Five million additional U.S. adults should be screened annually for lung cancer, according to updated guidelines released Wednesday by the American Cancer Society.

Previous guidelines, last updated in 2013, recommended screening only for adults ages 55-74 with at least a 30 pack-year history of smoking—and who hadn’t quit smoking or quit smoking 15 or fewer years ago.

A pack-year is equal to smoking a pack (about 20 cigarettes) a day for a year. Someone who smoked a pack a day for 30 years, or a pack and a half a day for 15 years has a 30 pack-year history.

Now, the guidelines have been expanded to include those ages 50-80. The organization dropped the pack-year requirement to at least 20 and says years since quitting should no longer be taken into account, as there is no point at which a former smoker’s risk of lung cancer drops to zero.

Recent studies have shown that modifying screening requirements “could make a real difference in saving lives,” Dr. Robert Smith, senior vice president of early cancer detection science at the American Cancer Society, said in a news release.

The recommended screening is a low-dose CT scan.

A leading cause of cancer deaths

Lung cancer is the leading cause of cancer deaths in the U.S. and the second most frequently diagnosed cancer. Overall, cancer is the No. 2 most common cause of death in the U.S., second only to heart disease. This year, nearly 240,000 new cases of lung cancer will be diagnosed—split nearly evenly between men and women—and more than 127,000 people with the disease will die, according to the ACS.

While lung cancer can be diagnosed at any age, it mainly occurs in those ages 65 and older. Those who meet the screening guidelines should discuss the potential benefits, limits, and risks of yearly screening, the ACS says. And those who smoke should still attempt to quit.

Adults with a limited life expectancy due to health conditions or who are unwilling or unable to receive treatment if diagnosed shouldn’t be screened, the organization says.

Of routine cancer screenings, lung cancer screening rates are the lowest, according to the Prevent Cancer Foundation, a U.S.-based nonprofit. While about 70% and 74% of Americans eligible for breast and colon cancer screenings have been screened, respectively, less than 6% of eligible individuals had been screened for lung cancer before guidelines were updated Wednesday.

Confusion contributes to low screening rates, according to the foundation. In a recent survey of more than 2,000 adults aged 21 and older, nearly half weren’t able to describe what a lung cancer screening entails and expressed confusion over the term “pack year.”