The well known dimer-dangling-bond surface state of the Si(001)2×1 surface is very sensitive to even a small amount of adsorption from the ambient pressure in an UHV chamber. The authors show the decay of the surface state intensity, obtained with ultraviolet photoelectron spectroscopy (UPS), as a function of time for two different ambient UHV chamber pressures typically considered to be sufficiently low for carrying out surface sensitive experiments. In ambient pressures greater than 1×10-10 Torr, the dimer-dangling-bond surface state decays completely in less than 1 h, even at the relatively “good” pressure of 4×10-10 Torr. The reason for the decay is likely due to the passivation of the dimer dangling bonds predominantly as a result of H adsorption from the ambient pressure in the UHV chamber. Furthermore, Ni-contamination-induced defects on the Si(001)2×1 surface alone do not cause the decay of the surface state. As long as unsaturated dimer dangling bonds remain on the surface, even in the presence of long range Ni-induced defects, the surface state produces a strong UPS signal.