Spacecraft modules that are last purged with clean air several months before they are entered by humans on orbit require careful management. The crew must not be exposed to harmful concentrations of air pollutants when they first enter. The magnitude of the pollution the crew will encounter depends on the volume of the module, the length of time since the last clean-air purge or scrub, the inherent offgassing rate of the materials in the module, the interior temperature of the module while offgassing occurs, and the system leak rate. The time of the last module purge or scrub can be several months before crew entry, so it is essential that the offgassing rate within the module be measured over a suitable interval of time to estimate pollution levels with confidence. Air samples were taken from the STS-74 Russian Docking Module, the STS-79 Spacehab, and the ISS Node 1 prior to launch to predict pollution levels at crew first entry. The samples were taken in pairs for baseline contaminant levels (time = 0) and at the following times: docking module, 20 and 43 days; Spacehab, 1.7 and 7.3 days; and Node 1.4, 3.9, 5.0, and 8.7 days. The samples were analyzed by gas chromatography and gas chromatography/mass spectrometry using a standard gas mixture for calibrations. Spacecraft maximum allowable concentrations (SMACs) for 7 days of exposure were compared to the measured offgassing of individual compounds to predict whether each module could present a risk to crew health during first entry. A simple linear extrapolation of the off-gassing rate at the end of the ground-based test was used to model pollutant concentrations. The results of the measurements and modeling also determined whether a ground-based or on-orbit purge was needed to ensure safe first entry.