. Today's smoke detector is common in homes and workplaces, which makes it not noticeable, but it is an invention that has saved millions of lives, as having an effective smoke detector in the home reduced the risk of death by fire by more than half, Thanks to the Swiss physiologist Walter Geiger. In the 1930s, Geiger was trying to invent a sensor to detect toxic gas. Instead, the device recorded the smoke emanating from his cigarette, a discovery that led to the invention of modern smoke detectors. Smoke detectors have been installed in industrial places since the 1950s, but their high cost prevented them from being installed in industrial areas. Their use became widespread in homes, and in 1977 more than 12 million smoke detectors were sold thanks to technological progress that greatly reduced the cost. Today, nine out of every ten homes contain smoke detector