The effect of metallic surface contamination on field electron emission is investigated for the first time using a three dimensional quantum mechanical model. The plane wave periodic version of the density functional theory is used to obtain wavefunctions and potentials. Local and averaged emitted current densities are obtained from them using time dependent perturbation theory. This method is used to study the effect of the presence of carbon adsorbates on emission from tungsten surfaces. Fowler-Nordheim plots which provide the dependence of the emitted current with the external electric field show that carbon contamination inhibits emission. Significant differences with the results of the analytical Fowler-Nordheim model are observed. Emissions images (i. e. the spatial dependence of the emitted current density) are presented to identify the important emission spots. These images are significantly different from the electronic density plots usually presented to model constant height scanning tunnelling microscope images. Analysis of the emitted current density energy distributions in the light of the projected local density of states provides a deeper understanding of the emission process.