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Environmental Protection Agency officials confirmed 99 of 135 beta-radiation sensors in its RadNet system—which monitors in all 50 states, the District of Columbia and Puerto Rico—aren’t working and have been turned off. Officials blame electromagnetic interference from sources such as cellphone towers and said efforts to resolve the problem have been unsuccessful.

EPA officials said the beta-detection problem cropped up in 2006 when they started putting the real-time monitors into the field.

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Some nuclear experts said that in an emergency, knowing as much as possible about whether beta or gamma emitters are present, and in which amounts, can be crucial for making decisions such as how large an area might need protective measures. In instances where only a beta emitter is present, the lack of a working monitor could leave officials unaware of potentially dangerous levels of contamination, they added.

The beta-monitoring issue could fuel critics who contend the EPA has been pulling back on its radiation-protection mission—an assertion the agency strongly disputes.

Gamma- and beta-emitting radionuclides can be carried by the wind long distances from a nuclear event, such as an explosion or power-plant accident. Gamma rays from those radionuclides can then travel hundreds of feet or more and penetrate objects, including human tissue, according to federal government websites. Beta particles generally travel only several feet from their emission source. While they can penetrate skin, their main health threat comes if inhaled or ingested in tainted food or water.

The EPA upgraded the RadNet system after 9/11 to monitor and transmit data on both types of radiation. While the attacks didn’t involve radioactive material, “one of the weaknesses identified in the post-9/11 reassessment of the RadNet air network was that decision makers were not receiving data quickly enough,” the EPA said in 2012.

Officials said they don’t know why some beta monitors still work, including locations in Phoenix, Dallas, Pittsburgh and Washington, D.C.

Real-time information on various types of radiation “is very important to the emergency-response community” because it could help determine the need for ordering evacuations or telling people to stay indoors, said Jim Hardeman, retired manager of the environmental radiation program at the Georgia Department of Natural Resources.

Federal officials said they can still obtain beta data by retrieving the filter from each monitoring station. These filters, which collect particles from air flowing through the machines, are sent to an EPA lab for analysis. The EPA said filters are typically changed once or twice a week.

Even with the beta-detection problem, the RadNet system, along with other government radiological resources, has enough capability to do the job, EPA officials said. “We can confidently say that this system is fully capable now and fully operational now with the current monitors it has to detect fairly minute levels of radiation,” said Jonathan Edwards, director of the EPA’s radiation protection division.

EPA officials acknowledged that one major radionuclide—strontium-90, which can get into people’s bones—emits only beta particles. However, they said, an event releasing a large amount of strontium-90 would also release large amounts of gamma-emitting radionuclides that could be picked up. Even with beta monitors, laboratory filter analyses would be needed to confirm the strontium, they added.

But some experts, inside and outside of government, argue that being able to separately and quickly detect the presence of a beta emitter such as strontium-90 could influence evacuation or other emergency plans. The 2012 EPA report also said gamma monitoring wasn’t sufficient to deal with the threat from strontium-90, which could “cause large-scale public health impacts.”

“If real-time beta measurements were unnecessary, why did the government spend money installing the capability in the first place?” asked Daniel Hirsch, a lecturer on nuclear policy at the University of California, Santa Cruz, and a longtime critic of federal radiation-protection efforts who has studied the RadNet system. The EPA’s explanation “seems like an after-the-fact rationalization when they discovered the monitors didn’t work.”