Why shouldn't you eat in a chemistry lab area, even when there is no lab?
You should not eat in a chemistry lab because of the threat of contamination. This threat of contamination has two parts. The first part is that you never know where chemical residue may be and ingesting it could be dangerous. Many chemicals leave no visible trace of their presence. You may touch a table, not knowing that a chemical is present, and then touch your food or your mouth. The second part of the threat of contamination is to the lab environment itself. While you are eating you may leave crumbs or residues on the lab surfaces. This may impact future lab projects and could lead to dangerous situations.
In addition to the first response, you should never eat in a chemistry lab simply because those are the rules. I have worked in many science labs and they all had stated "no food or drink allowed".
For example, thermometers are frequently used in labs. Thermometers are fragile and break often. They contain mercury which is very lethal when ingested. If you place food near on mercury that may have spilled on the counter top and then you eat that food you may become very ill.
Other chemicals that spill may also leave residue that may linger for some time.