Highly porous silicon reacts with water vapor in the air to create images on photographic plates within seconds. It is demonstrated that this process, known as chemography, can be used diagnostically to reveal inhomogeneity in porous silicon layers that is not always detectable by optical or electron microscopy. It is suggested that many dried highly porous semiconductors may constitute a significant health hazard owing to the liberation of high concentrations of toxic hydrides through hydrolysis.