What, if any, danger lies in a dirty microwave?

Not much danger unless it is caked on, inches thick.

First, the magnetron in the microwave (that creates the waves) is almost always aimed at the center of the oven. The sides get much fewer waves and therefore, stuff on the sides never cooks as fully as stuff in the middle. (Which is why a lot of microwave ovens have those rotating trays, to even out the dispersal of the microwaves)

Second (very simply put) , microwaves work on water, fat and other substances that are primarily water or liquid. (of course, there is more to it than I am describing, but for our purposes, this is enough).

The stuff on the walls of the microwave has no water content – it is dry splatter. Or more correctly, it had water content when it splattered, but it has no more water in it now. Therefore, the heating waves of the microwave magnetron have nothing to act on.

Since bacteria needs to have some level of moisture or humidity to grow, the dried splatter is not a fertile area for it to flourish. So there is no safety issue here.

Having said all of that above, it is hard to believe that a landlord would have the gall to rent someone an apartment with a dirty microwave. That’s just tacky. If I went into your place and saw that, I would walk right out and think – “what kind of a schlock landlord is this?”

So you may want to clean it to get to renters.

Here’s something to consider: very small quantities of water aren’t affected by microwaves in the same way that larger quantities are. For example, I just placed a single drop of water in our office microwave. After one minute at full power, the drop looked exactly as it did when I put it in. I then put 15cc of water (about ½ oz) in. After only 30 seconds, it had boiled away ⅓ of its volume. For this reason, the amount of water contained in single-cell organisms living on the oven walls probably doesn’t heat up enough to kill them.

The food crud in a microwave is dried, true. but it also gets rehydrated on a regular basis when the oven environment collects steam. It’s theoretically possible for bacteria to colonize that newly moistened crud, proliferate, and then go into spore form when the crud dries up again. Those spores can then survive pretty much any conditions the microwave is likely to serve up.

My own opinion on this, with no research whatever, is that there is “marginal danger” from that stuff.

Since the microwave is often filled with water vapor from cooking food, that stuff is frequently hydrated enough for something to grow, so it’s not completely benign. From time to time that food can become a good growing medium for bacteria, I’m sure.

On the other hand, as most of us cook food in a microwave we cover it with something, even if it’s just a paper towel. So there’s not a lot of danger from tightly caked “stuff” falling into our cooked food, is there?

It might be a good idea to post a disclaimer of some kind on the microwave that it should be cleaned more frequently, and that you won’t be liable for food-borne illness traced back to a dirty appliance. In addition, since so many college kids (boys in particular, I presume) have zero kitchen experience to begin with, it might be especially helpful if you also put a paper towel dispenser on the wall nearby and post easy to follow instructions about cleaning the microwave by nuking a soaking wet sponge to dampen the walls, and then wiping it down from time to time.

And remind them that “cleaning the microwave” is part of what their security / cleaning deposit does, and if they don’t do it, you will – at their expense.

Great answers from all!
@elbanditoroso Please be assured that I do clean and reclean the microwave before I even attempt to show the unit, along with every other major appliance in the places.
@thorninmud Interesting study, and since I assume they are splattering it to begin with, they are not covering it and so, at least theoretically, it could fall into the new dish at some point.
@CWOTUS You would be surprised at the number of young ladies that have either little experience in the kitchen or just don’t care. They are just as bad, and in some cases worse than the males. With the exception of in the bathroom, for some reason they are more willing to clean a shower/tub but are just as reluctant to clean a toilet. They are just more accurate. We do give them a complete list of what we expect them to do to get their deposit back and are quite explicit about what it will cost them if they don’t. But hey, most of the time is it momma’s money so they do not seem to care.
I have had some very irate parents who become a lot more understanding when we forward the photos to them. I have also had parents who come up to help clean and, as in a recent case, you can see why the kids did not keep the place up. What I consider clean and what they do are worlds apart but I will not rent a place that I would not live in myself so we do it my way. All I ask is that you leave it the way you received it. And there is a big difference between “normal wear and tear” and a lack of housecleaning.

If you clean it up I would not worry at all.

@thorninmud First, it is advised not to run a microwave empty, so one drop of water is pretty close to empty, so I would adise against doing that. Second, your expirement sounds much likepeople who think microwaves cook food from the inside out. Simply not true. I know you observed what you observed, but a drop of water is going to cook up like a lot of water. A lot of water taking longer. Not seeing something boiling does not verify temperature. There have beem cases of water exploding, because a container, a glass is so perfect the water does not appear to boil, and then when it is moved by the person to remove it from the oven the scalding hot water suddenly bursts. Don’t be fooled.

@JLeslie been there, done that. Not exploding but boiling over as soon as the cup was moved.

@JLeslie Actually, it has to do with the ratio of surface area to mass. The air in a microwave does not heat up. The drop has a huge surface area relative to the mass of the water it contains, so while the microwaves are causing the water molecules to oscillate and generate heat, that heat is very quickly lost to the much cooler air through all of that surface area. The 15cc sample had much less surface area, so the heat couldn’t escape into the cool air as easily.

A microbe has waaaaaaaay more surface area-to-mass than even the drop of water, so the heat would be dissipated almost as quickly as it forms.