NEW YORK (CNN/Money) - Even though Maribeth Jones works at a hospital, the Kansas City woman doesn't have health insurance.

"I can't afford it," explained the nurse assistant, who would have to pay $36 a week for coverage.

Jones's situation is not unusual. An estimated 45 million Americans, or 15.6 percent of the population, was uninsured in 2003, up from 15.2 percent in 2002, according to the U.S. Census Bureau's most recent data.

"The number of uninsured has been growing the last several years," said Catherine Hoffman, senior researcher and associate director of the Kaiser Commission on Medicaid and the Uninsured.

She blamed, among other things, the sluggish economy and growing cost of health care for the decrease in coverage. As the cost of health insurance has escalated, companies have opted to pass those higher premiums on to employees or to not provide coverage.

"We are in the midst of a weak economy," said Paul Fronstin, director of health research, at the Employee Benefit Research Institute. "That means fewer jobs and fewer people with coverage."

Job-based coverage -- one of the major sources of health insurance in the United States -- dropped from 61.5 percent in 2002 to 60.4 percent in 2003, according to the US Census Bureau.

"If you don't get it through your employer, you don't have many options for affordable coverage," said Sara Collins, senior program officer at the Commonwealth Fund. "That means you have huge numbers of people who don't have consistent, affordable access to the healthcare system."

Even when employers provide insurance, she said, a growing number of people opt out of coverage because they can't afford the premiums.

The cost of employer-sponsored covered increased by 13.9 percent between 2002 and 2003, according to the Henry J. Kaiser Family Foundation.

The likelihood of coverage increases with income, according to the U.S. Census Bureau.

Only 41 percent of workers who earn less than $10 an hour had employer-provided insurance while 88 percent of those earning at least $15 an hour received it.

That's a dangerous signal because a growing number of jobs are in the lower-paying service sector.

An estimated 80 percent of the uninsured are people like Jones who are employed or have a family member who is in the work force. They don't have coverage because their employer doesn't provide insurance or they can't afford the employee premiums.

According to Kaiser, the average monthly premium is $282 for an individual and $756 for a family. That means a family would spend more than $9,000 a year for healthcare coverage.

The lack of coverage has a huge impact on the health and productivity of the uninsured as well as on local hospitals and governments, which try to meet their needs.

The Institute of Medicine has estimated that providing health insurance to those without coverage would save between $65 billion and $130 billion a year because it would increase life expectancies and worker productivity and would lower healthcare costs. The Institute, a branch of the National Academy of Sciences, estimates that federal, state and local governments spend about $30 billion a year to pay hospitals and clinics for services they provide to the uninsured.

The Kansas City Free Health Clinic is typical of many of these programs. Started in 1971 by a group of concerned citizens, it provided more than 33,000 patient visits in 2004. The program has seen demand for its services soar in recent years, especially as companies lay off people who historically have had health insurance.

"We are seeing more people than we've ever seen in the past, and we're still turning down hundreds of calls a week," said Sheri Wood, executive director of the agency.

Many of its newest clients have never used a free health clinic but after losing their jobs at companies like Sprint, they are opting to pay for their mortgages instead of maintaining their health insurance.

Besides the increased volume for those seeking services, one of the greatest challenges for the clinic is that people without insurance often wait until it's absolutely necessary to get help before seeing a doctor. As a result, they rarely get preventative care and are much sicker when they get care.

That's what happened to Denis Wittmeyer, who lives in Parkville, Mo.

"I can't afford health insurance," said the self-employed painter. "I'm also a poor risk."

Two years ago, Wittmeyer, 54, refused to get medical care even though he often fell and lost consciousness. Friends eventually persuaded him to go the Kansas City Free Health Clinic.

The staff diagnosed a blocked artery and arranged for surgery at a public hospital. They also treated his diabetes and high blood pressure.

Since then, Wittmeyer has been able to return to work, but without insurance, he still relies on the clinic for the 11 medications he takes every day. Wittmeyer faces a daunting future, declining health and no insurance as his condition worsens. "I worry about not having insurance, but what I've gathered is my time is coming, and there is nothing I can do about it," said Wittmeyer.