A low-flying helicopter seen over Seattle this week is measuring radioactive materials found throughout the area to establish a baseline for comparison in case of a radioactive emergency similar to the one in Japan earlier this year.

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That low-flying helicopter you may have seen or heard in the skies above Seattle and Bellevue is gathering baseline data about radiation levels in the region.

The state Department of Health commissioned the flights to map the amounts and location of radioactive materials to provide a basis for comparison in case of an emergency similar to what happened in Japan earlier this year.

After Japan’s nuclear reactors were damaged, experts in that country were unable to determine how much radiation levels had risen because they had no baseline.

“With this information, we’ll be able to measure the extent of any radioactivity, how fast it’s spreading, what we have to do to keep the public safe,” said health-department spokesman Donn Moyer.

Moyer said the health department wants to be ready for anything from a toxic spill to a radiological weapon attack. “What happened in Fukushima drove home the importance of this work.”

The flyovers here started Monday in Seattle and Bellevue and will move on to other areas in King and Pierce counties, including Tacoma.

The helicopter is based at Boeing Field and will make daily flights until July 28. The flights run anytime from 7 a.m. to 7 p.m., with the frequency and location dependent on weather, fuels and the number of pilots on hand.

The helicopter’s detection equipment — massive pill-shaped capsules encasing a number of sensors flanking the aircraft — is measuring natural radioactivity from sources such as uranium, cesium and iodine, materials found in low concentrations combined with other elements in rocks, soil, water and dust. These materials, which are likely to increase during a radiation emergency, release X-rays or gamma radiation that can be easily measured from a low altitude.

The helicopter is flying at an altitude of 300 feet — about half the height of the Space Needle. The results of the aerial survey will be made available to the public by the end of the year, with the caveat that some information may be withheld for national-security purposes. Similar surveys have already been done in New York, Los Angeles and Atlanta.

Health officials expect to find natural radioactive materials throughout the area from licensed locations such as hospitals, laboratories and road-construction projects. Any levels of radioactivity high enough to pose health concerns will be investigated and residents will be notified.

“There is a certain amount of radioactive material everywhere. It’s been there since the dawn of time,” said Moyer. “So it’s important to know where and how much of it there is.”

Department officials say that the survey is not a direct response to the March earthquake and tsunami that crippled Fukushima’s Daiichi plant, adding that the flights had been in the works since 2009, a coordinated effort of more than 40 local agencies, including King County’s fire, police and health departments, to improve the state’s readiness for radiation emergencies.

The disaster overseas did, however, validate the need for this project in the eyes of state officials.

The reason for selecting King and Pierce counties was their sheer size, as the largest urban populations in the state. The project is funded by a grant from the Department of Homeland Security. Officials hope to continue this effort across the state but say it depends on additional federal funding.

Roberto Daza: 206-464-3195 or rdaza@seattletimes.com