3-5 ciggs a month has no effect on test or health. The cells in the lungs need a chance to repair. It's the people that smoke a lot everyday for long periods of time that have big problems. If you continually damage cells and never give them a chance to heal, they mutate.... otherwise known as cancer.
Most countries in the world smoke far more than we do and drink as much if not more. We are the kings of fast food though!