Two recent studies suggest that the declining fertility among men is caused by exposure to chemicals. 

Exposure to anti-adhesive, stain and water repellents

These chemicals known as perfluoroalkyl acids (PFAAs), a subset of a larger group of chemicals called polyfluorinated compounds (PFCs) are used in a variety of products such as carpets, paper food packaging and clothing. These include such products as Teflon(c) used in anti-adhesives in pots and pans, and Scotchgard(c) which repels stains in clothing, carpets and furniture.

The study conducted in Denmark found that men with high PFAA exposure had lower mean percentages and total number of normal sperms compared with men with low exposure.

Exposure to chemicals in the atmosphere, soil and water

A decline in male fertility is connected to chemicals, such as PAHs, a class of chemicals that are released in the atmosphere, soil and water as a result of incomplete burning of a range of substances including coal, oil, gas, wood, refuse and other organic substances. These have been classified as probable carcinogens.

The study conducted in China showed that infertile males with abnormal semen quality had higher concentrations of PAHs.