ChemRisk was a joint venture with McLaren-Hart ENV, Engineering. In 1996, Dennis became President and Chief Executive Officer (CEO) of McLaren-Hart Environmental, a nationwide consulting firm of 600 persons (number 100 in the US at the time). He was the youngest professional (age 44) in the U.S. to head a national environmental firm at $700M (revenue).

In 1997, Dennis joined Exponent and was a Group President responsible for leading the Health Sciences division (approximately 140 professors). In 2003, he left Exponent and he restarted ChemRisk as a separate enterprise. In 2015, the management team sold the firm to Cardno, an Australian based environmental services firm. He was President of Cardno-ChemRisk from 2015 until he left the firm in April 2019. He is currently President of Paustenbach and Associates (a firm devoted to Dennis’s historical interests in toxicology, occupational hygiene, engineering, and risk assessment).

Dr. Paustenbach has provided expert witness testimony in public meetings and as many as 500 depositions and three dozen trials concerning the health effects of chemicals in sediments, air, soil, consumer products, groundwater, and the workplace. He has been an adjunct professor at five universities.

He has been an invited peer reviewer for prominent journals and of proposed regulations. Dennis has published approximately 300 peer-reviewed articles and written more than 50 book chapters in the fields of industrial hygiene, human and aquatic toxicology, engineering, and risk assessment. His two textbooks on risk assessment are among the most popular that have ever been published and they have been adopted by a number of universities in various countries.