The group whose participation is most needed in the new exchanges under the Patient Protection and Affordable Care Act is unaware of what the exchanges will offer them, just weeks before they are supposed to roll out.

Just 27 percent of people ages 19 to 29 said they've heard of the exchanges, set to open for enrollment Oct. 1, while 70 percent hadn't, according to research from the Commonwealth Fund. The foundation polled almost 1,900 people between November 2011 and March 2013.

The lack of knowledge was greatest among those the administration most wants to enroll in the exchanges — those who had been uninsured in the past year, and those with the lowest incomes. Just 19 percent of young adults who had been uninsured during the year and 18 percent of low-income young adults were aware of the marketplaces.