Smoking, pollutants, obesity, exposure to radiation, specific chemicals, exposure to certain viruses like human papillomavirus, and family history are some of the risk factors listed in cancer literature. These factors can be called triggers for cancer. They themselves are not the cause.
Cancer involves uncontrolled cell division and tissue invasiveness (metastasis). These are caused by a series of mutations in the genes of proteins that regulate the cell cycle. These mutations typically involve either promotion of cell division or the inactivation of cell cycle suppression.
Cancer is a genetic disease; tumor cells differ from their normal progenitors by genetic alterations that affect growth-regulatory genes. There exist 2 classes of such cancer genes: the oncogenes, which function as positive growth regulators, and the tumor suppressor genes, which function as negative growth regulators.
If the individual’s genetic makeup has tumor suppressor genes, the oncogenes that cause cancers will not get triggered, and the individual will not get cancer. Probably the genetic composition of your grandfather had a preponderance of tumor suppressor genes.
Be that is it may, smoking may not cause cancer in such patients, but the deposits in the lungs will reduce the ability of the lungs to function efficiently and can lead to lung-related ailments.
References: Natural Solutions for Cancer