https://vocabularyserver.com/psh/en/ Polythematic Structured Subject Heading System National Technical Library National Technical Library 1998-12-31 en 2012-03-20 15:19:18 chemical warfare agents 2012-03-20 15:19:18https://vocabularyserver.com/psh/en/xml.php?skosTema=3987https://vocabularyserver.com/psh/en/xml.php?skosTema=3989