The answer is the letter 4.environmental. Cancer is caused by modifications in the genes that change the functioning of the cells. Some of these genetic changes are natural, however there are other changes that are produced as a result of environmental exposures that damage DNA. These environmental exposures include substances such as the chemicals in tobacco smoke, or radiation, such as ultraviolet rays emitted by the sun, which can be avoided, but others are more difficult to avoid, especially if they are found in the air we breathe. , the water we drink, the food we eat or the materials we use to do our jobs.