Microwaves are low-frequency electromagnetic radiation. There
are microwaves passing through us at all times, some emitted by cell
phones. Microwave ovens are effective for cooking food because they
emit a microwave frequency that excites the rotational (low-energy)
motion in water molecules; as the rotational motion of the water
molecules increases, they bump into each other and produce heat. If you
were to put a completely (>99.99% pure) nonpolar substance such as
hexane into the microwave oven, it wouldn't heat up at all...but don't
try this at home because even a little water contamination could cause
the hexane to ignite.
The ability of the microwave oven to sufficiently heat food (by doing work on the water molecules, much of which is subsequently dissipated as heat), depends not only on the frequency of the radiation emitted by the oven, but also on the power/cm^2. The microwave oven that I have in my apartment is 8"X12" and produces 700 W of power; in other words, my microwave oven supplies 1.13 W/cm^2.
Now, suppose I were to leave the door open. Estimating that my kitchen is about 12 feet X 12 feet, the power per square foot supplied by the microwave would then be 5.23 mW/cm^2, on average. According to researchers who studied the effects of microwave radiation on rats, 5.23 mW/cm^2 is definitely a nonfatal (and "nonhyperthermal") dose, and a dose almost 6 times greater (30 mW/cm^2) had a very small, though statistically significant, effect on the animals.
You might be thinking, "My kitchen is smaller than yours, and therefore the power per area must be much higher, and even fatal!" The area of your kitchen might matter to some degree, but if your cell phone works in your kitchen, then that means that microwaves can escape through the walls and windows, and any radiation from your microwave oven will likewise dissipate. Microwaves can penetrate through pretty much anything except for metal. Your microwave door contains microwave radiation because there is a metal netting between the glass panes.
Despite the fact that the average power per area in the room would be really low, injury would still be a possibility. The microwave oven with the open door would be analogous to a light bulb; the closer you get to the open microwave oven, the more intense the radiation would be. You might feel hot if you were standing really close to the microwave, but it definitely wouldn't cook everything in the room. If you were standing near enough to the open oven to feel hot, you would only have to back away to avoid damage.
And on a related note, if you had a metal object relatively close to the open microwave oven, you might see some arcing, though most of the time arcing happens when something is placed directly in the path of the microwaves, so that the microwaves are reflected back to the source (magnetron). For example, if you place a paper clip or some other sharp metal object really close to the front edge of the microwave oven (by the door) and then run the microwave oven for a while, you won't notice any arcing. If, however, you place the object in the middle of the microwave oven, sparks will fly!
With regard to the open microwave oven heating the surrounding area, this is unlikely. If the power per area is not capable of having a hyperthermal effect on your body, which is 70% water, then it is unlikely that it will heat the air, which has a very small number of water molecules per unit volume.
Try this experiment at home: heat a mug of water in your microwave for 1 minute. Open the microwave door and feel the temperature of the air inside with your hand (put your hand NEXT to the mug, rather than above, where there might be hot water vapors). Next, CAREFULLY stick a finger into the water to feel its temperature. How does the temperature of the air compare to the temperature of the water? Would the results of the experiment be different if you were using a conventional oven (which uses thermal radiation) rather than a microwave oven? (The answer is yes, because conventional ovens rely on conduction and convection rather than excitation.)
Finally, I must note that, according to the FDA, "there have been allegations of radiation injury from microwave ovens, but none as a direct result of microwave exposure." In other words, people have been injured by spilling hot, microwaved water on themselves, but the microwaves themselves have never caused any problems. I would suspect that there have been at least a couple of times in the history of microwave ovens where the interlock system has failed and microwaves have continued to be produced even when the door is open.
For more "Q&A" on the topic of microwaves, check out http://howthingswork.virginia.edu/microw... which is maintained by physics professor and author Louis Bloomfield.
The ability of the microwave oven to sufficiently heat food (by doing work on the water molecules, much of which is subsequently dissipated as heat), depends not only on the frequency of the radiation emitted by the oven, but also on the power/cm^2. The microwave oven that I have in my apartment is 8"X12" and produces 700 W of power; in other words, my microwave oven supplies 1.13 W/cm^2.
Now, suppose I were to leave the door open. Estimating that my kitchen is about 12 feet X 12 feet, the power per square foot supplied by the microwave would then be 5.23 mW/cm^2, on average. According to researchers who studied the effects of microwave radiation on rats, 5.23 mW/cm^2 is definitely a nonfatal (and "nonhyperthermal") dose, and a dose almost 6 times greater (30 mW/cm^2) had a very small, though statistically significant, effect on the animals.
You might be thinking, "My kitchen is smaller than yours, and therefore the power per area must be much higher, and even fatal!" The area of your kitchen might matter to some degree, but if your cell phone works in your kitchen, then that means that microwaves can escape through the walls and windows, and any radiation from your microwave oven will likewise dissipate. Microwaves can penetrate through pretty much anything except for metal. Your microwave door contains microwave radiation because there is a metal netting between the glass panes.
Despite the fact that the average power per area in the room would be really low, injury would still be a possibility. The microwave oven with the open door would be analogous to a light bulb; the closer you get to the open microwave oven, the more intense the radiation would be. You might feel hot if you were standing really close to the microwave, but it definitely wouldn't cook everything in the room. If you were standing near enough to the open oven to feel hot, you would only have to back away to avoid damage.
And on a related note, if you had a metal object relatively close to the open microwave oven, you might see some arcing, though most of the time arcing happens when something is placed directly in the path of the microwaves, so that the microwaves are reflected back to the source (magnetron). For example, if you place a paper clip or some other sharp metal object really close to the front edge of the microwave oven (by the door) and then run the microwave oven for a while, you won't notice any arcing. If, however, you place the object in the middle of the microwave oven, sparks will fly!
With regard to the open microwave oven heating the surrounding area, this is unlikely. If the power per area is not capable of having a hyperthermal effect on your body, which is 70% water, then it is unlikely that it will heat the air, which has a very small number of water molecules per unit volume.
Try this experiment at home: heat a mug of water in your microwave for 1 minute. Open the microwave door and feel the temperature of the air inside with your hand (put your hand NEXT to the mug, rather than above, where there might be hot water vapors). Next, CAREFULLY stick a finger into the water to feel its temperature. How does the temperature of the air compare to the temperature of the water? Would the results of the experiment be different if you were using a conventional oven (which uses thermal radiation) rather than a microwave oven? (The answer is yes, because conventional ovens rely on conduction and convection rather than excitation.)
Finally, I must note that, according to the FDA, "there have been allegations of radiation injury from microwave ovens, but none as a direct result of microwave exposure." In other words, people have been injured by spilling hot, microwaved water on themselves, but the microwaves themselves have never caused any problems. I would suspect that there have been at least a couple of times in the history of microwave ovens where the interlock system has failed and microwaves have continued to be produced even when the door is open.
For more "Q&A" on the topic of microwaves, check out http://howthingswork.virginia.edu/microw... which is maintained by physics professor and author Louis Bloomfield.
Source(s):
Jensh, Ronald P. 1997. "Behavioral teratologic studies using microwave
radiation: Is there an increased risk from exposure to cellular phones
and microwave ovens?"
FDA. "Microwave Oven Radiation." < http://www.fda.gov/cdrh/consumer/microwa... >
FDA. "Microwave Oven Radiation." < http://www.fda.gov/cdrh/consumer/microwa... >
