What does it mean to have a shadow on your lung in a scan?

On a chest xray,: The lungs are usually clear. A shadow (really an area of increased density compared to the air filled lung) can be 1 of 5 things. Fluid (water) - pleural effusion being the most common. Blood - from hemorrhage pus/infection, generally pneumonia, but lung abscess is possible cells - cancer and other tumors, some of which are benign (ex. Hamartoma) atelectasis - collapsed lung segment(s).