Over the last several decades, the rates of new cases of lung cancer have fallen in the United States. There were roughly 65 new cases of lung cancer for every 100,000 people in 1992. By 2019, that number had dropped to about 42.

But for all that progress, a disparity is emerging: Women between the ages of 35 and 54 are being diagnosed with lung cancer at higher rates than men in that same age group, according to a report published Thursday by researchers at the American Cancer Society. The disparity is small — one or two more cases among every 100,000 women in that age range than among men — but it is significant enough that researchers want to know more.

The report adds to a mounting body of evidence that emphasizes the lung cancer risks for women in particular.

Overall, lung cancer remains the leading cause of cancer death in the United States. The Centers for Disease Control and Prevention estimates that, nationwide, around 197,000 people are diagnosed with the disease each year.

There’s a common perception that lung cancer occurs only in older men who have smoked for decades, said Dr. Narjust Florez, a thoracic medical oncologist at the Dana-Farber Cancer Institute. But, every day, she said, hundreds of women are “dying of lung cancer in this country.”

What do we know about lung cancer disparities?

Researchers are trying to make sense of why lung cancer rates are higher among younger women, as well as the best way to help patients. But there aren’t definitive answers.

While lung cancer is still far more common in older patients, some doctors say they’re seeing more and more younger patients with the disease, even if they are not smokers — another puzzle they’re working to understand.

Cigarette smoking remains the leading cause of lung cancer, and while there have been widespread efforts to reduce smoking, women have generally been slower to successfully quit, said Ahmedin Jemal, senior vice president of surveillance and health equity science at the American Cancer Society and an author on the new study.

But about 15 to 20 percent of lung cancer cases in women are among those who have never smoked, he said. It’s tricky to tease out why these women develop the disease. They may be exposed to secondhand smoke. Or women might metabolize carcinogens differently from the way men do, said Dr. Jyoti Patel, medical director of thoracic oncology at the Lurie Cancer Center at Northwestern Medicine.

Environmental exposures could also play a role. Air pollution has been linked to lung cancer, and it’s possible that women could be particularly susceptible to it, for reasons researchers are still working to understand, said Dr. Patrick Forde, an associate professor of oncology at Johns Hopkins Kimmel Cancer Center.

Additionally, the C.D.C. cites radon — an invisible, naturally occurring gas that can build up in some homes — as the second leading cause of lung cancer. But data on residential radon exposure is mixed, Dr. Florez said.

Ultimately, there is no clear-cut explanation for the disparities. “The differences are really not obvious,” said Dr. Humberto Choi, a pulmonary medicine doctor at the Cleveland Clinic. “This is definitely an area for future studies.”

What are the early signs of lung cancer? And who should get screened?

In 2021, the U.S. Preventive Services Task Force broadened its lung cancer screening recommendation: Anyone ages 50 to 80 who smoked at least a pack of cigarettes a day for 20 years or more, and who currently smokes or who has quit within the last 15 years, should get a CT scan annually. Medicare and most insurance plans fully cover this screening.

Still, less than half of those eligible actually get screened, said Dr. Charu Aggarwal, a lung cancer specialist at Penn Medicine’s Abramson Cancer Center. That may be because of barriers to access, stigma associated with lung cancer or fear of what a screening may find.

Gender bias can also affect testing, Dr. Florez said. Women, and particularly women of color, are less likely to be offered tests for lung cancer, she said.

“I have women that have come with chest pressure and leave the office with Xanax,” she said. “And then when they start coughing up blood, that’s when somebody listens to them.”

In light of the disparities, experts urged women to get screened if they qualify. If you have a close family history of lung cancer not associated with smoking, you should also talk to your doctor about evaluating your cancer risk, Dr. Forde said.

And everyone should be aware of symptoms of lung cancer. Lung cancer is typically not detected until it is late stage, Dr. Forde said: partly because the symptoms go underrecognized, and partly because screening is underused. Early signs can include a cough that lasts for longer than six weeks, upper back pain, shortness of breath and unexplained weight loss, Dr. Florez said. Some people may also develop a hoarse voice; in severe cases, they may cough up blood, Dr. Patel said.

“We’re seeing the demographics change,” Dr. Patel said. “People shouldn’t ignore symptoms that could lead to lung cancer diagnosis.”