The balloon borne experiment PRONAOS has measured the sub-millimetre emission from the diffuse interstellar medium to a quiescent filament in the Taurus molecular cloud. We model the output emission of this filament using a radiative transfer code and an independent tracer of the total column density (near infrared extinction). We show that inside the filament the transiently heated particles responsible of the 60 μm emission are removed and the dust sub-millimetre emissivity is increased by a significant factor, 3.4 +0.3-0.7 (for typically nH> 3 ± 1 × 10 3 cm -3, AV> 2.1 ± 0.5). This induces a deficit in the IRAS I60 μm/ I100 μm flux ratio in comparison with the diffuse interstellar medium and a decrease of the grain equilibrium temperature in excess to that predicted by extinction.
