<P> To place the increased risk in perspective, a plain chest X-ray will expose a person to the same amount from background radiation that people are exposed to (depending upon location) every day over 10 days, while exposure from a dental X-ray is approximately equivalent to 1 day of environmental background radiation . Each such X-ray would add less than 1 per 1,000,000 to the lifetime cancer risk . An abdominal or chest CT would be the equivalent to 2--3 years of background radiation to the whole body, or 4--5 years to the abdomen or chest, increasing the lifetime cancer risk between 1 per 1,000 to 1 per 10,000 . This is compared to the roughly 40% chance of a US citizen developing cancer during their lifetime . For instance, the effective dose to the torso from a CT scan of the chest is about 5 mSv, and the absorbed dose is about 14 mGy . A head CT scan (1.5 mSv, 64mGy) that is performed once with and once without contrast agent, would be equivalent to 40 years of background radiation to the head . Accurate estimation of effective doses due to CT is difficult with the estimation uncertainty range of about ± 19% to ± 32% for adult head scans depending upon the method used . </P> <P> The risk of radiation is greater to a fetus, so in pregnant patients, the benefits of the investigation (X-ray) should be balanced with the potential hazards to the fetus . In the US, there are an estimated 62 million CT scans performed annually, including more than 4 million on children . Avoiding unnecessary X-rays (especially CT scans) reduces radiation dose and any associated cancer risk . </P> <P> Medical X-rays are a significant source of man - made radiation exposure . In 1987, they accounted for 58% of exposure from man - made sources in the United States . Since man - made sources accounted for only 18% of the total radiation exposure, most of which came from natural sources (82%), medical X-rays only accounted for 10% of total American radiation exposure; medical procedures as a whole (including nuclear medicine) accounted for 14% of total radiation exposure . By 2006, however, medical procedures in the United States were contributing much more ionizing radiation than was the case in the early 1980s . In 2006, medical exposure constituted nearly half of the total radiation exposure of the U.S. population from all sources . The increase is traceable to the growth in the use of medical imaging procedures, in particular computed tomography (CT), and to the growth in the use of nuclear medicine . </P> <P> Dosage due to dental X-rays varies significantly depending on the procedure and the technology (film or digital). Depending on the procedure and the technology, a single dental X-ray of a human results in an exposure of 0.5 to 4 mrem . A full mouth series of X-rays may result in an exposure of up to 6 (digital) to 18 (film) mrem, for a yearly average of up to 40 mrem . </P>

Which device is used to measure the wavelength of x rays