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I've just rented a house that came with an equipped kitchen, and I've found that the microwave has a rather weird behavior - opening the doors seems to trigger it to start working, which the owners claim "has always been like that". Yet, I've been placing anything I want to heat as fast as possible and without completely opening the door for fear of exposure to microwaves - and I'm curious, assuming there is something wrong with the device, how dangerous is this type of short exposure?

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    \$\begingroup\$ Over time it'll cause cataracts and blindness at the very least. Definitely replace that microwave and stop using it. \$\endgroup\$ – Hearth May 5 at 19:12
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    \$\begingroup\$ Unplug it. Don't use it. Have it replaced. \$\endgroup\$ – JRE May 5 at 19:19
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    \$\begingroup\$ It's not good for you, and it's not good for the microwave. Switch of at the wall before you open it, or better still, switch off at the wall and then replace the microwave. \$\endgroup\$ – Neil_UK May 5 at 19:23
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    \$\begingroup\$ What make and model is the unit? Are you sure it is actually turning on and not just powering the lights and fan? Microwave ovens generally have at least two safety devices to prevent them from running when the door is opened. It's extremely unlikely that they would all fail. \$\endgroup\$ – Bruce Abbott May 6 at 0:12
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    \$\begingroup\$ When you say start working, do you mean fan spinning and turns lights on? Old Philips makes microwaves used to do that. \$\endgroup\$ – winny May 6 at 6:30
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It is dangerous to humans because it heats up things that contain water. You contain water. Some parts of you can't deal with this added heat (your eyes) and will be damaged first.

It also fries sensitive electronics like the embedded radio chips in your phone and laptop when they are close enough. (meters away, not centimeters)

Also, there are laws for for the amount of leakage of a microwave, and being on with open door certainly violates this law. Please unplug and don't use it, ask the landlord to replace it.

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    \$\begingroup\$ Extending @Jeroen3 answer, it can also mess with cardiac pacemakers and other medical devices! Replace the door interlock switch, and do not use until fixed. \$\endgroup\$ – DrMoishe Pippik May 5 at 19:35
  • \$\begingroup\$ You should replace it, however this is ee.se and not diy.se so we can split these hairs. Technically in some cases, assuming the magnetron is actually on and it isn't just lights and fan , it will be less effective at transferring energy from the magnetron to water molecules. It's not a magic ray emitter but a resonant cavity and the closed structure of the microwave case is required to perform it's function . It will demolish your WiFi signal though since they share a frequency \$\endgroup\$ – crasic May 5 at 19:48
  • \$\begingroup\$ @crasic for what is worth, I noticed that the plate doesn't turn and that there is no change to the WiFi signal strength when opening the door, so maybe it's just the fan and light turning on. \$\endgroup\$ – joaocandre May 5 at 20:28
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    \$\begingroup\$ @joaocandre Note that crasic is right, but dangerously so: you clearly underestimate the dangers of a microwave. Throw that thing out. DO NOT USE IT. You risk going blind. crasic's point is that the magnetron, the thing that generates the microwave radiation won't work at nearly 100% efficiency while the door is open. But even 5% of a microwave can permanently damage your eye. WiFi is not a suitable measurement method. (for many reasons, most of which are that magnetrons are narrowband) THROW THAT THING OUT. 100$ FOR A NEW MICROWAVE ISN'T WORTH YOUR EYESIGHT. \$\endgroup\$ – Marcus Müller May 5 at 21:00
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IF the oven heats with the door open then
THE OVEN IS HORRENDOUSLY LETHALLY DANGEROUS AND
IT MUST NOT BE USED

It is possible that the light and fan operate when the door is open BUT that the oven doesn't. Answers to these four questions will clarify the matter:

Q1. Can you cook/heat with the door closed?

Q2 Can you cook/heat with the door partially open?

Q3 If you open the door, place a cup of cold water in the microwave and then close the door to a slit so it remains open, after 1 minutes, is the water any warmer?

Q4 What brand and model is the oven? |

Your 4 answers are ...?

If the answer to Q2 and/or Q3 is "Yes" then "you have problems".

If the answer to Q1 is "No" then someone has probably rewired the control switches.

If Q1 Q2 Q3 answers are Yes No No then all is well.

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If there IS a problem then:

I've been placing anything I want to heat as fast as possible and without completely opening the door

This MAY be more dangerous than opening it more fully.
The partially opened door may form a "slot radiator" and project a semi-focused beam of energy.
This is NOT just an academic possibility - slot radiation from unintended sources is a common problem.

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@crasic's initial comment is so lethally misleading that it should be removed. While there is SOME technical merit in the answer it is partially wrong and a user could suffer severe injury or death if believed.

There is no certainty re how much of a load or resonant structure that will be formed with an open door - and a door opened to just enough to allow the cutout to (mal)function could forma slot antenna that could produce a beam of energy. eg

A child standing by the open oven could receive horrific eye and/or head irradiation injuries.


Note: This fault is so unusual and so impossible to occur in a properly constructed over that it sounds as if it is the product of somebody 'tinkering' with the circuitry. The danger is so high that special care is taken (and required by regulations) to ensure that this cannot happen.

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    \$\begingroup\$ Or maybe it's just the light coming on - I'd have thought if it were really radiating, it'd have killed OP's cellphone already and they'd have mentioned that \$\endgroup\$ – pericynthion May 6 at 5:41
  • \$\begingroup\$ @pericynthion If it's just the light, and he cooks with the door open, it must be a VERY bright light indeed :-). \$\endgroup\$ – Russell McMahon May 6 at 6:24
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    \$\begingroup\$ OP doesn't say s/he cooks with the door open.. \$\endgroup\$ – pericynthion May 6 at 16:18
  • \$\begingroup\$ @pericynthion On re-re-re-reading the OP's post I see that it is ambiguous AND that she PROBABLY means what you say. HOWEVER my answer is correct and useful as it is - it establishes whether the magnetron is operating when the door is not closed. I believe that it is not doing so - but, my answer covers all eventualities and allows a proper conclusion to be arrived at. No? \$\endgroup\$ – Russell McMahon May 7 at 11:35
  • \$\begingroup\$ Sure - it covers all the bases now :) \$\endgroup\$ – pericynthion May 7 at 15:30

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