 . `` We know that it 's not coming from our plant , '' said Progress spokesman Drew Elliot . Had the U.S. nuclear plants been responsible for the radioactive iodine , other isotopes would also have been found , he said . The levels detected were so low that authorities do not require they be reported , he said . Sensors in Maryland have also reported elevated levels of I-131 in air samples . `` None of these levels pose a risk to health , '' the state 's Department of Health said . The Maryland secretary of health said Monday that microscopic amounts were also discovered Friday in rainwater . He said the levels found posed no risk to public health . The Massachusetts Department of Health said Sunday that a monitoring station in Boston detected I-131 in rainwater on March 22 , but had not detected any in air . In a question-and-answer page on its website , it says the amount detected should not concern residents . On Monday , Pennsylvania Gov . Tom Corbett said rainwater collected Friday from his state 's nuclear power plant facilities contained low levels of iodine-131 `` likely originating from the events at Japan 's damaged nuclear plants . But weekend tests of drinking water found no elevated levels of radioactivity . '' The levels reported `` are still about 25 times below the level that would be of concern , '' Corbett 's office said in a statement . Similar testing in other states , including California and Washington , has shown comparable levels of iodine-131 in rainwater samples . Trace detections were found in the air in Oregon , Colorado and California . Duke Energy spokeswoman Rita Sipe said Duke Energy had detected trace radioactive elements , likely to have originated from Japan 's Fukushima plant , in North Carolina and South Carolina -- but at a level `` far below '' reporting requirements . 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 . '' A spokeswoman for the EPA said Monday that 90 percent of the 124 RadNet monitors were working .
A growing number of U.S. states are affected , but none at levels that threaten public health . At least 20 states report mercury contamination in food or water or both . ` ` The levels detected are far below levels of public health concern , '' EPA says .