An environmental scan is a search of the patient's home and/or workplace to look for toxins or other possible environmental causes for their symptoms. House routinely orders his team to perform environmental scans. He also routinely tells them not to ask the patient first, usually requiring them to break in or to resort to some other subterfuge (such as stealing keys) in order to enter the patient's home. In Pilot, House explained to Foreman that he can't ask the patient for permission just in case the patient is a drug dealer or something similar. In Pilot, the patient was a kindergarten teacher and Foreman was incredulous. However, Foreman started agreeing with House after his experience in Euphoria (Part 1) when he did an environmental scan on a police officer's apartment and found a marijuana grow-op.
The environmental scan is often fruitless, but it has either provided the important clue for the diagnosis, or indirect clues, for example:
In Airborne, Chase discovered during the environmental scan that the neighbour's home was being fumigated and the patient's home was connected by an old electro-mechanical service from when the properties were joined.
Again in Airborne, House found a receipt for scuba diving in the patient's wallet, leading him to be able to prove the correct diagnosis.
In Love Hurts, Chase noted that the patient had a whole case of mints, leading House to come to the correct conclusion that the patient was suffering from persistent bad breath. This also led to the correct diagnosis.