HOUSTON (KTRK) -- When a fire truck leaves a Houston fire station, every firefighter on that apparatus is being paid a salary by Houston taxpayers. However, at any given time, there may be a firefighter on a truck who is making hundreds of dollars in cash, on top of his salary."The way it is, you're getting paid, on top of getting paid," says a veteran Houston firefighter. "I guess that's the best way to describe it."The firefighter did not want to identify himself for fear of being criticized by his coworkers. He approached Eyewitness News two weeks ago because he said he was frustrated with the way fire stations are being staffed around the city. The firefighter says the practice of using so called cash riders is very common. The entire process is managed by fellow firefighters, who advertise for riders and shift traders on several Facebook pages, a text message service, and a website.The firefighter tells Eyewitness News that while trading a shift is perfectly legal, now it is difficult to find firefighters willing to trade a shift, unless there is additional cash payment involved. He says the going rate is $250 to $350 per shift for most days. However, the price could sky rocket if a firefighter wants to find someone to work for him during a holiday or for example, during the Super Bowl."You're potentially making an extra twelve grand a year untaxed; you're kind of screwing everybody else."Eyewitness News was directed to a Facebook page called HFD Extra Shifts. In the description, it specifically tells members not to talk about prices on the page. On another Facebook page dedicated to HFD spouses, we were shown a number of postings where wives asked for riders. One typical posting read: "My husband is looking for a rider tomorrow... In case anyone's husband is looking to make some extra money."The practice surprised Houston City Council Member Dave Martin. "This isn't trading shifts; this is selling shifts, selling shifts for cash."Martin, who has an accounting background, says he's heard that firefighters paid each other cash for work, but had no idea how widespread the practice is, until he saw the websites. Martin is concerned about the legal ramifications of firefighters paying each other with cash because if the payments are not reported to the IRS, there are legal ramifications."The most alarming thing, in my opinion, is reading the Facebook posts, and looking at the website," said Martin. "If someone's getting cash, from another firefighter, that is tax fraud, because they're not reporting that cash as ordinary income."Martin has reason to be concerned. In 2014, 13 Cleveland, Ohio firefighters were indicted in a cash-for-shift scandal that engulfed the city. The firefighters eventually plead guilty, and were given suspended sentences. Across the country, a number of fire departments, large and small, have dealt with the issue. Sometimes no action is taken, and sometimes firefighters get fired, or in the case of Cleveland, face criminal charges.The practice also poses ethical challenges because when a firefighter pays someone else to work his shift, he is still accruing the same vacation, retirement, and other benefits as if he is actually at the fire house. In other situations around the country, senior firefighters were found to being accruing more benefits while paying a more junior firefighter to work for them.The Houston Fire Department says it only monitors shift swaps among firefighters to make sure all stations are staffed, and does not get involved in money transactions among its firefighters."A firefighter's shift is difficult. It's not always aligned with anniversaries, birthdays, and graduations and members are allowed to trade shifts," said Senior Captain Ruy Lozano, who says trading of shifts is allowed. "No firefighter should exceed 460 hours of shift trading in a calendar year."When asked about the cash for shift issue, Lozano says the city has no policy, one way or the other. "We don't condone it; our policy doesn't regulate or even mention the trade for cash."The firefighter who spoke to us says everyone at HFD knows that cash-for-shift happens daily. He says most firefighters see it as an unwritten perk of the job. "It's an accepted practice; I believe a blind eye is turned to when it happens."