||This is the first part of a study of the historical growth of light pollution at Mount Wilson Observatory. The night sky brightness at Mount Wilson due to light pollution from Los Angeles basin was calculated for the years 1910 to 1990, neglecting changes in lighting technology, and without any air pollution ('smog'). The very large effect of population growth is shown. We made a simple extension of our night sky brightness program to include a layer of smog. Two possibilities of constant density and finite thickness. The ground level density is determined by the visibility. We assume a smog layer whose density increases from zero in 1920 to appropriate values for the years from 1950. We added this layer to out model and repeated the Mount Wilson calculations. An average smog layer reduces the visual brightness at the present time by about 6 percent.