Most people don’t invest their HSA savings, but use them like a bank account.

Health savings accounts offer perhaps the best tax advantages compared to other investment accounts.

But most account holders use them in ways that dilute their benefits, data shows.

Only 19% of HSA participants invest their account assets, according to a new survey from the Plan Sponsor Council of America, a group that represents employers. These investments can be, for example, a stock mutual fund.

Others park their money in cash, treating their HSA like a bank account.

This behavior goes against the advice of financial experts: invest and grow HSA assets as a sort of retirement account, like a 401(k) plan, to cover future healthcare costs.

“There’s a fair amount of health care you can expect to pay for in retirement, and this is just a more efficient way to pay for it,” said Lee Baker, an Atlanta-based certified financial planner and member of the CNBC advisory board.

HSAs are tax-advantaged accounts for healthcare expenses and are only available to consumers enrolled in a high-deductible health plan.

They enjoy a three-pronged tax benefit: Contributions to the account are tax-free, and investment growth and withdrawals are also tax-free if used for qualified medical expenses.

“An HSA is a perfect investment vehicle, if we dare say such an animal exists,” said Baker, founder and president of Apex Financial Services. “There is nothing better under current tax law.”

There is a long list of healthcare costs that qualify for HSA use. Even if used for a nonqualified expense, the account’s tax benefit remains similar to that of a traditional 401(k) or individual retirement account: A withdrawal would be taxed as income.

HSAs do not require “use it or lose it” money each year, unlike many healthcare flexible spending accounts.

The optimal way to use an HSA is to keep cash in the account equal to the annual insurance deductible and invest the rest, Baker said.

Consumers would pay current healthcare costs out of pocket, if possible, allowing HSA money to grow for the future. In the event that consumers have a large health bill, they can use HSA money to cover the annual deductible, if they are unable to pay it out of pocket, Baker said.

Of course, “that’s just not the reality for everyone,” said Hattie Greenan, research director at PSCA.

Most people probably can’t afford to pay current medical bills out of pocket, so they continually draw on their HSAs instead of investing the assets, Greenan said.

“If they need to draw on it to pay for everyday health care expenses, they’re not using it as an investment vehicle,” she said.

Additionally, according to the PSCA survey, approximately 40% of employers don’t even offer HSA investment options to their workers. They only offer cash options.

However, there is a workaround: Unlike 401(k) accounts at work, employees are not beholden to HSA options offered by their employers; they can open an HSA account elsewhere with another provider to access investments.