It might not be Black Friday, but Oct. 1 should be a shopping day to remember. For the first time, millions of uninsured Americans will be able to go online to compare health insurance plans, find out if they are eligible for federal aid and buy coverage.

what happens on Oct. 1 and why?

Open enrollment begins for the online health insurance marketplaces, the centerpiece of the 2010 Affordable Care Act, open for business

People can begin shopping for insurance plans on the exchanges. Lower- and moderate-income people will be able to apply for tax subsidies to offset the cost of premiums or enroll in Medicaid in states expanding the program.

The initial open enrollment period for these marketplaces, which operate as interactive websites, runs from Oct. 1 through March 31. Under the law, most Americans are required to have insurance by Jan. 1, 2014, or face a fine, but officials have said those who are insured by April 1 won’t be penalized. Insurance bought by mid-December will take effect Jan. 1; coverage bought by mid-March is effective April 1.

Insurers must meet new requirements. For the first time, they’ll be barred from rejecting people with pre-existing conditions or charging them more.

Each state will have its own online marketplace, or exchange, and offer in-person help.

what will you find on the exchanges?

The marketplaces will tell people if they are eligible for Medicaid, which many states are expanding, or if they should sign up for private plans.

If someone doesn’t qualify but earns less than four times the federal poverty rate — $45,960 for an individual and $94,200 for a family of four — that person may qualify for subsidies to reduce the cost of the premiums. Small businesses will also be able to sign up their employees for policies on the marketplaces in many states.

Individual policies will be organized into four tiers — bronze, silver, gold and platinum — depending on their coverage and costs. People with employer-sponsored coverage generally won’t buy on the exchanges, nor will those on Medicare, Tricare or other veterans health care plans.

mark these big dates on the calendar

Jan. 1, 2014: Coverage obtained on the exchanges goes into effect. Most individuals are required to have health insurance. Penalties for those who fail to do so kick in, beginning at $95 a year per person, or up to 1 percent of a family’s income, whichever is greater. (The penalty will increase in future years.) The penalty for an uninsured child is $47.50. Insurance companies cannot deny coverage on the basis of pre-existing conditions nor charge higher rates based on an applicant’s sex or medical history. Policies must offer a basic package of benefits.

March 31, 2014: Open enrollment period ends for the year. It will reopen in October 2014.

April 1, 2014: If you don’t have insurance, you’ll face a fine.

Jan. 1, 2015: Businesses with 50 or more workers must provide health benefits or pay fines.

who can and can’t buy from exchanges

Yes: They are aimed at people who are uninsured, people who don’t get coverage through their employer and those whose employer-based coverage is too costly and/or lacking in benefits. Legal immigrants are permitted to use the marketplaces and may qualify for subsidies if their income is no more than 400 percent of the federal poverty level (about $46,000 for an individual and $94,200 for a family of four).

No: People who are in the country illegally will be barred.

Maybe: For businesses with 50 or fewer workers, the federal marketplaces and most states will have a Small Business Health Options program, or SHOP, that will give employees more options than they now have. Most marketplaces will allow only businesses with 50 or fewer workers to buy through the exchanges initially.

how does the process really work?

If you have a computer, go to Healthcare.gov or to your state-run exchange (It’s mnsure.org in Minnesota) and create an account. Then you would fill out an application and provide information such as household size, location and income.

Then the exchange takes over. It determines whether you are eligible for Medicaid; if so, it will refer you there. If not, it will tell you how much of a subsidy you can receive. It will show you a list of health plans and their premiums and out-of-pocket costs, including deductibles and co-payments. If you decide to buy one of those plans, in most cases, you will be directed to the insurer’s website to make the payment. Consumers may have to make their first premium payment to the exchange and then further monthly payments to the insurer.

You can also fill out paper applications or apply over the phone. The federal and state exchange sites have toll-free numbers, see box at left, where consumers can find information about getting help in person.

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