R.A.
Environmental just means that the cause isn't within the body. Pnemonia, specific viral or bacterial infections, allergies,spores,etc are all environmental causing conditions which would contribute to an abnormal chest xray..A CT scan is used to detect any abnormalities, and picks up on anything that isn't normal. Lung cancer can be detected by using a CT scan, or any other cysts, absesses, or lumps. If they don't think the cause is internal or cancer related, they will need to rule it out by doing a CT. It's a safety precaution more or less. If the cause is environmental, then antibiotics, or other medications will be given to treat it.