Can x-ray radiation be compared to background radiation? I've been trying to learn about the possible effects of x-ray radiation from dental x-rays and most of the resources I come across compare the exposure to that of natural background radiation. 
Here's an example (not specific to dental x-rays): http://www.hpa.org.uk/webw/HPAweb&HPAwebStandard/HPAweb_C/1195733826941?p=1158934607708
I know electro-magnetic-radiation comes in different wavelengths and some are ionizing and some are not. Is this a valid comparison?
Update:
Thanks for the helpful responses so far. Looking at the chart here from @anna v, is comparing x-rays with background radiation comparing EMR that is 10^-10 with wavelengths that are longer than infrared? That's what I still don't understand. Wouldn't those different types of waves have different properties?
 A: Roughly yes.
Radiation is broadly divided into two from a safety point of view.      
Ionising radiation can break chemical bonds and so has an obvious way to cause damage to your body - how much depends on the energy, how much radiation you absorb and where in your body it gets to.
Both X-Rays and particles from radioactive material are ionising, as is the UV in sunlight,  and all these can cause health issues.
Non-ionising radiation is that where the energy of the individual particles is too low to directly break a chemical bond. This is true of most visible light and radio waves. It's still possible to damage your body with this, but only by some mechanism such as direct heating eg. a microwave oven or an industrial laser.
A: First of all terminology:
When physicists speak of radiation they primarily speak of electromagnetic radiation. When health physicists speak of radiation they include radiations of other types, alpha  and beta and neutrons in addition to gamma and xrays. They have developed a system where  radiation is given in Becquerel  ignoring the particular source.
So when somebody says that one xray is equivalent to background radiation, they compare the becquerels that one gets from one xray to the equivalent becquerels one would get from the ambient surroundings. This background comes from cosmic ray muons (about  1 per cm^2 per second) to natural radioactivity of stones and materials, to gases in the atmosphere released by volcanoes  etc. Natural radiation is mainly non electromagnetic, since high energy photons produced by close by radioactive decays are easily absorbed by the materials intervening, and photons coming from the cosmos are absorbed or interact high in the atmosphere ( another important point for life, to have an atmosphere).
Cosmic background radiation is another story, and is not health physics related :it is photons left over from the Big Bang and has very little energy, it is in the microwave part of the electromagnetic spectrum( mm)  whereas x-rays are of higher energy, in the range of nanometers. 
The comparison  is valid because it is the result of painstaking studies of calibration and measurements.
Response to updated question:

Wouldn't those different types of waves have different properties?

Matter responds differently to the different wavelengths of photons, due to the increasing energy they carry which is proportional to their frequency and inversely proportional to their wavelength.
The column on the far right gives the energy of the photon. A micron wavelength is in the electron Volt range and can affect molecular distances and cohesion and living matter. Below that the interaction with matter is in bulk, not individual molecules and cells after the Ultraviolet level. The electromagnetic radiation that can affect health is ultraviolet and smaller wavelengths. The smaller the wavelength the larger the possibility of destruction of living cells which is the study of health physics: by, as the frequency increases, heating in depth,breaking of chemical bonds, ionizing, and finally destroying complete cell structures when going to MeV energies.
A: This very good article on radiation explains that one unit of uSv measures the amount of damage done by that dose of radiation. If this dose is put entirely in one small part of the body then presumably the damage is confined to that small space. If this is the brain and the mouth then this would appear to be a much bigger potential problem than background radiation of the same quantity that would cover the entire body, especially if it includes radiation that it blocked by the skin etc. 
http://documents.manchester.ac.uk/display.aspx?DocID=26897
