About one in five U.S. adults visits the emergency room at least once per year, according to a new report.

The report, based on a national survey done by researchers at the Centers for Disease Control and Prevention (CDC), found that 18 percent of American adults said they visited the ER at least once in 2014, and about 6.5 percent said they visited the ER two or more times.

Women were more likely than men to make an ER visit; about 20 percent of women said they went to the ER, compared with 16 percent of men.

Young people were also slightly more likely to end up in the ER. In 2014, 20 percent of people ages 18 to 29 said they visited the ER at least once, compared to 17 percent of people ages 30 to 44, and 17.5 percent of people ages 45 to 64.

Of adults who went to the ER, most (77 percent) said they went because of the seriousness of their medical problems. However, 12 percent said they went because their doctor's office wasn't open, and 7 percent went because they didn't have access to other health care providers, the report said.

In the past, researchers have found differences in ER use depending on the type of insurance that people have; those with private insurance are less likely to visit the ER than are people with Medicaid or people who are uninsured. Because nearly 8 million people gained health insurance between 2013 and 2014 as a result of the Affordable Care Act, the researchers also looked to see whether there were changes in ER visits during this period.

They found that, overall, ER use among adults did not change between 2013 and 2014. But the percentage of adults who visited the ER two or more times dropped slightly, from 8 percent to 6 percent. [7 Facts You Should Know About Health Care Reform]

However, there were still differences in ER use depending on insurance type. In 2014, about 14 percent of adults with private insurance visited the ER, compared to 35 percent of adults on Medicaid and 16.5 percent of adults without insurance.

"The findings indicate that, so far, there have been no changes over time [in ER visits], and disparities between groups persist," the researchers said. More research on ER visits could help identify ways to reduce inappropriate ER use, they said.

The high use of the ER among adults who are on Medicaid may be because this population is generally in poorer health than are people with private insurance or those who are uninsured, the researchers said. Adults on Medicaid were the most likely to say that their ER visit was due to the seriousness of their medical problems, compared to people with private insurance or people without insurance, the report said.

The report is published Feb. 18 as a CDC National Health Statistics Report.

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