The rankness of a fart can be expressed in decibel-Farts (dBF), a logarithmic scale where dBF = 10 log10(F1/F2).

Characterizing F in standard units is a bit of a chore, but broadly speaking the resolution concentration of hydrogen sulfide (H2S) in air for humans-- the concentration at which 50% of people can detect its odor-- is about 4.73ppb, or 4.73 x 10^-9.

Air at sea level (1atm or 760mmHg) at 15°C contains about 2.53 x 10^25 molecules of mixed gasses, so the resolution volume of a H2S in humans is about 1.19 x 10^17 H2S molecules per cubic meter at 1atm at.15°C. This is value is your F2.

A typical human fart has a concentration of anywhere between .001ppm and 1ppm of H2S concentration, or between 1x10^-6 and 1x10^-3, thus between 2.53x10^19 and 2.53x10^22 H2S molecules.

Therefore the rankness of a typical fart could be expressed as falling somewhere between -60dBF and -30dBF.

This is a useful expression of rankness for modeling attenuation over time, accounting for dispersion, wind drift, and distance from the zero point.