Government workers in 21 states are using an obscure perk to retire early or to boost their annual pensions by thousands of dollars, which can cost taxpayers millions more in payments to retirement funds, a USA TODAY analysis shows.

The practice, called buying "air time," lets state, municipal and school employees pay to add up to five years to their work history so they are eligible to retire and collect a lifetime pension. Workers already eligible for retirement can buy extra years to boost a pension by up to 25%.

It's called "air time" because workers buy credit for non-existent work, in contrast to policies that let workers buy credit for military service or government jobs in a different state.

Dan Pellissier, a former adviser to California's previous governor, Republican Arnold Schwarzenegger, paid $75,000 in 2004 for five years of work credit. When he turns 55 in 2015, he will get a California pension of $61,536 a year - nearly $13,000 more than if he hadn't bought air time. That's $320,000 extra by the time he is 80.

"They give away the store here," says Pellissier, now president of California Pension Reform, which is pushing to cut state retirement costs.

Air time is coming under scrutiny as states try to curb retirement spending and make their pension systems resemble private-sector plans. Federal law allows air-time purchases only in government pension plans.