A flow visualisation of the interaction between human respiration and heat plume. In a still room with ambient temperature of 24 degrees and skin temperature of 32 degrees, bouyancy forces drives natural convection (Boussinesq approximation). A heat plume is produced and a person becomes a 'candle' which is undetected with a naked eye. The exhaled air of 0.4ms^-1 pushes the thermal boundary layer outwards before being convected into the plume.
The Boussinesq approximation only applies if β(T - T0) << 1. A temperature difference (air) of 15 degrees will only result in 1% error.