There are unbleached and untreated flours and breads on the market. The public perception of white bread is that it has to have a white crumb. This led to the bleaching of flour. Most common used one used to be chlorine dioxide, it 's use limited by law and measured in parts per million. So it will not kill you. Modern mixing processes and dough moulding (shaping) technology means that you can achieve a whiter crumb from untreated flours.