Young women who smoke may have an increased risk of a common type of breast cancer, according to a new study.

Researchers found that women between 20 and 44 years old who had smoked a pack of cigarettes per day for at least 10 years were 60 percent more likely than those who smoked less to develop so-called estrogen receptor-positive breast cancer.

Smokers were not more likely to develop a less common form of breast cancer known as triple-negative breast cancer, which tends to be more aggressive.

"I think that there is growing evidence that breast cancer is another health hazard associated with smoking," Dr. Christopher Li told Reuters Health.

Li is the study's senior author from the Fred Hutchinson Cancer Research Center in Seattle.

Previous research has found links between smoking and breast cancer, Li and his colleagues note in the journal Cancer. The studies looking at breast cancer among younger women have produced conflicting results, however.

They also say there are still questions about whether smoking is linked to an increased risk of some types of breast cancer but not others.

"I think there is a growing appreciation that breast cancer is not just one disease and there are many different subtypes," Li said. "In this study, we were able to look at the different molecular subtypes and how smoking affects them."

He and his team analyzed data from young women in the Greater Seattle area who were diagnosed with breast cancer between 2004 and 2010.

Of those women, 778 were diagnosed with the more common estrogen receptor-positive type and 182 had the less common but more aggressive triple-negative type.

The researchers also included information from 938 cancer-free women for comparison.

According to the National Cancer Institute, about one in every eight American women will eventually develop breast cancer - but the risk is lower at younger ages. Only about one in every 227 30-year-old women - or less than half a percent of them - will develop breast cancer before the age of 40, for example.

In this study, young women who had ever smoked were about 30 percent more likely to develop any type of breast cancer, compared to women who had never smoked.

When the researchers looked at each type of breast cancer separately, there was no link between smoking and triple-negative breast cancer.

But women who were recent or current smokers and had smoked for at least 15 years were about 50 percent more likely to have estrogen receptor-positive breast cancer, compared to women who had smoked for fewer years.

And those women who reported smoking at least one pack a day for 10 years were 60 percent more likely to have that type of cancer, compared to lighter smokers.

It could be that some of the substances found in cigarettes act like estrogens, which would promote estrogen receptor-positive breast cancer, the researchers write.

"There are so many different chemicals in cigarette smoke that can have so many kinds of effects," Li said.

Geoffrey Kabat cautioned that some of the effects found in the new study are small and not clear-cut.

Kabat was not involved with the study, but has researched the effects of smoking on breast cancer risk. He is also an epidemiologist at the Albert Einstein College of Medicine of Yeshiva University in Bronx, New York.

He told Reuters Health the findings of previous studies are not "very consistent."

"We know smoking is bad for you and the earlier you smoke and the more often you smoke the worse off you're going to be in terms of many outcomes, but the role of smoking in breast cancer is not clear," Kabat said. "There may be something going on and it may be a modest effect in some subgroups."