The rates of disintegration of potassium-40 and carbon-14 in the normal adult body are comparable (a few thousand disintegrated nuclei per second).
This is small compared to the doses from potassium-40 (0.39 m Sv/year) and radon (variable).
Production rates vary because of changes to the cosmic ray flux caused by the heliospheric modulation (solar wind and solar magnetic field), and due to variations in the Earth's magnetic field.
The latter can create significant variations in The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere.
Such deposits often contain trace amounts of carbon-14.
These amounts can vary significantly between samples, ranging up to 1% of the ratio found in living organisms, a concentration comparable to an apparent age of 40,000.), or other unknown secondary sources of carbon-14 production.
Liquid scintillation counting is the preferred method.