Story updated Oct. 11, 2019.

Rocky Flats used to be a nuclear weapons plant in Northern Colorado. Now, parts of the site that used to be a security perimeter around the plant have been turned into a wildlife refuge (the former plant itself remains closed to the public). The site went through years of cleanup, but a number of groups are suing the U.S. Fish and Wildlife Service about safety concerns, and ongoing soil sampling has shown varying levels of radiation, continuing the debate over whether it's safe to visit the refuge.

In August, when laboratories were in the process of analyzing 250 soil samples from Rocky Flats, the Colorado Department of Public Health and Environment put out a statement saying that one of those soil samples had come back "with an elevated level of plutonium."

Now, an 25 additional samples requested after that unexpected result have come back looking pretty benign. According to the Hazardous Materials and Waste Management Division of CDPHE, each one was tested twice, using different laboratory methods, and the highest plutonium reading came back at less than 3 picocuries per gram, well below the cleanup standard of 50.

"In the case of Rocky Flats — and this is true of many environmentally contaminated sites — you have all the factors that make the perception of the risk a lot higher than the reality of the risk," said Linsday Masters, an environmental protection specialist with CPDHE. "There's an element of dread that radiation in particular inspires … It's an uncomfortable thing to think about but it's quite a low risk."

Masters has crunched the numbers, and she said that even if you lived at Rocky Flats for 13 years and the soil everywhere had plutonium concentrations of 20 picocuries per gram, your risk of cancer would likely only increase by about one in 100,000. Put another way, Masters said, "We all have a normal or average risk of getting cancer — about one in three Americans will be diagnosed with cancer over their lifetimes."

That means we all have about a 30% chance of getting a cancer diagnosis at some point in our lives. If you lived on site at Rocky Flats for 13 years, Masters said, that chance of getting cancer would likely bump from 30% to somewhere around 30.001%.

David Ropeik has written a number of books on the psychology of fear and risk. He said measurements like this — whatever the number — can be misleading, because in general people really struggle with understanding risk.

"Radiation is a perfect teaching example of this," he said. "There's a huge amount of emotional baggage that goes off instinctively, subconsciously when we hear words like 'radiation' that overwhelms our ability to just be objective about what the facts say."

According to Ropeik, as he wrote in the New York Times, the facts indicate there's not a lot of reason to be worried. For example, he said, researchers followed thousands of survivors who were exposed to lots of radiation during the Hiroshima and Nagasaki bombings.

"While it is true that the rate of cancer was increased by almost 50% for those who had received 1 Gy of radiation, most of the survivors did not develop cancer and their average life span was reduced by months, at most 1 year," wrote molecular biologist Bertrand Jordan in the journal Genetics.

Researchers looking at people who actually worked at Rocky Flats Nuclear Facility report that 16 people died of brain tumors, but they said that because the group was so small, they could find "no statistically significant association between brain tumor death" and exposure to plutonium on the job. Another group concluded that cancer risks couldn't be ruled out, but that "fewer deaths than expected were found for all causes of death, all cancers, and lung cancer." Others wrote, "In contrast to the findings from Russia, studies of plutonium workers in other countries have produced only suggestive evidence for cancer risks."

However, looking at a broader set of studies, the International Agency for Research on Cancer concluded that plutonium exposure increases the risk of lung, liver and bone cancer. The same report noted that a much more commonly encountered substance, radon, "is the leading cause of lung cancer after tobacco smoke."

"In Colorado we actually have a lot of uranium in the rocks," said Lindsay Masters with CDPHE. When uranium breaks down, it releases radon into the air, which can build up inside a home. "About half of all homes in Colorado exceed the recommended radon mitigation threshold and that's much more risky to people in their day-to-day lives than Rocky Flats."

We come into contact with radioactive material in other parts of everyday life, too. Bananas emit radiation (albeit from potassium, not plutonium) at about 3.5 picocuries per gram. That's a higher concentration than the vast majority of the Rocky Flats soil samples.

"A risk that you take voluntarily feels emotionally way different from one that's imposed on you," Ropeik said. "People want choice. And when you don't have choice — like, they dumped all that stuff on the ground at Rocky Flats — the risk feels different."

To run with the banana example, the "elevated" level (264 picocuries per gram) found in one result from one soil sample is about the radiation you could expect from 75 bananas. The second result from that same soil sample clocked in at 1.5 picocuries per gram — in the less-than-half-a-banana range — and is in line with other measurements at the site.

People have argued that bananas are not a good way to conceptualize radiation dose because plutonium and potassium are different, and our bodies are good at getting rid of excess potassium. So, let's take another example, using different units that are meant to express an actual dose, rather than just a concentration.

According to the National Cancer Institute, a regular chest CT scan delivers about 7 mSv of radiation. Assuming the concentration of plutonium in the soil at Rocky Flats Wildlife Refuge is 1.1 picocuries (which is in line with most of the soil samples), CDPHE has calculated that "the estimated exposure to plutonium in surface soil in the area of windblown contamination by a refuge visitor is 0.3 millirem [0.003 mSv] for an adult and 0.2 millirem [0.002 mSv] for a child" if they make 100 visits to the refuge per year, and spend 2.5 hours there on each visit (which is about the same as spending 10 days there straight).

So, an adult would have to spend more than 60 years at Rocky Flats in order to receive the same radiation dose as a CT scan.

That would not be the case if the soil had elevated plutonium levels like the 264 picocuries per gram — then, it would take about three months at Rocky Flats to reach the radiation of a CT scan. But, again, that one reading of one sample was radically different from the others.

CDPHE's Laura Dixon says a full report on soil sampling at Rocky Flats is expected to come out later this year about soil results from about 300 locations. That includes testing done by the U.S. Fish and Wildlife Service, which looked at multiple radionuclides at 48 sites along proposed trails and found none to be concerning.

Still, Chuck Newby isn't happy with the sampling results so far, especially given the plans to build a toll road on the eastern edge of the refuge.

"If you're going to build a toll road there you're gonna need to sample the heck out of that entire area," said Newby, who is president and CEO of Colorado Environmental Analytics. "Envision a 300-foot-wide right of way where construction crews are coming through with earth movers; they're moving a lot of dirt around, they're disturbing a lot of the radionuclides in the soil. When you do that and you have wind conditions or other situations where you're moving a lot of earth around, then that's where you get into trouble."

Newby conducted a pro bono study of the site a few years ago that included a look at wind patterns and velocities at Rocky Flats.

"We argued that the best thing to do is to just leave the refuge — just leave it to the plants and animals. Don't try to mix people in with this contaminated soil," said Newby. "It's always better to err on the side of caution."

And here is where we come full circle, back to the perception of risk. Many people choose to get in a car every day, even though statistics show it's a really bad health choice.

In some ways, living on the Front Range involves a risk.

"You're the better part of a mile high, so there's less atmosphere between you and the infrared radiation coming from the sun and you're at greater risk than if you're at sea level," said Ropeik.

It's possible to see risk everywhere, nowhere and at many points in between, he said: "You can see it wherever you think it's gonna keep you from getting to tomorrow."

This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Boise State Public Radio in Idaho, KUER in Salt Lake City, KUNR in Nevada and KRCC and KUNC in Colorado.