Thursday nights duty crew gets back to back calls. First dispatched to an investigation at the Neshaminy Falls. Crews found an unattended camp fire. Companies E 4-1 n E 84 used two cans and some hand tools. Second call dispatched for an alarm system to Neshaminy Interplex bldg. 6. Companies on scene E4 E4-1 and DC4. Crews encountered a heavy concentration of some type of fumes in the lobby up to the fifth floor. After an extensive search the crews found out that the cleaning crew was in the process of cleaning the elevators with a strong solvent. The odor of the chemical set off the alarm. Crews had to pressurize the north stairwell and ventilate each floor out up to the roof top access door. After the odor dissipated we went floor by floor and monitored them with our multi gas meter for Co. The all clear was given at 2132hrs.