"The NRC adopted the 100 mrem per year dose limit from the 1990 Recommendations of the International Commission on Radiological Protection (ICRP). The ICRP is an organization of international radiation scientists who provide recommendations regarding radiation protection related activities, including dose limits. These dose limits are often implemented by governments worldwide as legally enforceable regulations. The basis of the ICRP recommendation of 100 mrem per year is that a lifetime of exposure at this limit would result in a very small health risk and is roughly equivalent to background radiation from natural sources (excluding radon) (ICRP, 1991). Thus, the ICRP equated 100 mrem per year to the risk of riding public transportation – a risk the public generally accepts (ICRP, 1977). The U.S. National Council on Radiological Protection and Measurements (NCRP) also recommends the dose limit of 100 mrem per year (NCRP, 1993)."
There is little we can do to reduce or eliminate natural background radiation exposure. What we can do is accept and respect the fact that radiation exposure is a normal part of our lives, and that levels will vary significantly according to geographic location.
Due to the Fukushima crisis, so-called "safe" and "acceptable" levels of radiation exposure are the topic of much heated discussion, with the well-wishing out to educate the paranoid and vice versa. As contrary claims are bounced around, keep in mind that each individual has a comfort zone when it comes to their own radiation exposure. Since "safe" doses have not been clearly defined by scientists, it is a matter of personal preference, thus, one individual can't presume his/her conviction of what is "safe" upon another.
In all circumstances, safety, practicality, and reasonableness should prevail when considering personal risk.
100 mrem/yr = 1 mSv/yr = 0.11415μSv/hr