Precursor
Any chemical reactant, which takes part in the production of a toxic chemical, is a precursor. The most important precursor is called key component.
See also: toxic chemical
Glossary Source
JRC EU CBRNe Glossary
(
EU Glossary on Chemical, Biological, Radiological, Nuclear, and Explosive risks
)
https://publications.jrc.ec.europa.eu/repository/bitstream/JRC128863/JRC128863_01.pdf