Potential Source Inspection. This important second step is also often overlooked by investigators more intent upon measuring contaminants than conducting a comprehensive diagnostic investigation. In order to diagnose a problem and prescribe mitigation one must have an idea of the different sources of contaminants in the building. This step involves a walk through of the building, the building ventilation system, and adjacent outdoor locations, during which one collects a detailed inventory of potential sources of contaminants. Sources of contaminants may arise from indoor sources such as off gassing of new materials installed during a building renovation, outdoor sources such as motor vehicle exhaust from parking garages and loading docks, and from underground sources such as gasoline from contaminated underground soil and water. A contaminant source inspection of the building ventilation system should include a description of any sources of contaminants which may be entrained into the outside air inlet (e.g. sewer vents or fume hood exhausts) and any standing water or visible microbiological contaminants (e.g. slime or mold) in the cooling coil condensate drip pan or inside the duct work downstream of the cooling coils. Also the cooling coil drain should be checked to see if it is properly trapped. In addition the prior use of the building should be investigated to determine if there is any potential of exposure to contaminants associated with the previous use (e.g. lead dust in a firing range being renovated to office space).