The affects of cosmic or solar radiation on humans has never been studied. All of the "Limits" suggested for flight crew are based on other forms of radiation. So basically we are flying guinea pigs.
This is not actually true. Cosmic particles hitting the upper atmosphere (far above where aircraft can fly) causes cascades of other particles toward the ground. Those particles and their interactions are well known so we actually do know something about the effects on humans. Secondly, exposure limits always err on the side of safety - if we did not know how cosmic or solar radiation affects humans we would assume a worst case scenario and set the exposure limits based on that until better information is obtained. To say we are flying guinea pigs is simply incorrect.