The previous graph (5-50 mR) has been compressed by a factor of 10 and is illustrated as the lower portion of the above bar.
The average natural background in Illinois is approximately 82 mR per year. This value varies from state to state and reaches 170 mR in Colorado. The average background in the United States is approximately 100 mR per year. This estimate excludes the exposure that is estimated from radon. Interestingly, there are places in the world (India and Brazil) where the background radiation is 5000 mR per year. This is partially due to increased Thorium in the soil. Even at doses that are 50 times the natural background in the United States, no increased incidence of cancer or genetic malformations is seen in these countries that is attributable to the additional radiation.
Medical diagnostics accounts for approximately 93 mR per year to the general population. This takes into account the very ill patient that receives several chest x-rays per week and those who are healthy and do not receive one for years at a time. The amounts of exposure attributable by the specialties of radiology and nuclear medicine are 79 and 14 mR respectively.
The federal government has set the maximum allowable exposure to a member of the public that does not routinely work with radioactive materials at 100 mR. Previously, the Nuclear Regulatory Commission used 500 mR as their limit to the general public. The benefits of this decrease are only theoretical, but results in significant costs to the radiation user and ultimately, the consumer. As mentioned above, there are places where the annual exposure is 5000 mR, and no ill effects have been deterined in those regions.
Radon is estimated to account for 200 mR per year of exposure. This value is somewhat uncertain and varies widely by region. There has been a great deal of speculation on the health effects of radon. It should be remembered that radon has been around for a lot longer than mankind and further research needs to be done to better identify the health risks.
Smokers receive the equivalent of about 20 chest x-rays (280 mR) in excess radiation for every year that they smoke one pack per day. This is because the tobacco plant concentrates polonium, a natural isotope. As the cigarette is smoked, the inhaled polonium exposes the lungs to additional radiation.