Heating With Microwaves

[Eugbug]

Main Question or Discussion Point

Could it be possible to heat the people in a house with long wavelength microwaves rather than heating all the air in the building ?

 

[Pengwuino]

I can only imagine the physiological problems associated with that. Remember, normal heating of a person in a household comes from convective heating. The body is heated from the outside. With a microwave, the radiation will penetrate the entire body and heat up internal organs, blood, everything, all at once. I can't imagine that's any good :)

 

[Drakkith]

I can only imagine the physiological problems associated with that. Remember, normal heating of a person in a household comes from convective heating. The body is heated from the outside. With a microwave, the radiation will penetrate the entire body and heat up internal organs, blood, everything, all at once. I can't imagine that's any good :)

If it isn't excessive I don't see why it would be bad. I heat up my whole body just by going outside and running around.

 

[Pengwuino]

If it isn't excessive I don't see why it would be bad. I heat up my whole body just by going outside and running around.

Not using penetrating radiation. Consider this. A nice little room heater will run at around 1,000W. A microwave oven typically produces the same amount of power. Would you want to sit in a room with an exposed 1000W magnetron?

 

[Drakkith]

Not using penetrating radiation. Consider this. A nice little room heater will run at around 1,000W. A microwave oven typically produces the same amount of power. Would you want to sit in a room with an exposed 1000W magnetron?

I have no idea. Is the radiation going to harm me? If not, then I would be perfectly fine with it.

 

[Eugbug]

Presumably the wavelength of the microwaves would be critical and the microwaves would have to be "stirred" to avoid nodes or areas of constructive interference and localised overheating in the body ?

 

[Drakkith]

Something to consider: Many times the point of heating and cooling is just comfort. Heating someones body using microwaves just makes them feel like they just ran a mile and came into to the 50 degree AC cooled air. Not that pleasant in my opinion. Your body doesn't need to be heated up itself unless you have hypothermia, it just needs to be kept insulated.

 

[QuantumPion]

Something to consider: Many times the point of heating and cooling is just comfort. Heating someones body using microwaves just makes them feel like they just ran a mile and came into to the 50 degree AC cooled air. Not that pleasant in my opinion. Your body doesn't need to be heated up itself unless you have hypothermia, it just needs to be kept insulated.

By the same token, while you could use microwaves to selectively heat up people in a room, I don't think it would be comfortable because you would still feel the cold air in the room convecting heat away from you. Like running outside in a sweater, your body may be warm but your skin can still feel cold.

 

[russ_watters]

I can only imagine the physiological problems associated with that. Remember, normal heating of a person in a household comes from convective heating. The body is heated from the outside. With a microwave, the radiation will penetrate the entire body and heat up internal organs, blood, everything, all at once. I can't imagine that's any good :)

I'm not sure if the premise of that last sentence is true, but even if it is, why would that be bad? You seem to be assuming the radiation would be excessive. I don't think that is a reasonable assumption. What if you assume the amount of heat absorbed is the same as with an IR radiative heater?

In fact, it seems to me that you may even have it backwards. But that's a biology issue, not a physics one...

 

[Pengwuino]

I'm not sure if the premise of that last sentence is true, but even if it is, why would that be bad? You seem to be assuming the radiation would be excessive. I don't think that is a reasonable assumption. What if you assume the amount of heat absorbed is the same as with an IR radiative heater?

In fact, it seems to me that you may even have it backwards. But that's a biology issue, not a physics one...

I think the issue is how you are heated. Microwaves would be able to penetrate and I would imagine IR radiation would get absorbed fairly quickly at the skin.

Of course, one overriding factor in how this would never be allowed would be the massive interference it would cause with wireless devices :P

 

[SteamKing]

If you didn't want to heat your whole house in the winter, you could just crawl inside your conventional oven and turn the heat on low.

 

[Bobbywhy]

Danger! Before any attempt to heat people using microwaves is planned a thorough study of the possible hazards must be done. The two below paragraphs contain a few references to known safety limits. Human eyes and testes are the most sensitive organs, and many more body parts may be damaged even at low power levels.

“Biological hazards:

The best understood biological effect of electromagnetic fields is to cause dielectric heating. For example, touching or standing around an antenna while a high-power transmitter is in operation can cause severe burns. These are exactly the kind of burns that would be caused inside a microwave oven.

