A case study of an indoor environmental quality investigation was presented. A NIOSH health hazard evaluation was requested in identifying a petroleum like odor in an office in Ohio that had first been occupied in August 1991. In late fall 1991 the employees began to notice the odor which gradually worsened and in March 1992 the building management installed an exhaust fan to vent the air directly to the outside. At the same time NIOSH investigators conducted general area air sampling for volatile organic chemicals (VOCs). A HNu photoionization analyzer was used to locate sources of VOCs within the office and hallway areas. Seven employees who spent a significant portion of the work week in the office were interviewed. Results showed that while no identifiable petroleum based product was present, the chromatograms were distinctly different from samples collected in other office suites of the building. The field samples were similar to a bulk diesel fuel sample, but there were no peaks for aromatics. Hydrocarbons ranging from C4 to C12 were indicated, but were less than 10% of NIOSH recommended exposure limits for stoddard solvent, but more than 20 times higher in some suites and in the ground floor office. The time weighted average of total airborne hydrocarbons ranged from 5.9 to 17.5mg/m3. The photoionization results in conjunction with the long term sampling results suggested that the VOCs emanated from the ground beneath the building slab. The employees reported frequent headaches, nasal congestion, eye irritation, throat irritation, severe fatigue, nausea and bouts of dizziness which were more severe on the days that the odor was stronger. The NIOSH investigators concluded that the soil beneath the site seemed to be contaminated with petroleum hydrocarbons and made recommendations for fan installations and other soil investigations.