A soil sample with an elevated level of plutonium taken along the eastern edge of Rocky Flats National Wildlife Refuge is more than five times the cleanup standard established for the radioactive substance at the former nuclear weapons plant northwest of Denver, state health officials said Tuesday.

But Colorado Department of Public Health and Environment toxicologists “do not believe there is an immediate public health threat,” the department said in a letter penned to community members.

“We do believe that further sampling and analysis is needed to assess what this elevated sample may mean for long-term risks, and whether it is an isolated instance or a sign of a wider area of relatively high contamination,” said Jennifer Opila, director of CDPHE’s Hazardous Materials and Waste Management Division. “We are taking the sample result seriously because it is much higher than previous samples in the vicinity and higher than the cleanup standard.”

CDPHE said the soil sample in question, taken on the west side of Indiana Street about a mile north of 96th Avenue, returned a result of 264 picocuries per gram of soil of plutonium. The standard for cleanup of the Rocky Flats plants, where triggers for nuclear weapons were manufactured over a 40-year period, was 50 picocuries per gram.

Opila said officials in charge of the yet-to-be-built Jefferson Parkway have taken approximately 250 soil samples along the west edge of Indiana Street, just east of the Rocky Flats National Wildlife Refuge fence line, in preparation for building the four-lane tollway that would connect Broomfield to Golden and nearly complete the beltway encircling metro Denver.

She said many of those samples are “still being analyzed by the laboratory” but that highway officials will “share that data with the department, and the department will share it with the community, as it becomes available in the upcoming weeks and months.”

The CDPHE in its Tuesday letter said the high reading came from a single sample. One portion of the sample registered the 264 picocuries-per-gram reading, while a second portion of the same sample registered only a 1.5 picocuries-per-gram reading, which is “within the range of anticipated levels.”

Jefferson Parkway officials have committed to conducting “additional detailed testing in the immediate vicinity” of the sample with the elevated level, Opila wrote.

She said CDPHE has shared the plutonium reading with the U.S. Department of Energy and the U.S. Environmental Protection Agency.

According to the EPA, plutonium emits radioactive alpha particles that are not very harmful outside the body but can be “very damaging” when inhaled.

“When plutonium particles are inhaled, they lodge in the lung tissue,” the EPA says. “The alpha particles can kill lung cells, which causes scarring of the lungs, leading to further lung disease and cancer.”

The state of Colorado says in an online fact sheet about plutonium at Rocky Flats that the “larger the ‘dose’ in the body, the greater the toxicity.”

Bill Ray, executive director of the Jefferson Parkway Public Highway Authority, said in an email to the CDPHE written Friday but released Tuesday that the technical consultants working with the authority believe the elevated reading is coming from a particle of plutonium approximately 8.8 microns in size — or about one-tenth the width of a human hair.

He said the firm analyzing the samples, Fort Collins-based Engineering Analytics, is “satisfied that their Quality Control measures were appropriate.”

“They performed a re-test of the same sample and came up with a similar result,” Ray wrote.

Retired Northern Arizona University chemistry professor Michael Ketterer, who is in the midst of analyzing soil samples taken last month along Indiana Street in connection to trails that have been proposed for the refuge, said finding a plutonium particle as large as the one reported last week is “a low-probability event” given the thousands of soil samples that have been taken on and around the refuge that showed no cause for concern.

Yet, it deserves attention, he said.

“The overriding message is that this result points to a relatively large particle of plutonium dioxide in the sample,” Ketterer said. “We need to study this further and make a determination about the health risk it presents.”

Rocky Flats had a long history of environmental contamination with hazardous radionuclides, including plutonium, as fires at the plant and winds blowing and water flowing in an easterly direction dispersed pollutants from the plant toward Denver.

State officials estimate that more than 14 tons of plutonium were left at Rocky Flats after the plant was closed in the early 1990s. The site was cleaned up over a 10-year period starting in 1995 at a cost of $7 billion.

In 2016, two firms that contracted to run Rocky Flats agreed to a $375 million settlement to be paid to thousands of homeowners living east of the plant as compensation for a decline in home values as a result of living downwind of the plant.