This work explores Hall effect sensing paired with a permanent magnet, in the context of pulmonary rehabilitation exercise training. Experimental evaluation was performed considering as reference the gold-standard of respiratory monitoring, an airflow transducer, and performance was compared to another wearable device with analogous usability - a piezoelectric sensor. Overall, the proposed sensor outperformed the piezoelectric sensor with a mean ratio, precision, and recall of 0.97, 0.97, and 0.95, respectively, against 0.98, 0.90, and 0.88. Characterization of the proposed sensor shows adequate monitoring capabilities for exercises that do not rely heavily on torso mobility. See more.