Could need a cleaning, room too small for combustion air if door is closed, gas pressure could be off. If you live in an area that sees temps of 20 during winter, the pressures could be ok during warmer weather but when temps drop to 20 or less the natural gas company puts propane into the lines to help increase pressure(this is called shaving) and this throws off the tune up. Yes its a concern because improper combustion produces carbon monoxide. Get a c/o detector.