Lawrence Livermore National Laboratory (pdf), Sept 28, 2015 (emphasis added):
- Ambient Air Radioactive Particulates
- Composite samples were analyzed by alpha spectroscopy for plutonium-239+240, which was detected in 2 out of 206 samples taken in 2014. Detections at the Livermore Site and Livermore off-site locations [in California] for plutonium-239+240 are attributed to a number of factors including the following: resuspension of plutonium-contaminated soil (see Chapter 6); ambient air from historical operations; resuspended fallout from previous atmospheric testing; or fallout from the Fukushima nuclear accident.
- The highest values and percentage of the DCS for the plutonium-239+240 detections were as follows: Livermore Site perimeter: 13.4 nBq/m3 (0.36 aCi/m3)… Livermore off-site locations: 10.4 nBq/m3 (0.28 aCi/m3).