If one is in a place that there is waste behind them or on the side of them and they want to recite a Davar Shebikedusha, they must distance themselves four amos away (aprox. 6 and a quarter feet) from the point that the odor is no longer evident. If it is in front of a person that they can see it without turning their head, then even if it is more than four amos away, they must distance themselves far away enough that it is out of  their range of vision. However, if one can only see it if they turn their head, the Poskim question whether it is considered to be on the side or in front them, and therefore one should turn so that it is certainly behind them.