Black phosphorus (BP) is a promising two-dimensional material with a direct band gap, making it suitable for optoelectronic applications. However, its environmental instability is a major challenge. This study investigates the environmental instability of few-layer BP under ambient conditions. Continuous AFM measurements show that BP flake volume increases by over 200% due to moisture absorption. Long-term exposure leads to layer-by-layer etching, reducing flake thickness to single-layer (phosphorene) thickness. BP's strong affinity for water significantly affects the performance of field-effect transistors (FETs) in ambient conditions. Initial exposure causes a shift in threshold voltage and reduced drain current, while prolonged exposure leads to p-type doping from water absorption. BP FETs degrade and break down after several days due to etching. The degradation is attributed to water absorption and oxygen physisorption. BP's hydrophilic nature, due to its out-of-plane dipole moment, facilitates water interaction, leading to significant structural changes. The study also shows that thinner BP flakes absorb water faster, enhancing degradation. The results highlight the need for controlled environmental conditions to preserve BP flake quality. The study demonstrates that BP FETs show significant environmental sensitivity, with initial effects from physisorption and later degradation from water absorption. The findings emphasize the importance of encapsulation and controlled environments for BP-based devices.Black phosphorus (BP) is a promising two-dimensional material with a direct band gap, making it suitable for optoelectronic applications. However, its environmental instability is a major challenge. This study investigates the environmental instability of few-layer BP under ambient conditions. Continuous AFM measurements show that BP flake volume increases by over 200% due to moisture absorption. Long-term exposure leads to layer-by-layer etching, reducing flake thickness to single-layer (phosphorene) thickness. BP's strong affinity for water significantly affects the performance of field-effect transistors (FETs) in ambient conditions. Initial exposure causes a shift in threshold voltage and reduced drain current, while prolonged exposure leads to p-type doping from water absorption. BP FETs degrade and break down after several days due to etching. The degradation is attributed to water absorption and oxygen physisorption. BP's hydrophilic nature, due to its out-of-plane dipole moment, facilitates water interaction, leading to significant structural changes. The study also shows that thinner BP flakes absorb water faster, enhancing degradation. The results highlight the need for controlled environmental conditions to preserve BP flake quality. The study demonstrates that BP FETs show significant environmental sensitivity, with initial effects from physisorption and later degradation from water absorption. The findings emphasize the importance of encapsulation and controlled environments for BP-based devices.