Plutonium in the Dust: Dr. Carl Johnson’s 1976 Warning About Rocky Flats

A 1976 Science paper asked whether Colorado was measuring plutonium contamination in the way that mattered most for potential human exposure.

In August 1976, Science published a paper by Jefferson County public health director Dr. Carl J. Johnson and U.S. Geological Survey scientists Ronald R. Tidball and Ronald C. Severson examining plutonium contamination in surface soil downwind of the Rocky Flats nuclear weapons plant. Their concern was remarkably practical: It wasn't simply how much plutonium was in the soil. It was how much plutonium was present in the fine particles of surface dust small enough to potentially become airborne and be inhaled.

The researchers reported that plutonium-239 concentrations in this “respirable” fraction of off-site soils downwind of Rocky Flats reached as much as 380 times the background concentration at the locations they sampled. That finding led them to question whether Colorado's method for evaluating contaminated land adequately represented a potential inhalation hazard—particularly as residential development was being proposed near Rocky Flats.

What is “respirable dust”?

Johnson and his colleagues defined respirable dust for the purposes of the study as the fraction of soil material with an effective particle size of 5 micrometers or smaller.

Why separate those tiny particles from the rest of the soil? Because a soil sample can contain everything from extremely fine dust to particles far too large to become airborne and be inhaled. The researchers argued that averaging plutonium across all of that material could obscure what was happening in the fraction most relevant to inhalation. As the authors explained, these fine particles were subject to suspension by air currents and therefore represented a potential pathway for human exposure.    

They weren't studying hypothetical contamination

The paper placed the research within the already documented history of plutonium releases from Rocky Flats. The authors discussed the 1957 and 1969 fires, plutonium released from contaminated cutting oil stored outside the plant, and another accidental release to the air in 1974. They cited an earlier Atomic Energy Commission survey that had documented off-site plutonium contamination in soils around the plant.  The map reproduced on page 2 of the paper is particularly striking. It shows Rocky Flats, Arvada, Westminster and Broomfield together with plutonium-contamination contours—and an area identified as proposed housing development immediately downwind of the plant. That development was central to Johnson's concern. The authors wrote that residential development was being proposed within roughly 5 kilometers of the plant, potentially involving as many as 3,000 homes and about 10,000 residents.    

What the researchers actually measured

The researchers selected sampling locations within proposed residential-development areas downwind of Rocky Flats. They collected loose surface soil—approximately the upper 0 to 0.5 centimeters—and then separated out the fine-particle fraction for plutonium analysis. A control sample collected about 23 kilometers south-southeast of Rocky Flats was used to estimate background. The results varied substantially depending on location.

The background sample contained 0.45 disintegrations per minute per gram (dpm/g) in the respirable fraction. In Section 7, immediately downwind of Rocky Flats, samples contained between 59 and 170 dpm/g. The authors calculated those concentrations as approximately 130 to 380 times their measured background.

Farther away, concentrations generally declined, although the pattern also varied with wind direction, topography and disturbance of the soil.The table on page 3 makes the geographic pattern especially clear: the highest measurements occurred in Section 7, immediately downwind, while sites farther from the plant or away from the principal wind direction generally had lower measurements.  

The Colorado standard Johnson questioned

At the time, Colorado had established an interim standard of 2 dpm of plutonium per gram of whole soil for contaminated land. Land exceeding that level would require treatment before residential development could be approved. Johnson and his coauthors questioned whether a whole-soil standard was the right way to evaluate potential inhalation hazards. Their concern was straightforward: a whole-soil sample includes larger particles that are unlikely to be suspended in the air or inhaled. The authors argued that measuring the plutonium specifically in the respirable fraction could provide a more meaningful measure of the potential hazard from airborne contaminated dust. That distinction remains one of the most important ideas in the paper: The amount of plutonium in the soil and the amount of plutonium in the portion of that soil capable of becoming respirable dust are not necessarily the same question.

