Releases of particulate matter may be one reason why monitoring after the TMI incident does not seem to have fully reflected the health risks and resulting cancer rates.  Depending upon particle size, they would have been deposited downwind at various points, and may not have been detected during fly-bys and other radiation monitoring events.

One key factor here is included in an old document made available by the UN Scientific Committee on the Effects of Atomic Radiation (annex d-1), which states in Paragraph 85:

Aerosols (which are particles) originate particularly from primary coolant leaks…