The Pentagon dramatically underestimated the amount of radioactivity to which U.S. Armed Forces members were exposed during Cold War-era atomic testing and explosions, a National Academy of Sciences panel announced yesterday. However, the panel found that the “ionizing radiation” to which the majority of the veterans was exposed — either during testing or in the vicinity of Hiroshima and Nagasaki — is not a serious carcinogen, so the revised exposure estimates will not significantly affect government compensation for veterans suffering from cancer. The government recognizes 21 kinds of cancer as “presumptively” caused by radiation exposure; of the roughly 4,000 former service members with other forms of cancer or other diseases who applied for compensation, all but 50 were turned down. Despite the new findings, their cases are unlikely to be reviewed again.