Eighty-nine million Americans were without health insurance for at least one month during the period from 2004 to 2007, and 23 million lost coverage more than once during that time, according to researchers at Penn State and Harvard University.

"These findings call attention to the continuing instability and insecurity of health insurance in our country," said Pamela Farley Short, professor of health policy and administration, Penn State. "With more than a third of all Americans under age 65 being uninsured at some point in a four-year period, it's easy to see that the problem of being uninsured is a big one that affects lots of people."

To do their analysis, the researchers used data from the U.S. Census Bureau's Survey of Income and Program Participation, which collects information from the same individuals every four months over a four-year period. The team examined changes in insurance coverage among people ages 4 to 64 from 2004 to 2007, which is the most recent period for which four-year data are available.

The researchers found that, of the 89 million people who were uninsured during the period from 2004 to 2007, 12 million were continually uninsured; 11 million gained coverage at some point; 11.5 lost coverage; 14 million experienced a single gap in coverage; and 6 million had a temporary spell of coverage, but were otherwise uninsured. In addition, 23 million people lost health insurance more than once during the four-year period.

The results of the analysis appeared online in a recent issue of Medical Care Research and Review.

"There is clear evidence that people who are uninsured use fewer services than people who have insurance; they postpone prevention and ignore serious problems because they don't feel they can afford the care," Short said. "As a result, some even die for lack of insurance."

But not only do people with gaps in their coverage suffer; those who remain insured and pay premiums suffer as well.

"When people get caught without health insurance, hospitals and emergency rooms are still required to care for them," Short said. "Someone has to pay for those services."

According to the survey, low-income people are particularly susceptible to periodic losses of health insurance coverage. The survey revealed that a little over 64 percent of adults and nearly 60 percent of children who are below 200 percent of the federal poverty level -- equivalent to $46,100/year for a family of four -- were uninsured for at least one month during the four-year period.

"Even though low-income people are disproportionately affected by gaps in health insurance, none of us is really safe," Short said. "Any one of us could be afflicted with a serious health problem that could cause us to lose our jobs and our access to employment-based insurance, which is how most of us get insurance."

In addition to losing or changing jobs, gaps in insurance coverage can occur when people divorce and when children age out of their parents' plans or public insurance programs.

"We all have a stake in this problem of providing everyone with continuing access to affordable insurance," Short said. "Promoting stability and minimizing uninsured gaps should be high priorities as federal and state officials proceed with the implementation of national health care reforms."

The Commonwealth Fund supported this work.

Deborah Graefe, research associate at Penn State; Katherine Swartz, professor of health policy and economics at Harvard University; and Namrata Uberoi, graduate student in health policy and administration at Penn State, also were involved with the research.