"Smog" is a good example of a Frankenword. Others are "spork" (spoon + fork) meaning a runcible spoon and "skort" (skirt + shorts) meaning a wrap-around skirt with short legs.

The problem with giving in to portmanteau words like "smog" is that it is a short slide down the slippery slope to words like "snizzle" (snow + drizzle) and "clumid" (cloudy + humid).