The following data is from an industry source believed to be reliable. However, you should verify the following government data  research the NIH (National Institute of Health). Additionally, "acceptable" levels of radiation exposure are somewhat subjective, so we encourage you to make your own conclusions as to the maximum levels of radiation exposure which would be acceptable to you: The US Government recommends that you limit your dosage or exposure of radioactivity to the following:

Occupational 5R/yr = 5,000mR/yr 

NonOccupational .1R/yr = 100mR/yr 
+ 100mR/yr (background)*
= 200mR/yr total
*estimated annual exposure to background radiation

First measure your dose rate using a Geiger counter that reads out in mR/hr. 

Understand that that dose rate includes both the radiation from the particular source of your focus that is emitting radioactivity, along with ever present background radiation. 

Then figure out the number of hours in a year that you are exposed to that dose rate. 

Then multiply the dose rate, expressed in mR/hr, by the number of hours of your exposure over the course of a year. 

Finally, compare your annual exposure to the NIH standard above. 

Handling Radioactive Ore: 
1.) Assume the dose rate from a high grade, 1" radioactive sample of Uraninite equals 100 mR/hr.
2.) Assume exposure time of just 2 hours for the entire year.
3.) Therefore, annual dosage from the radioactive ore equals 200 mR (100 mR/hr x 2 hours per year).

Airplane Flights: 
1.) Assume a commercial airline pilot flies at a ceiling of 40,000', and at that altitude, where cosmic rays are more intense, receives a dose rate of .3 mR/hr.
2.) Assume 20 hours of flight time per week, or 1,040 hours per year.
3.) Therefore, annual dosage from high altitude flights equals 312 mR (.3 mR/hr x 1,040 hours per year).

Granite Countertops: 
1.) Assume the dose rate from a granite countertop in a kitchen is .05 mR/hr.
2.) Assume close proximity to the countertop of 3 hours per day, or 1,095 hours per year.
3.) Therefore, annual dosage from the granite equals 55 mR (.05 mR/hr x 1,095 hours per year).

High Altitude Living: 
1.) Assume a Sherpa living in Tibet at 12,000', which high altitude means less atmospheric and electromagnetic shielding of cosmic rays, receives a constant dose rate of .04 mR/hr.
2.) Given that 365 days a year x 24 hours per day equals 8,760 hours per year of exposure.
3.) Therefore, annual dosage from high altitude living equals 350 mR (.04 mR/hr x 8,760 hours).
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