This heating effect varies with the power and the frequency of the electromagnetic energy. A measure of the heating effect is the specific absorption rate or SAR, which has units of watts per kilogram (W/kg). The IEEE[3] and many national governments have established safety limits for exposure to various frequencies of electromagnetic energy based on SAR, mainly based on ICNIRP Guidelines,[4] which guard against thermal damage.”

http://en.wikipedia.org/wiki/Electromagnetic_radiation_and_health

 

[sophiecentaur]

You don't so much need to raise the temperature of someone's body in order to 'warm them up' - so microwave heating, as well as being dodgy, isn't really what you need. A body is producing lots of surplus heat (ignoring hypothermia situations) and comfort is based on your surface temperature.
If you want people to 'feel warm' in a room then you need to warm up their surface. Infra red heaters can do this quite effectively after only a short time (i.e. not a lot of energy), which is why they are used so often in bathrooms and pub gardens. You don't actually need to heat all the surrounding air and walls (standing in the Sun on a chilly day can make you feel very comfortable).

Our systems did not evolve to be able to detect RF heating effects so we can't 'know' what damage, if any, is being done to us by RF fields but we are aware of surface heating from the sun or hot objects. So we can regulate our own exposure to danger from 'heat' better than other sources. Heating with radiating surfaces is ok as far as it goes but warm air, although expensive, avoids the problem of warm face cold back.

 

[russ_watters]

I think the issue is how you are heated. Microwaves would be able to penetrate and I would imagine IR radiation would get absorbed fairly quickly at the skin.

Right, so if you are actually cold (your core body temp is a touch low), wouldn't it be better for the heat to penetrate a little? You could apply more heat without burning if you aren't just heating your skin.

 

[nasu]

The problem is: when you feel "a little cold", does your internal temperature decreases?
Talking about mammals, I doubt it. I did not find any specific sources yet. Maybe we should try it.
On the other side, increasing the internal body temperature by 1-2 degrees means fever, discomfort and related problems.
I think that the normal sensation of cold or hot is given by the rate of heat transfer rather than internal body temperature. This may be different for extreme cases like hypothermia but we are talking about the normal, "keep it comfortable", situation.

It may work for reptiles, though.

 

[sophiecentaur]

The original post talks of "heating people" which, to me, sounds like making the inhabitants comfortable. In that case, you want to give the subjective feeling of warmth - not introduce heat into their bodies which they will try to get rid of asap.

As a house-heating system, I should say that RF, in general, would not be a viable idea.

@nasu: I believe that people who's body core temperature is well below normal actually start to feel "a little warm", in fact, and may feel like removing clothing. It's all in the mind.

 

[microwaveguru]

In 1980. the concept of a "people heater" was put forth by Professor R.V. Pound of Harvard. His suggestion was to heat people rather than an entire room as a means of conserving energy. That way, when a person went from one room to another the microwave energy would cease in the first room and come on in the second, thereby again saving energy. The idea is not as far fetched as some may think. In an interesting series of experiments carried out by colleagues in the late 1980s, they exposed themselves, full-body, to microwave radiation and demonstrated that a field intensity of approximately 20 to 30 milliwatts/cm2 produced a comfortable warmth. Note that an equivalent amount of sunshine is about 85 milliwatts/cm2. Now, 20 - 30 mw/cm2 is far to little to cause injury, and the only injury that could be caused by microwaves is heat-induced injury. Microwaves are very low energy EM waves - look at the electromagnetic spectrum - there is more energy in visible light than microwaves, and I doubt that any of you is so concerned about light that you sit in the dark. Here are some useful numbers representing the energy content, in electron volts (eV) of various EM sources: microwaves = 0.00001; visible light = 0.3 to 0.5; UV = 0.5 to 20; gamma radiation = > 3x104

 

[Drakkith]

Hmm, wikipedia is showing 1.6 to 3.4 eV as the photon energy of visible light.
http://en.wikipedia.org/wiki/Electron_volt

 

[microwaveguru]

The fact that it will probably never happen is true - after all, when people are terrified that cell phones could be causing brain cancer, they will never accept microwave-people-heaters. My point was simply that the idea is not so far fetched, and that many people are unjustifiably worried about microwave radiation. There are other practical drawbacks to the idea - assume the room temperature is 40 - 50 F, but the person is microwave-warm. But most of the objects in the room will remain cool, possibly uncomfortable cool. Other problems could be metallic objects arcing when brought close together.

 

[Drakkith]

And what about all the electronic items people have? Cellphones, computers, pacemakers, etc?

 

[microwaveguru]

Pacemakers were never a problem - that's a myth, and even if true, the early ones were improperly shielded. They don't exist any more. With early pacemakers, there were recorded incidents due to lawnmower & automobile engines, elevators switches and some other things - but never a microwave oven.
Your comment on cell phones and computers is likely correct, but I can't say that for sure.