Our cat has always been a totally indoor cat. I don't think there's ever been a time that we even considered just opening the door and letting her roam. Now, there *was* the time when my hubby saw a neighbor walking a cat with a harness to keep it safe and decided he'd try the same. I'm laughing right now remembering the thoughts I had about *that* little brainstorm working... I was right. :-)
Bottom line, I never let anyone tell me that cats "need fresh air" or somehow have to run outside where there are so many dangers so they stay healthy. I've never thought those things were true... and our cat is perfectly happy to sit on her perch and watch the squirrels and birds. During spring, summer, and part of fall we have a screen in that window, and that's plenty of fresh air as far as I'm concerned.