Time-reversal symmetry relation for nonequilibrium flows ruled by the fluctuating Boltzmann equation

► Nonequilibrium dilute or rarefied gases are ruled by Boltzmann's equation. ► Particle and energy currents carried by such gases manifest fluctuations. ► Remarkably, these fluctuations obey a time-reversal symmetry relation. ► This symmetry relation is here established from the fluctuating Boltzmann equation. ► A consequence of the symmetry relation is the non-negativity of entropy production.