Light of any wavelength can cause heating. It depends on the material which absorbs it. Few materials absorb light present in microwave range while others simply reflect them . Consider a material which absorbs light at microwave frequencies. If the intensity of the light wave at microwave frequencies is increased then heating effect will also be observed. This phenomenon is due to light waves exciting electrons in the medium which absorbs it.
Black body radiation is the phenomenon dealing with absorption and re-radiating the absorbed light. where ε is At any given wavelength λ, emissivity is defined as the ratio of the actual emitted radiance, Rλ, to that from an ideal blackbody, Bλ
ελ = Rλ / Bλ .
Emissivity is a measure of how strongly a body radiates at a given wavelength; it ranges between zero and one for all real substances. A gray body is defined as a substance whose emissivity is independent of wavelength. In the atmosphere, clouds and gases have emissivities that vary rapidly with wavelength. The ocean surface has near unit emissivity in the visible regions.
For a body in local thermodynamic equilibrium the amount of thermal energy emitted must be equal to the energy absorbed; otherwise the body would heat up or cool down in time, contrary to the assumption of equilibrium. In an absorbing and emitting medium in which Iλ(intensity in layman terms) is the incident spectral radiance, the emitted spectral radiance Rλ is given by
Rλ = ελBλ = aλIλ ,
where aλ represents the absorptance at a given wavelength. If the source of radiation is in thermal equilibrium with the absorbing medium, then
Iλ = Bλ , so that ελ = aλ .
This is often referred to as Kirchhoff's Law. In qualitative terms, it states that materials that are strong absorbents at a given wavelength are also strong emitters at that wavelength; similarly weak absorbents are weak emitters.
Coming to microwave heating of human bodies one needs to keep in mind that free radicals cause cancer. Microwave heating increases the number of ions. Your project must find a link between microwave heating and concentration of free radicals. Though microwave frequencies donot have enough energy per quanta to ionize molecules it is still worth a try.
Edit 1- @Floris Yes you are right. I have changed it. Free ions cause cancer, antioxidants reduce them. Never mind I hope this explanation might be useful. But microwave waves just heats the body fluids. The heat may increase number of free ions but not the microwave radiation. Gamma rays and x-rays are powerful enough to create free ions. Maybe the heating might cause some effects but there is no support that microwaves cause cancer.