Bioavailability is the amount of a substance (like heavy metals) that gets into the bloodstream after ingesting it. In other words, it is the percentage that could actually cause harm.
We measure bioavailability in the lab by putting the soil samples in a fake stomach and intestine, then measure the concentration of heavy metals. This gives us a much better estimate of the potential health effects than by just measuring the soil alone.
Bioavailability is the amount of heavy metals that gets into the bloodstream after ingesting soil.