Tornadoes only since April 1999. Before that day, they also set them off for severe thunderstorm warnings. That led to people not taking siren activations seriously, and when an F4 tornado hit Montgomery/Blue Ash on 4/9/1999, 4 people died. I think that's how it went, anyways.
Actually, that’s not how it went. Hamilton County actually stopped sounding the sirens for severe thunderstorm warnings with a tornado watch in 2011. They began sounding the sirens for severe thunderstorms in 1986, and because of people not taking the sirens so seriously, they changed it to where there also had to be a tornado watch in 1992. At the time of the tornado, tests were only on the 1st Wednesday of each month, and emergency activations were 3 minutes long. Starting in early 2000, the sirens were tested the 1st and 3rd Wednesday each month, and emergency activations became 5 minutes long. Because no other surrounding counties were testing their sirens on the 3rd Wednesday, it led to some confusion for people in those counties, and Hamilton County stopped 3rd Wednesday tests in 2006. And like I mentioned, Hamilton County continued to activate the sirens for severe thunderstorm warnings with a tornado watch until 2011, as people still weren’t taking the sirens seriously. That certainly seems to have helped, and starting sometime recently, they now activate the sirens for 3 minutes with 7 minute intervals of silence during emergencies.
Resident of a county with big a mixture of sirens, but in the process of being replaced.