Cancerous tissue from the body of a smoker was afflicted with up to 50,000 genetic mutations, according to a new study that compared tumors to healthy tissue.

Reuters reported May 26 that researchers at the biotechnology firm Genentech examined tissue from a 51-year-old male cancer patient who had smoked 25 cigarettes daily for 15 years. Study author Zemin Zhang said that the mutations are “likely associated with the smoking history of the patient. It is very alarming.”

Some studies have suggested that genetic mutations may occur every time a cigarette is smoked; Zhang estimated that the subject of his study developed one genetic mutation for every three cigarettes he had smoked in his lifetime.

“If you imagine over a lifetime you are going to develop this many mutations in the genome, some people may think twice about it,” he said.

The findings were published in the journal Nature.