"Precursor chemicals" refers to substances that are used to make drugs, explosives, or other illegal or dangerous substances. They are like the building blocks or ingredients that are combined to create such products. Full definition
Factors that affect ozone formation include heat, concentrations of precursor chemicals, and methane emissions. (cdc.gov)
French researchers have incorporated molecularly imprinted polymers (MIPs)-- dubbed «plastic antibodies» — into a cosmetic product to capture precursor chemicals in human sweat that cause bad smells. (scientificamerican.com)
Methane is of interest as a simple precursor chemical for life. (spaceref.com)