As we all know, contact with a dead animal or person brings ritual impurity. But we also know that "dust we are and to dust we shall return." Most of the soil around us is organic matter that once composed living things that then decomposed. At what point in the process of decomposition is a (former) corpse no longer a defiling corpse?
See the second chapter of Mishna Ahilot מלא תרוד רקב– Double AA ♦May 20, 2022 at 17:34
In the first mishna of the second perek of Maseches Ohalos, it states that a מלא תרווד רקב (a spoonful of the rot of a dead body in a literal sense) has the same level of tuma as a dead body in that it defiles one present under the same roof with that amount of rot or someone who lifts that amount of rot (even without touching it directly). The commentaries explain that this "spoonful" is the amount of two handfuls.
The Rambam in his peirush to the mishna explains that this רקב is very similar to dirt or dust: ורקב הוא העפר אשר ישאר מגוף המת כאשר יכלה לחותו יתרקבו העצמות והוא דומה לעפר
So as long as it's known or suspected that a dust-like matter had it's origins in a dead body, it seems to still have the capability to cause defilement.
1This is missing the important caveat that this rule is only relevant if the complete dead body decomposed without anything else mixed in (eg. clothing) which is basically never May 20, 2022 at 18:06
@DoubleAA And what if the only thing included in the dust (decayed body) is the remnant of the head? Which is equated with the whole (body)…And if the issue of tumah pertains to a prohibition ‘mDoraita’, how are you supposed to hold? May 20, 2022 at 19:32
@YaacovDeane Then nothing May 20, 2022 at 19:34
@DoubleAA What does that (Then nothing) mean? May 20, 2022 at 19:37
@YaacovDeane It means if the only thing is decayed head then nothing happens. May 20, 2022 at 19:39