On a CO call, we encountered 400 ppm of CO in the single family residence. All gas operated appliances were in working order, producing zero CO. We found an Oxygen/Acetylene welding tank set-up in the garage. The Acetylene tank was leaking, causing a reading on the multi gas detector. Having a very low Lower explosive limit, we were in an environment that had the potential to be very explosive. Not to mention we were operating all of the appliances to check for CO, possibly causing a spark to light off the Acetylene accumulating in the house!