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I was cooking something in the microwave and opened the door early to check on it and the microwave didn't stop. I didn't realize this for a few seconds and when I did I shut the microwave off but I'm concerned what I could have been exposed to for the few seconds that it was open.

As far as I can remember I still heard the humming and the light was on so I assume it was still running. I didn't think it was even possible for it to keep running with the door open. The microwave is a fairly new model as well.

Is there radiation or other things that could have caused some damage in those few seconds?

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    $\begingroup$ If you were close enough for long enough you could get a burn. But you don’t have to worry about the type of cell damage that can occur with gamma rays or something. Microwave radiation is lower energy than lots of other radiation you’re exposed to all the time, like sunlight. $\endgroup$ – Ben51 Feb 15 at 2:32
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    $\begingroup$ @Pieter that was my biggest concern after doing some research but I wasn't sure if I should get checked out after those few seconds. $\endgroup$ – Mr.Smithyyy Feb 15 at 3:32
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    $\begingroup$ I'm posting this as a comment, because it's not about physics. Last time it happened to me, it was the office microwave. Someone set it on "combined" mode (which means it was both microwaving and grilling). I thought it kept running, when in fact it wasn't (a colleague explained it's the ventilation from the grill mode, which keeps the light and the sound going). I don't know whether or not you are in the same situation, but I thought I would drop it here. $\endgroup$ – Clockwork Feb 15 at 13:06
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    $\begingroup$ Not an answer but go online and search for the exact same make and model. Try adding different words to the search such as "problem". The chances are that someone else will have the same question. It's worth checking that there isn't a manufacturer's recall notice on the model as well. $\endgroup$ – chasly - supports Monica Feb 15 at 13:20
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    $\begingroup$ My combined oven+microwave sometimes does keep the light on and keeps humming for several minutes after either I open the door or let the timer finish. I say "sometimes" because for short durations (less than 90s) both the light and the humming turn off immediately. It must be some self-cooling mechanism that triggers only above a certain usage time. $\endgroup$ – ris8_allo_zen0 Feb 15 at 15:54
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The very first thing you should do is stop using your oven and have it checked out by an authorized repair service.

If in fact the oven was operating with the door open, there was a failure of the door interlocks to turn the oven off and a failure of the backup system intended to permanently shut the oven off in the event the interlocks failed which, although extremely unlikely, is nonetheless possible.

The next thing you need to know is that microwave "radiation" is not the same thing as x-radiation, gamma rays, or other forms of nuclear radiation, which are referred to as "ionizing radiation" and are associated with injury such as cancer if excessive. Microwave radiation is non-ionizing radiation which, to date, has only been conclusively associated with thermal injury, i.e., injury to tissue due to heating (that's how it cooks food).

Since you were only potentially exposed for a few seconds, unless you are experiencing discomfort, you are probably all right since the temperature of your tissue was probably not raised all that much. In the early days of commercial microwave ovens, before multiple safety features were required, there were incidents where workers were exposed to multiple exposures and had some loss of muscle function due to heating damage. But those were the cumulative effects of many exposures. A few seconds of exposure is unlikely to cause that effect on you.

In any case, I urge you to stop using your oven now.

Hope this helps.

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    $\begingroup$ It may be that the light, turntable, and fan were running, but the magnetron was not firing. I recently disassembled a late model microwave and there were no less than three independent microswitches physically disconnecting power to the magnetron transformer that were only able to all be closed if the door was closed and locked. $\endgroup$ – Tristan Feb 15 at 14:45
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    $\begingroup$ @Tristan That certainly is possible, which is why I carefully chose my words to say "If in fact the oven was operating with the door open". But why take a chance. It was nearly 40. years ago when I tested microwave ovens for safety. Then there were two independent interlock switches and a separate monitor, called a crowbar circuit, that would blow an internal fuse if one or both of the interlock switches were closed with the door open. The interlock safety system has probably evolved since then. $\endgroup$ – Bob D Feb 15 at 14:55
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    $\begingroup$ It took me a little while to realize it, but my last roommate's microwave will keep the fan running for a while if the microwave was used for more than 2 minutes. It sounded like it was still running when it wasn't. I finally confirmed what was going on by reading the sticker inside the door, which stated the expected fan behavior $\endgroup$ – anjama Feb 15 at 16:21
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    $\begingroup$ @BobD I suspect the evolution has been in the direction of lower cost (maintaining adequate safety for regulatory purposes), not higher safety $\endgroup$ – Chris H Feb 15 at 16:55
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    $\begingroup$ @StianYttervik Need to be careful here. In the incident I referred to in my answer the persons were putting in and taking out food while the oven was operating without really any discomfort. They didn't realize they were undergoing cumulative tissue damage until they started losing muscle function. At that point, it was too late. $\endgroup$ – Bob D Feb 15 at 18:29
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When reading your post, it seemed to me that the most likely explanation for your experience is that the microwave did turn off, but the ventilation did not. You may have mistaken this for the microwave not turning off.

