SAN FRANCISCO >> The Environmental Protection Agency has ratcheted down the nation’s radiation monitoring program for rain, drinking water and milk in response to a consistent drop in the levels of fallout detected in the wake of the Japanese nuclear crisis.

Extremely low amounts of radioactive iodine tied to the crippled Fukushima Dai-ichi nuclear power plant showed up in milk samples taken in California, Colorado, Connecticut and Massachusetts over the last two weeks, but agency officials said Friday the levels were so miniscule they were not harmful to public health.

Numerous radioactive particles have been detected in milk, water and air tests nationwide since the magnitude-9.0 earthquake and tsunami struck the power plant on March 11.

After seeing the levels drop in recent weeks, EPA has resumed sampling water and milk once every three months, a move some critics felt was premature given that the world’s second-worst nuclear accident is still unfolding.

"Throughout this and other radiation accidents it has always turned out that more radiation was involved than we initially thought," said Ira Helfand, a co-founder of Physicians for Social Responsibility and a physician who practices internal medicine in Springfield, Mass. "The U.S. should continue to monitor milk and rainwater until we can be sure that the plant is under control and there are no further emissions."

The nuclear plant lost its power and cooling systems in the earthquake and tsunami, triggering fires, explosions and radiation leaks. Radiation leaking from the Fukushima plant has forced 80,000 people living within a 12-mile radius to leave their homes, and many still are living in gymnasiums and community centers.

Federal officials use the EPA’s RadNet monitoring system to validate the impact of nuclear incidents, then alert local governments and the public. Launched after the Cold War and upgraded following the Sept. 11 attacks, it also measures radiation levels through dozens of air monitors that periodically suck in air samples and pump out real-time readings about radioactive isotopes. 

Some of the nation’s air monitors were out of service as the public braced for possible exposure to the fallout from Japan in mid-March. EPA said at the time that the malfunctioning monitors weren’t a problem because the system had more than enough units to safeguard the country against a threat that did not materialize.

This week, EPA officials also announced they were weighing whether to keep operating additional air monitors that were sent out in March to increase the network’s geographic coverage. There are currently two each in Alaska and Hawaii, and one in Guam, Saipan and Idaho. 

"No decisions have been made. Those monitors are still deployed and continue to transmit data to EPA scientists," said agency spokesman Brendan Gilfillan. "We would likely see signs of elevated radiation levels in the air before seeing it in precipitation, drinking water or milk."

People typically are exposed to natural sources of radiation every day — most of it from radon in the air and, to a lesser extent, from cosmic rays. Foods we eat also contain low levels of naturally occurring radioactivity, including bananas, carrots and red meat.  

The type of radioactive iodine found in the milk samples is short-lived and decays fairly quickly, becoming harmless. Cows could have ingested the particles through the air, or from eating tainted feed or drinking puddles of rainwater containing it, but that does not pose any threat to the milk supply, said Mike Payne, a veterinarian and food safety specialist at the University of California, Davis.  

"Those levels are really inconsequential to human health, and I think probably (the EPA’s) resources in terms of radiation monitoring are best spent elsewhere right now," Payne said.