[Wientje]

Radiation is not a static. If a particle happens to decay at your detector when it is detecting, you'll get a massive spike. As the radiation changes between hours, places and the specific isotopes decaying, it's near impossible to get fast and accurate results for average dose recieved.

[neutronicus]

    If a particle happens to decay at your detector when it is detecting, you'll get a massive spike.

This is not true. The energy deposited by a single quantum of radiation is negligible, and at this point most of the activity is from long-lived isotopes and so shouldn't change from day to day.

I would guess that wind patterns are responsible for the variability in measurements, not nuclear physics.

[eliasmacpherson]

The way they monitored people during the human plutonium injection trials[1] for the WW2 nuclear program was monitoring the radioactive content of the human excrement and urine, not sure how practical that is. They injected people that had terminal illnesses and short life expectancies with ~5 micrograms of plutonium(and other elements) to ascertain the effects, so the dangers to nuclear scientists working on the program would be known. The study didn't find much in the way of damage (i.e not fatal or cancerous) to those studied, but the isotope injected seems to be of high importance as some isotopes are more radioactive than others.

[1]http://inpp.ohiou.edu/~massey/pdf/10_human_Pu.pdf

[Tichy]

I would think one decaying event is one blip, and higher readings indicate more decaying events. A single particle can not decay very often and hence can not result in a spike.