It is reported on that Hanford alert as though soil subsidence above and along the location of a buried rail tunnel (used to transport plutonium during its production process) has indicated the tunnel collapsed. Therefore, it is assumed for safety sake, that dangerous pollutants in the tunnel, may have been expelled as dust, prompting the lock-down indoors.
Plutonium dust is about as deadly a material as humans can come in contact with in such a situation. A single one micron dust particle lodged in a lung will kill you.
The PFP is in the final stages of decommissioning. The buildings have been demolished and site restoration is ongoing.
A micron is a measure of length. I will gladly ingest a micron of plutonium so long as there is no width nor depth involved.