A new concept was proposed for a personal warning device for use by miners who may be exposed to health hazards from inhalation of alpha- emitting radionuclides, especially radon-222 and its short-lived daughters. Two successive prototype models of the proposed instrument were designed, fabricated, and tested. The personal alpha alarm, in its final form, weighs less than 90 grams (3 ounces), and can easily be mounted on a miner's safety cap. It is powered by electricity drawn from the cap-lamp battery and consumes less than 100 nanoamperes at 4 volts direct current. It is entirely passive and has no moving parts. Alpharadiation originating in the air around it is detected by a 5-cm2 area semiconductor-type sensor that was developed especially for this purpose. The presence of alpharadiation in the air is communicated to the wearer by a small flashing light--a light-emitting diode--clipped to the brim of the safety cap. When the intensity of alpharadiation exceeds a predetermined threshold, the flashing mode changes to a continuous illumination, thus providing an alarm signal to the wearer. Because the detector senses all alpha-emitters in the air, including 222rn, it is not able to measure working levels accurately.