Benzene and carbon tetrachloride are Class-1 Solvents. They are present as contaminant in many organic solvents used in manufacturing of pharmaceuticals. Their traces are detected in pharmaceuticals, even when they are not used in synthetic processes. The lecture discusses the origin of these solvents in pharmaceuticals and strategies to tackle the presence of these solvents.

https://www.youtube.com/watch?v=-D4aKgHFTrQ&t=13s