Whether a person gets cancer is a complex combination of body chemistry, genetics and previous exposure to cancer-causing agents, Dr. Mike Franklin, a professor of pharmacology and toxicology at the University of Utah, told health workers at the annual meeting of the Utah Public Health Association Thursday in Park City.

Some of the chemicals in industrial processes, prescription drugs and ingested items like food or cigarette smoke can cause the change in genetic material that is a precursor to cancer.But other chemicals are needed to "promote" the genes to override a natural "stop-dividing" command and produce the uncontrolled cell growth that results in a tumor or leukemia.

For instance, he said, saccharin is associated with bladder cancer in laboratory animals but only if those animals are first exposed to a chemical that changes their genetic material. That makes saccharin a promoter and not in and of itself carcinogenic.

A chemical found in molds on peanuts or grains, Aflatoxin B-1, is capable of causing changes in genetic material and so is labeled an "initiator." Aflatoxins are particularly bad because they are also capable of promoting tumors in cells that have been genetically altered by another initiating chemical.

Unless someone has been exposed to initiating chemicals first, no cancer will form when the same is exposed to promoters. The phenomenon among individuals within a population will seem to differ widely in their tolerance to known carcinogens.

Although that seems complex enough, Franklin said, the amount and activity of different enzymes in each person's body can control whether a possible carcinogen actually turns into a initiator or promoter.

Potential cancer-causing agents are not usually in their worst form when they enter the human, whether it be through the skin, mouth, intestines, or nose and lungs, said Franklin. The body's enzymes must first metabolize or break the chemicals down into more dangerous components. Usually those enzymes convert the dangerous components or the chemical itself into something the body excretes as harmless, but sometimes it switches them to an initiator or promoters.

How often that happens depends on a person's genetic makeup, for example, whether there is an abundance of the enzymes that coverts it to a carcinogen. But it is also related to how many other toxic chemicals have entered the body recently, Franklin said.

He said if a person had smoked cigarettes recently his lungs might have a larger-than-average amount of the enzymes that metabolizes cigarette chemicals. When a person smokes his next cigarette he is more likely to have one of the components switched to an initiator or promoter than a first-time smoker. The person would also be more likely to develop cancer from any other related carcinogen broken down by the particular enzyme - nitrosamines from bacon for example.