A system and method can detect ethylene oxide in a sample of gas, such as air. The system includes a light source operating at a wavelength where molecules typically found within air absorb light at a substantially lower level than ethylene oxide molecules. Exemplary wavelengths are in the range of approximately 1.6–2.2 μm, and in particular at 1.6 μm, 1.645 μm, 1.692 μm, 2.195 μm, 2.2 μm, 2.216 μm, passes through the sample of gas to be detected by a detector. In one variation, the light source is a tunable diode laser or a VCSEL and the ethylene oxide level is determined using harmonic spectroscopy.