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.