Formaldehyde in exhaled human breath serves as a pivotal biomarker for the detection of nasal cancer. In the context of nasal cancer, the non-invasive detection of formaldehyde, which is a primary biomarker, using a chemiresistive sensor has gained significant attention. However, selectivity and sensitivity are still challenges. This study focuses on a non-invasive Bi₂O₃ porous nanosheet-based chemiresistor for the detection of nasal cancer via exhaled human breadth. Here, a Bi₂O₃ porous nanosheet was synthesized using a one-step hydrothermal approach. Rietveld refinement analysis confirmed its polycrystalline nature. The gas sensing properties of the Bi₂O₃ porous nanosheet towards formaldehyde were studied via a static system in a range of 1-20 ppm at a temperature of 100℃. The Bi₂O₃ chemiresistor showed 8.53 sensor response at 1 ppm, with fast response and recovery times of 2.74 s and 4.64 s, respectively. The enhanced sensor response of Bi₂O₃ was attributed to the formation of a porous nanosheet with higher active sites and a large surface area of 60.95 m²g^-1. The N₂ adsorption–desorption isotherm confirmed that a type-II isotherm exists and the mesoporous structure had a mean pore diameter of 60.73 nm. The Bi₂O₃ chemiresistor is highly selective towards formaldehyde compared to other VOCs. Furthermore, exhaled human breadth was used for the detection of nasal cancer in static conditions. The sensor's response values were higher for the breath of diabetic patients, indicating its potential for use in nasal cancer monitoring and clinical diagnosis.