On the eve of full implementation of President Barack Obama's Affordable Care Act, the Census Bureau is reporting that 72,546 residents in Sonoma County and 25,938 in Santa Rosa lack health insurance.

County officials are hoping to get half of the roughly 50,000 eligible residents who lack insurance covered by the end of 2014. About 20,000 of the county's uninsured are undocumented immigrants who are ineligible for Obamacare, official estimates say.

The data released today comes from the census bureau's 2012 American Community Survey, which polls more than 3.5 million households across the country annually. The annual survey produces results for areas with populations of 65,000 or more.

The number of residents in Sonoma County and Santa Rosa with no health insurance has remained steady at about 70,000 since 2011. In 2011, there were 69,421 county residents without medical insurance, and in 2012 there were 72,546, but because of margins of error the change was not statistically significant.

The number of uninsured did not change in the county, despite a federal program that allowed counties to conduct an early expansion of Medi-Cal two years in advance of Obamacare.

"You can have a real change, but you may not be able to demonstrate that there is a statistical significance," said Mary Maddux-Gonzalez, chief medical officer of the Redwood Community Health Coalition, a network of clinics and health centers.

Under the county's early expansion of Medi-Cal, 6,600 residents became insured through a program called Path2Health. On Jan. 1, these residents will be transferred to Medi-Cal.

However, most of these residents, about 4,900, already were covered by the County Medical Services Program, or CMSP, for indigent residents. Only 1,700 previously uninsured county residents became newly insured through Path2Health.

Maddux-Gonzalez said that while Path2Health affected only a small population of the uninsured, the real changes will come after Jan. 1.

"We will be seeing the much larger enrollment of currently uninsured individuals over the next couple of years through the Medi-Cal expansion and Covered California," Maddux-Gonzalez said.

The county hopes to insure at least 25,500 Sonoma County residents by the end of Obamacare's first year of full implementation. About 13,500 of these residents will be enrolled in Medi-Cal, while another 12,000 are expected to become insured through the state exchange, officials said.

Under Obamacare, Medi-Cal eligibility is being increased from up to 100 percent of the poverty line<NO1><NO> to up to 138 percent<NO1><NO>. The state health exchange will be open to all, with tax credits available to those making between 138 and 400 percent of the poverty level.<NO1><NO>

Some subsidies will be available to low-income residents who may have trouble paying for out-of-pocket co-pays and deductibles.

The federal poverty level for an individual is $11,490 and for a family of four is $23,550, so 138 percent of the poverty level would be $15,856 for an individual and $32,499 for a family of four.<NO1><NO>

The data released today do not contain information for areas with populations less than 65,000. However, a three-year analysis provides a snapshot of how many uninsured residents there are in four of the county's largest cities, as well as how many people will qualify for expanded Medi-Cal and subsidies through the insurance exchanges.

From 2009 to 2011, there were 2,671 uninsured Petaluma residents who made less than 138 percent of <NO1><NO>the poverty level; Rohnert Park had 1,502; Santa Rosa had 9,906 and Windsor had 389.