The study investigates the environmental instability of few-layer black phosphorus (BP) flakes. Continuous atomic force microscopy (AFM) measurements over several days reveal that BP flakes absorb moisture from the air, leading to a volume increase of over 200%. Long-term exposure to ambient conditions results in a layer-by-layer etching process, reducing the flakes to single-layer thicknesses. The strong affinity of BP for water significantly affects the performance of field-effect transistors (FETs) fabricated from BP. Initial exposure to air causes a shift in the threshold voltage due to physisorbed oxygen and nitrogen, while prolonged exposure leads to strong p-type doping from water absorption. Continuous measurements show that the channel material degrades and eventually breaks down after several days due to the layer-by-layer etching process. The study highlights the environmental challenges faced by BP-based devices and suggests that controlling atmospheric conditions could be a method to mitigate these issues.The study investigates the environmental instability of few-layer black phosphorus (BP) flakes. Continuous atomic force microscopy (AFM) measurements over several days reveal that BP flakes absorb moisture from the air, leading to a volume increase of over 200%. Long-term exposure to ambient conditions results in a layer-by-layer etching process, reducing the flakes to single-layer thicknesses. The strong affinity of BP for water significantly affects the performance of field-effect transistors (FETs) fabricated from BP. Initial exposure to air causes a shift in the threshold voltage due to physisorbed oxygen and nitrogen, while prolonged exposure leads to strong p-type doping from water absorption. Continuous measurements show that the channel material degrades and eventually breaks down after several days due to the layer-by-layer etching process. The study highlights the environmental challenges faced by BP-based devices and suggests that controlling atmospheric conditions could be a method to mitigate these issues.