The waiting room at CommuniCare Health Center in Davis hums with the trills of children and the chattering of adults.

With a sliding economy, the buzz is louder at the clinic, one of five in Yolo County that provide care to the uninsured.

Paula Hutchison of Woodland was there last week because she’s gone without insurance since she started her Paula Pooh Housecleaning service. She’s been lucky, she said, but there have been close calls.

Like last year, when she knocked against an iron railing. Her shoulder turned red, then a yellowish blue and purple. She bought a sling and continued working. It still looks bruised.

Hutchison is among the uninsured in Yolo County, a proportion of the total county population among the highest in California.

Yolo is separated from Placer by a strip of Sutter County. But it’s a world away statistically.

Placer’s combination of demographics makes it one of the best for insurance coverage: wealthier, older residents employed by large companies.

In contrast, Yolo County residents are diverse in occupation and age.

Disparities in coverage, created by the nation’s reliance on employment-based health insurance, are at the heart of the debate about overhauling health care. Those disparities are most evident, locally, in Placer and Yolo counties.

According to new census data, Placer County in 2006 had the lowest rate of uninsured people under age 65 in California: 13.7 percent. Yolo County’s rate was 22 percent.

The uninsured present an immense fiscal and public health challenge: 18,000 Americans die each year because they aren’t covered, according to the Institute of Medicine, a nonprofit research organization. This is because having insurance is closely tied to health outcomes: The uninsured won’t see a doctor regularly, and if they seek care it is likely to be inadequate or too late.

Moreover, the uninsured are a cost for society: One economist recently estimated the tab at $56 billion per year, 75 percent of which is paid by governments. In cash-strapped California, that cost is critical: 6.6 million residents went uninsured in 2007, more than in any other state, according to the California HealthCare Foundation.

Today, the majority of the insured are covered through work, a tradition born of the Great Depression and World War II. During the war, the government limited wage increases, but provided incentives for private companies to add benefits.

But when it comes to insurance, not all employers are equal.

Placer and Yolo counties have had similar unemployment rates in recent years, yet a gap in insurance coverage persists.

What makes employers more, or less, likely to offer insurance is size and industry. Yolo County relies heavily on agriculture, a business with low rates of insurance benefits, said Catherine McLaughlin, a health economist who studies the uninsured at Mathematica Policy Research in Princeton, N.J. Additionally, a significant portion of workers in Yolo are in service and food industries, where coverage is less likely.

At the other end, white-collar workers are most likely to have employer-based insurance, according to the Center on Policy Initiatives, a social policy advocacy group.

In Placer, the financial industry employs 12.4 percent of workers, and professional and business services employ 16.3 percent, according to the Sacramento Regional Research Institute. In Yolo, the financial industry employs 4.9 percent, according to census statistics.

Placer County also counts high-tech companies such as Hewlett-Packard and Oracle among its major employers. High-tech firms are more likely to offer solid health insurance, McLaughlin said.

Yolo County is teeming with small businesses, which often do not offer health benefits, said Pat Billingsley, Yolo’s deputy director of medical services. Smaller businesses face higher overhead costs in purchasing insurance and lack leverage to negotiate with insurers, he said.

In California, businesses with fewer than 10 workers offer insurance 63.6 percent of the time, while businesses with more than 50 workers offer coverage 96.2 percent of the time, according to the California HealthCare Foundation. Census statistics show Placer County has 487 businesses with more than 50 employees; Yolo County has 242.