Polymers are composed of long molecules successively repeated along a chain. These long structural entities are called mer units. Mer originates from the Greek word meros, which means part. A single mer is called a monomer. It denotes a repeat unit in a polymer chain while monomer is used in context of a molecule consisting of a single mer. Many mers together make a polymer.

This section describes the toxic and hazardous properties of some of the widely used monomers employed in synthesizing polymers. Traces of these monomers often remain in commercial polymer supplies at levels generally less than 1 ppm by weight. It is unusual, but not impossible for the TLV to be reached during the normal processing and fabrication operations.

As an example, in some operations during the milling of Nitrile Rubber, concentrations of Acrylonitrile between 2 to 3 ppm on the mill rolls have been detected. Acrylonitrile is a carcinogenic material and workers should proceed with caution when working with nitrile rubbers to ensure that operator exposure is well below the TLV.

Other polymers can contain even higher concentrations of monomers. Fabricators, compounders, and mill operators should always check to ensure that exposures to residual monomers do not exceed the TLV. Another example of a high-level exposure risk is Polychloroprene Latex, where several thousand ppm by weight of Chloroprene monomer can be created during processing operations.