An on-demand system for preventing and/neutralizing bioactive substances employs real time monitoring of the fluid for bioactive substances. Once a specific bioactive substance is detected, then a specific neutralizing substance is selected, such as an oxidizing substance, intense ultraviolet light, a neutralizing substance or other substances selected for particular threats. In another example of the invention, a device neutralizing a substance would include a container that would be capable of containing each of the plurality of precursors, a collecting space for receiving each of the plurality of precursors selected by the selector, and a dispensing mechanism for dispensing the neutralizing substance. In one example the neutralizing substance is formulated by the reactions of the precursors in order to neutralize the substance. The device may alternatively select a plurality of precursors to produce an aroma-producing substance and alternatively may neutralize the aroma-producing substance if it harmful or lethal, for example.