75 picocuries, or 2.775 becquerel of I-131 = 100 mrem/yr
2000 picocuries, or 74 becquerel of Cs-134 = 100 mrem/yr
5000 picocuries, or 185 becquerel of Cs-137 = 100 mrem/yr.
Each of these amounts will max out your allotted extra exposure from natural sources per year. The total ballpark figure to not exceed is 500 millirem/year, but keep in mind - there are no safe exposures to radiation. Every photon entering your body is a dice throw, capable of causing cellular damage that can lead to cancer. You don't want to be paranoid of course, but you want to keep cumulative exposures as low as reasonably achievable.