The issue is that the lungs doesn't always manage to clean the asbestos fibers at all, so some may be stuck in the lungs forever. One stuck fiber might lead to

The issue is that the lungs doesn't always manage to clean the asbestos fibers at all, so some may be stuck in the lungs forever. One stuck fiber might lead to a cancer. It's very unlikely of course, but it could happen. A thousand stuck fibers are a thousand times more likely to cause cancer, but just one could still be enough to do it. With cigarettes for example it's different, since the self-cleaning process will clean and remove the remains of the cigarettes from the lungs fairly quickly unless the smoking is repeated over a long period of time and the system gets over-saturated and gradually breaks down.