As Tax Day approaches, an underused part of many health care plans could offer a chance to significantly reduce your tax bill.

A health savings account, or HSA, can help pay for some medical expenses, if you qualify to have one. And they offer three valuable tax breaks: Money is deposited pretax, can grow tax-free and is not taxed when you spend it, as long as the expenses are eligible.

It is rare for so many tax advantages to be wrapped into one benefit, advisers say.

"It's a great deal," said Neal Van Zutphen, a certified financial planner in Tempe, Ariz., "even if you don't invest the money."

There's a catch, though: The accounts are available only to people with health insurance plans that meet specific criteria, such as a high deductible, which is the amount a person pays for nonpreventive medical care before insurance. For 2020 and 2021, the amount is at least $1,400 for an individual or $2,800 for family coverage.

The high deductibles could make HSA-eligible plans (which are usually labeled as such) unattractive for those with chronic conditions or costly health needs, even if monthly premiums are lower. But the accounts could also significantly reduce your tax bill.

Tax Day is the deadline for making deductible contributions for 2020 to health savings accounts. This year, the date was pushed back to May 17 because of the pandemic. But the IRS has not confirmed that the contribution deadline also will be delayed, although last year it was.

A person does not have to itemize personal deductions on a tax return to claim the HSA benefit. It is reported with "adjustments" that reduce taxable income. (Two states — California and New Jersey — do tax HSA contributions as capital gains.)

Even though the number of HSAs has been growing, experts say they are underused, and that more could be done to encourage their use. There were 30 million HSAs holding about $82 billion at the end of 2020, according to Devenir, an HSA services firm.

The accounts can pay for a variety of medical and health expenses, including doctor visits, hospital stays, surgery, and vision or dental care. The money can also go toward long-term-care insurance premiums and services.

The federal government's pandemic relief program expanded what HSAs can pay for, including nonprescription medicine like pain relief and allergy pills, and menstrual products like tampons and pads. (The IRS has a full list of eligible items.)

Some employers match contributions to HSAs. But self-employed people and contractors can open them, too.

People often confuse HSAs with other types of health accounts, such as flexible health spending accounts. But unlike FSAs, health savings accounts are portable: If you change jobs or leave the workforce, you keep the account. Contribution limits are higher for HSAs, and there is no deadline to spend the cash. Unspent money can be invested for health needs in retirement.

A study published last summer in JAMA Network Open, a journal from the American Medical Association, found that many people with high-deductible insurance did not have a health savings account. And more than half who had one had not contributed to it in the previous year. People with health plans bought through a government exchange were more likely to not have an HSA, even though the average deductible in the federal marketplace is high enough.

The findings suggest that health plans, employers and financial advisers could do more to explain how HSAs work, simplify their use and encourage contributions, said an author of the study, Dr. Jeffrey T. Kullgren, associate professor of internal medicine and health management and policy at the University of Michigan. "Anything that makes it easier would be a good thing," he said.

HSA providers increasingly are cutting fees and using technology to encourage use. Fidelity Investments this month will test an app that will allow clients with employer HSAs to track account balances, contributions and spending. The app will also let users scan products to check if they are HSA-eligible.

Others are focusing on workers in the gig economy. Starship, a startup, promotes its accounts through affiliations with ride-hailing and delivery companies. Its app allows workers to automatically invest contributions in low-cost index funds and exchange-traded funds. Because there is no required minimum balance, users are able to invest all their contributions. But that would also leave no cash available to cover medical costs, unless the investments are sold. Starship sets the default minimum threshold before investing at $2,000, but users can lower it to zero, said Sean Engelking, the company's CEO.