The only way that I allow my cats to sit in a open window (with a screen on it) is if I'm in the room w/ them. I also only open the window a little bit...enough to get some fresh air in the house, but not enough for them to escape if the screen was to fall out somehow . I also only open it a little bit so that if another cat happens to walk by the house when the window is open, that it can't go after one of mine.