Colorado's winds mattered

The researchers also considered the environment around Rocky Flats itself. They wrote that winds in the area exceeded 30 km/hour for 500 to 600 hours each year, with much higher wind speeds occurring and prevailing winds generally carrying material east and southeast toward the Denver metropolitan area. The wind rose on page 3 illustrates the direction and average velocity of winds measured at Rocky Flats from 1953 through 1970. This mattered because the authors were concerned not only about the contamination already present at a particular location, but also about the possibility of resuspension and redeposition of contaminated surface material.

What happens when contaminated soil is disturbed?

This may be the most relevant part of the paper to today's conversations about Rocky Flats.

Johnson and his colleagues specifically discussed mechanical disturbance of soil.

They wrote:

“Mechanical disturbances such as those produced by plowing, vehicular traffic, construction work, or street sweeping can resuspend surface dust.”     

They also questioned whether simply plowing contaminated soil under was an adequate remedy. At the time, mixing contaminated soil by plowing was accepted as one technique for treating land that exceeded Colorado's interim standard. The researchers argued that this was insufficient because the plutonium remained in the soil. Future construction or gardening could disturb that soil and potentially bring contaminated particles back to the surface. They also raised the possibility of treated areas being recontaminated through material transported from more contaminated areas upwind. That is an important historical point when considering present-day discussions about soil disturbance around Rocky Flats. It documents that soil disturbance and resuspension were already recognized as potential exposure questions in scientific research around Rocky Flats in the 1970s.

Johnson was thinking about ordinary life

Perhaps the most striking section comes near the end of the paper. The researchers weren't thinking only about industrial workers or unusual accidents. They considered how contaminated surface dust might interact with everyday residential life.

They described potential pathways involving:

• children playing in dusty areas;

• adults working outdoors;

• dirt carried into homes on shoes and clothing;

• dust entering through windows and ventilation;

• pets tracking fine particles indoors;

• playground activity suspending dust;

• gardening exposing previously buried soil;

• construction disturbing contaminated ground.    

These were presented as potential exposure pathways, not measurements demonstrating that individual residents had received particular doses or developed illnesses as a result.

That distinction matters.

What this study does—and does not—tell us

This paper did not study cancer rates. It did not follow the health of people living near Rocky Flats. And it did not establish that the plutonium concentrations the researchers measured caused disease in nearby residents. What it did was measure plutonium in a specific fraction of contaminated surface soil and argue that this fraction deserved particular attention when evaluating potential inhalation hazards. That makes this paper an important companion to Johnson's later epidemiological work. Before Johnson was asking whether cancer patterns differed around Rocky Flats, he was already asking a more fundamental environmental-health question: How should we measure contamination if we want to understand how people might actually encounter it?

Nearly 50 years later, the question still matters

The landscape around Rocky Flats has changed dramatically since Johnson, Tidball and Severson collected their samples.

The plant is gone. A partical cleanup has occurred. Land uses have changed. Residential development has expanded. Portions of the former Rocky Flats property became the Rocky Flats National Wildlife Refuge. A 1976 soil study cannot simply be used to describe environmental conditions in 2026. But historical research matters because it tells us what was measured, where contamination was found, what scientists were concerned about at the time, and what questions they believed deserved attention as development moved closer to Rocky Flats. Johnson and his colleagues ended their paper by recommending that plutonium standards consider the respirable fraction of surface soil because those small particles had the greatest potential for suspension and inhalation. Almost half a century later, their paper remains part of the documented scientific history of Rocky Flats—and part of the history that should inform conversations about contamination, development, soil disturbance and public health.

Read the Original 1976 Study

Carl J Johnson - Plutonium Hazard in Respirable Dust on the Surface of Soil (1).pdf

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Carl J. Johnson, Ronald R. Tidball and Ronald C. Severson“Plutonium Hazard in Respirable Dust on the Surface of Soil”Science, Vol. 193, August 6, 1976, pp. 488–490.