Maybe because of safety? From [1]: > An ICSD smoke detector is composed of a housing made of polyvinylchloride or poly-styrene plastic, a small electronic ala

Maybe because of safety? From [1]: > An ICSD smoke detector is composed of a housing made of polyvinylchloride or poly-styrene plastic, a small electronic alarm horn, a printed circuit board with an assortment of electronic components, and a sensing chamber and reference chamber, each containing a pair of electrodes and the radioactive source material. [1] http://www.madehow.com/Volume-2/Smoke-Detector.html#:~:text=An%20ICSD%20smoke%20detector%20is,and%20the%20radioactive%20source%20material http://www.madehow.com/Volume-2/Smoke-Detector.html#:~:text=....