The Southwest has the lowest health insurance coverage in the country, with 30 percent of nonelderly adults and 18 percent of children uninsured, according to a new government study.

And New England – with a rate of uninsured people less than half that of the Southwest – has the largest proportion of its population covered, the study found.

The study marks the first time the Atlanta-based Centers for Disease Control and Prevention has compared different regions of the country by health insurance status, said Robin Cohen, the lead researcher.

Cohen declined to theorize why Arizona, New Mexico, Texas and Oklahoma together have higher rates of uninsured people than other parts of the country.

But another expert said it likely comes from a combination of factors, including state policy decisions and the fact that many jobs in the Southwest are service, construction or other jobs without good health benefits.

Aggressive steps by states such as Massachusetts to increase coverage of their uninsured may widen the gap between regions like New England and the Southwest, said the expert, Ken Thorpe of Atlanta’s Emory University.

“There are substantial inequities in coverage depending where you live, and they seem to be getting worse,” said Thorpe, a health policy researcher.

The CDC study’s results are based on a national, in-person household survey of more than 106,000 families in 2004 through 2006.

The researchers focused on noninstitutionalized people under the age of 65, the age when Medicare insurance for the elderly kicks in.

The study presented estimates for the 41 states that had at least 1,000 respondents. But the researchers pooled data from the other states as well to come up with regional estimates.

Among those states for which there were data, Oklahoma had the highest percentage of people uninsured – more than 33 percent. Hawaii and Massachusetts were tied with the lowest percentage, at 9.5 percent.

The Southwest has a large American Indian population served by the Indian Health Service. The Indian Health Service was not counted as a form of insurance, in keeping with definitions used in other health insurance studies. But even if it had been counted as coverage, the Southwest still would have had the highest uninsurance rates, Cohen said.

In the six-state New England region, 11 percent of nonelderly adults were uninsured, as were a little under 4 percent of children.

Next best? Three regions that include the Great Plains, Great Lakes in the upper Midwest and the Northeast each had uninsurance rates of 14 to 15 percent for adults and about 6 to 7 percent for children.

The Southeast was the second-worst region, with nearly 23 percent of adults uninsured. The Rocky Mountain states were second worst for children, with nearly 12 percent uninsured.

For the nation as a whole, nearly 17 percent of people under 65 were uninsured at the time they were interviewed.

A second study by the CDC, also released Wednesday, presented 2007 data from the same annual survey. It found about 16.5 percent of Americans were uninsured at the time they were interviewed that year. That survey only covered 20 states and did not give a regional breakdown.

The CDC estimates that as many as 54 million Americans went uninsured during at least part of the year, and nearly 31 million were uninsured for a period of more than a year.