If indeed the microwave didn't turn off, you would have felt pain and other sensations immediately. Get a new microwave in that case, and throw out the old one.

Edit: With other effects, I was referring to the discoveries of Allan Frey. Also, being exposed directly to a couple of hundreds of Watts is a different story than your local wifi. The heat will be felt immediately. The frequencies are also selected to penetrate food including meat and human tissue. This would be very unpleasant soon.

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    $\begingroup$ No, one would not feel pain. Not until it the microwaves had overheated some tissue. Our heat receptors are almost all in the skin. $\endgroup$ – user137289 Feb 15 at 10:39
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    $\begingroup$ I would expect to feel heat, not pain (agreeing with @Pieter). A microwave oven puts out a few hundred Watts of microwave radiation. An infrared heater puts out a few hundred Watts of radiation at shorter wavelengths, and can be felt quite strongly and immediately from a couple of metres away. The microwave penetration depth is probably greater than for IR (less heating where the nerve endings are), but the OP was probably about half a metre away. That said I suspect it was humming but not emitting, so +1. $\endgroup$ – Chris H Feb 15 at 10:48
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    $\begingroup$ ... I'd test the power consumption when I open the door with it running because I have a mains power meter and more curiosity than fear. It should drop from ~1kW to a few W as fast as the power meter can update. $\endgroup$ – Chris H Feb 15 at 10:50
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    $\begingroup$ Don't throw out old appliances! Donate them to a recycler or your local neighbourhood hackerspace. They may be able to repair it or turn the parts into something else. Do put a big sticker saying the safety interlock is broken though. Throwing things out in the landfill is a waste of natural resources. $\endgroup$ – user253751 Feb 15 at 12:00
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    $\begingroup$ While I do not agree with the second part (which is simply wrong), the first one makes a lot of sense, especially if it is a new microwave OP bought. My microwave makes basically the same sound when open because of the cooling (and it says Cool on the display) $\endgroup$ – WoJ Feb 15 at 15:38
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I mentioned this in a comment, but will post as an answer as well.

Apparently, if you run it long enough, some microwaves will keep the fan running for cooling even though the microwave is no longer producing microwaves. My last roommate's microwave did this at the 2 minute mark, and even though the microwave was done cooking, the running fan made it sound like it still was running.

You should be able to confirm the behavior by looking in the owner's manual that came with it (or looking up the manual for your model online). Alternatively, the behavior may be described on one of the stickers on the microwave itself (which was the case with my former roommate's microwave). The sticker may be on the back of the machine, or it may be placed along the frame hidden by the door when it's closed.

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If you can't find the info on a sticker on the product or in a manual online...

Call the manufacturer and ask

Call their customer service and tell them what you observed. They will either tell you that A) it was expected behavior because e.g. as others have suggested the fan keeps running for ventilation after running it a while, or B) that there may have been a significant safety failure, and you should stop using it. In the case latter case they may be able to advise you on the likelihood that you suffered personal injury.

Bottom line is they will know your model best, and have skin in the game when it comes to your safety, so while the other answers provide good general advice, they will be best able to advise you with regard to your specific model.

And until you clear this up, I'd stop using it. Just in case.

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    $\begingroup$ may be able to advise you on the likelihood that you suffered personal injury In today's litigious society, I highly doubt that. The customer service people (who are not management/top/etc.) are almost certainly told to not say: "You might be injured" because that would them up to a lawsuit and not say "You're fine" because if you were injured then they would have that "disinformation" added to the lawsuit. So they'll be nice and certainly tell you if it is a feature (cooling fan), but otherwise, if out-of-warranty, probably just tell you to replace it. $\endgroup$ – manassehkatz-Moving 2 Codidact Feb 16 at 20:05
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    $\begingroup$ True. I considered that--so most likely they'd comment on the product, and you'd have to read between the lines. $\endgroup$ – bob Feb 16 at 20:10

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