Atherton resident Evan Collins lives in the nation’s second priciest ZIP code, where $4 million barely buys you a fixer-upper.

Yet when it comes to health insurance, Collins — like many Bay Area residents of all incomes with a pre-existing medical condition — said he’s “extremely worried” about President-elect Donald Trump’s vow to repeal and replace the Affordable Care Act, better known as “Obamacare.”

The 54-year-old retired financial planner has tendonitis and asthma — conditions that he said previously increased the cost of his insurance. And now he also has ulcerative colitis, a chronic disease of the large intestine that often prevented Americans from being insured at all. That was before Obamacare prohibited insurers from discriminating against anyone who has had cancer, heart disease and diabetes, among other serious illnesses.

Collins has plenty of company.

A stunning analysis released this past week by the Menlo Park-based Kaiser Family Foundation found that 52 million American adults under age 65 — including 5.8 million Californians — have pre-existing health conditions that would most likely make them uninsurable under underwriting practices in place in most states before Obama signed the Affordable Care Act into law six years ago. Many Obamacare opponents — including Trump — say the pre-existing condition provision is one of the few parts of the Affordable Care Act worth keeping, but it’s far from clear how their alternative plans could afford to retain it.

“They are not going to take me back if Obamacare goes away,” Collins, a registered Republican, said of insurance companies in the deregulated health care world that Trump and the Republican Congress are envisioning.

“The companies don’t care,” said Collins, who voted for Hillary Clinton. “That’s why I’m a big fan of Obamacare.”

An unsubsidized Health Net plan now costs his family $1,700 a month, which will increase to $2,000 next year.

“It’s expensive,” he said. “But I’m covered.”

While the national rate of “declinable” pre-existing conditions is 27 percent, the numbers are notably higher — ranging from 32 percent to 36 percent — in several Republican-controlled Southern states. In heavily Democratic California, the rate is 24 percent.

The Kaiser Family Foundation study says millions more could have other conditions – from asthma to migraines to peptic ulcers — for which some insurers have been known to charge higher prices.

Cynthia Cox, one of the study’s authors, noted that the vast majority of non-elderly with pre-existing conditions in America have coverage through their employers or Medicaid, the nation’s health care program for the poor.

“But circumstances change,” she said, “and these people may one day need to be in the individual market.”

The state and federal insurance exchanges established under the Affordable Care Act tried to solve that problem by creating huge pools of insured people to dilute the costs of insuring people with serious health problems. The pools are large because Obamacare mandates that virtually everyone in the country have health insurance or pay a steep penalty.

Three years ago, San Jose resident Thomas Sutton suddenly found himself searching for an individual policy after his wife’s employer dropped family members from her plan. It happened about the same time the 60-year-old businessman developed chronic back pain that was later diagnosed as a type of degenerative arthritis — a pre-existing condition that insurers declined to cover.

But Sutton’s timing was spot on: In late October 2013, just as Obamacare kicked in, he bought a nonsubsidized plan through Covered California, the state’s health insurance exchange, established through Obamacare. And the next year, he switched to a similar plan purchased through a private insurance agent.

“Now I’m worried that they will drop Obamacare and it will affect me,” Sutton said. “There’s no way you can cover pre-existing conditions without the mandate.”

Most health care experts say covering pre-existing conditions with no health insurance mandate would cause premiums to skyrocket for everyone.

The Kaiser Family Foundation’s Cox said the replacement plans Republicans are talking about would ultimately do away with protections against pre-existing conditions.

In one proposal by House Speaker Paul Ryan, insurers would have to cover pre-existing conditions. But if for any reason someone dropped a health insurance policy, the individual could not return to the insurance market without being penalized for a pre-existing condition.

“High-risk” pools for sicker individuals also are being discussed by Republican leaders. Many states, including California, had established these pools before the Affordable Care Act. But the insurance policies were unaffordable to a large percentage of Amercians.

“The key aspect of such a pool is that the coverage has to be heavily subsidized,” Cox said. “But if there is not enough money going into the pool, what we have seen is that only the sickest people are buying – which makes the cost of premiums a barrier.”

But Sally Pipes, president and CEO of the San Francisco-based Pacific Research Institute, a nonprofit that promotes limited government, said Ryan and Dr. Tom Price, Trump’s choice for secretary of health and human services, have answers.

“All of us are concerned about people with pre-existing conditions,” said the health care economist, who has penned several books critical of Obamacare and is now advising Ryan and Price.

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Pipes believes that after Obamacare is repealed, each state’s high-risk pools will be much better funded by the federal government, making premiums more affordable for the sickest people. That way, she said, the billions of dollars they cost other Americans — who are now paying higher insurance prices to support them — will be reduced, encouraging more young, healthy people to buy a plan.

And, she added, those with pre-existing conditions who cannot afford to keep their current coverage won’t be penalized. Instead, she said, they will join their state’s high-risk pool.

Kathryn Johnson, of Capitola, said she finds the whole debate disheartening.

Before 2014, when most of the law’s provisions took effect, the self-employed psychiatric nurse practitioner had shelled out $31,044 annually in premiums — a steep price for having asthma and chronic back pain. This year, her Obamacare policy cost $7,800.

While she agrees that Obamacare is far from perfect and needs to be tweaked, she said that her coverage is still “far superior” to her policy before the health care law took effect — and at a fraction of the cost.

“I am now 61,” Johnson said. “I should be looking at the home stretch as far as needing to pay exorbitant monthly health care premiums.”

‘DECLINABLE’ CONDITIONS

Here are some of the common diseases and conditions that insurers cited to decline health coverage before the Affordable Care Act took effect.

AIDS/HIV

Alcohol abuse/drug abuse with recent treatment

Alzheimer’s/dementia

Arthritis (rheumatoid), fibromyalgia, other inflammatory joint disease

Cancer within some period of time (e.g. 10 years, often other than basal skin cancer)

Cerebral palsy

Congestive heart failure

Coronary artery/heart disease, bypass surgery

Crohn’s disease/ulcerative colitis

Chronic obstructive pulmonary disease (COPD)/emphysema

Diabetes mellitus

Epilepsy

Hemophilia

Hepatitis (Hep C)

Kidney disease, renal failure

Lupus

Mental disorders (severe, e.g. bipolar, eating disorder)

Multiple sclerosis

Muscular dystrophy

Obesity, severe

Organ transplant

Paraplegia

Paralysis

Parkinson’s disease

Pending surgery or hospitalization

Pneumocystis pneumonia

Pregnancy or expectant parent

Sleep apnea

Stroke

Transsexualism

Source: Kaiser Family Foundation