This work investigates the response to humidity of Quartz Tuning Forks (QTFs) coated with a sol-gel iron-doped amorphous magnesium silicate sensing film. Thanks to the reduced film thickness, the investigated sensors exploit a short time response thus being suitable for applications where a fast humidity change is expected such as in breath monitoring. The sensor working principle takes advantage on the quartz resonance shift due to additional loading mass caused by humidity absorption. Structure and morphology of the sensing film were characterized by X-ray Diffraction (XRD) and scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). The sensors were experimentally tested in static conditions by comparison with a reference hygrometer and the results showed that the sensor sensitivity is proportional to the thickness of the hygroscopic layer. Preliminary breath monitoring tests confirmed the effectiveness of the proposed approach in the presence of fast humidity changes.