In theory you could, in practice it's fine, very unlikely to happen to you especially at your age having done it for such a short time. Many people, most people in cities in fact walk or cycle to work in busy traffic and very few of them proportionately get lung cancer. I would be more worried about the fact that you obviously feel so tense and worried about it. Stress and anxiety is far more damaging to your health than walking to school in a busy area. Try to relax, see your Dr when you have a moment, explain to him or her your worries and concerns and try to put your mind at rest and not worry so much. I'm 51 and although I now live in the country I spent all my life practically in either Belfast or London, and I, along with most other folk who live and work there have not developed anything nasty because of it.