1. Microwaves, like other forms of
electromagnetic radiation, can propagate through
space (i.e. they don't require a medium like air
or water to travel through). In general, when an
atom or molecule absorbs energy, one or more
electrons in the atom or molecule can
become "excited." When the electron(s) return to
what is called their "ground state energy" (the
lowest energy state) the atom or molecule can
produce electromagnetic radiation. It turns out
that certain "transitions" to particular excited
states and falling back down to lower or ground
states will result in the formation of different
types of electromagnetic radiation (such as
microwaves, infrared, etc.).
2. The short answer to this question is
that when the molecules in food absorb
microwaves, they vibrate and the resulting
friction between the molecules heats up the food.
3. This is a good question. I would imagine
that if microwaves hit our skin, they would have
a similar heating effect as they do with food.
It would probably be harmful for parts of the
body that can't conduct heat away quickly.
4. When you say "functioning" microwave
oven, I understand that to mean that the
microwave works as it is supposed to and has the
appropriate screen to prevent "leakage" of
microwaves from the oven. If there is no leakage
from the microwave oven, then I think the chance
of getting brain cancer or any other kind of
cancer is pretty low. Even if there is leakage,
I think the main health risk would be tissue
damage from heating, and not cancer.
Microwaves are a "non-ionizing" form of
radiation. What this means is that when they are
absorbed by molecules, the molecules do not
become ionized (they don't become charged). It
is typically the ionizing forms of radiation,
such as X-rays, that are associated with cancer.
If you're wondering why microwaves are non-
ionizing whereas X-rays are ionizing, take a
look at where these kinds of radiation lie on
the electromagnetic spectrum. X-rays are much
higher frequency. We know from Planck's equation
that frequency and energy are proportional (i.e.
if we increase frequency, then we increase
energy). If we think about it, radiation with
higher energy has a higher chance of kicking
electrons into all kinds of excited states,
possibly even "ejecting/liberating" an electron
and thus leaving a charge on the molecule (i.e.
ionizing the molecule -- making it an ion).
Something like this can be detrimental in the
body, where certain molecules are supposed to be
charged while others are not. But luckily for
us, microwaves are non-ionizing!
5. This is a pretty controversial topic as
far as I can tell. I don't think there have been
enough studies to show microwaves' affects on
the brain, let alone health in general.
According to a study by Volcow et al., 2011,
microwave radiation from cell phones increased
brain glucose metabolism. But we should be
careful not to jump to conclusions here. It's
important to be thorough! The microwave
radiation from cell phones occurs at a different
frequency (i.e. have different wavelengths) than
those produced in microwave ovens. Furthermore,
I don't believe it's well-understood what kinds
of consequences increased brain glucose
metabolism could actually have. I realize this
is a foggy answer to your question, but I think
there's just not enough evidence out there about
microwave ovens to really say anything one way
or the other.
6. I am a chemistry graduate student. I
earned my Bachelor of Science degrees in
chemistry and biology.