The Smoke Bloon requires 15 hits to be popped. If after 5 seconds of him on-screen he is still not popped, he will explode and cover the entire screen with smoke making anything not visible (except the tabs for selecting towers and special agents and other things like the exit button) for 5 seconds. They also return in Bloons 3, where they leave permanent smoke in a 3x3 area when popped.