In 1985, David Steinman was writing for the LA Weekly when he learned that fish in the Santa Monica Bay were tainted with DDT and PCBs. He began to wonder how many poisons were in other foods he ate. He started doing research into the levels of industrial pollutants and pesticides in human blood and published his team’s findings in the Journal of Environmental Toxicology and Chemistry. The resulting human blood study led to the Heal the Bay movement that carried out a cleanup of Santa Monica Bay.

As a journalist, Steinman won awards for his reporting from the California Newspaper Publishers Association, Sierra Club, and the Society of Professional Journalists (“Best of the West: Environment and Natural Resources Reporting”).

In 1986, Steinman testified before the Congressional Subcommittee on Health and the Environment as an expert witness on the levels of chemical contaminants in the blood of fishermen and women eating locally caught fish from the Santa Monica Bay. From 1989 to 1991, Steinman served as a member of the Committee on Evaluation of Safety of Fishery Products for the National Academy of Sciences’ Institute of Medicine. Steinman was also a contributing author of Seafood Safety (National Academy Press, 1991).

Steinman’s contributions to Seafood Safety led to his controversial book Diet for a Poisoned Planet (Crown 1990, Ballantine 1992; Thunder’s Mouth Press 2007). Steinman made hundreds of requests under the Freedom of Information Act for government studies on food safety and reviewed tens of thousands of chemical analyses on food. In Diet, he identified foods with the lowest and highest toxicity levels by classifying them as green-, yellow- or red-light foods. The book recommended the Purification Rundown, a "detoxification" regimen created by L. Ron Hubbard as part of the scripture of Scientology. It was criticized by sectors of the food industry as well as by the U.S. Food and Drug Administration (FDA) and by C. Everett Koop, the former Surgeon General of the United States. The California Raisin Advisory Board spent over $500,000 on a PR campaign to directly counteract Steinman’s warnings about industrial chemical and pesticide residues in raisins.

In 1995, Steinman wrote Safe Shopper’s Bible: A Consumer’s Guide to Nontoxic Household Products, Cosmetics and Food (Macmillan) with a foreword by Ralph Nader, and which included a comprehensive review of products from pet supplies to carpeting to auto products. He held a press conference with Ralph Nader challenging 12 manufacturers and distributors of the most dangerous products, calling them “the Dirty Dozen” in an effort to force manufacturers to reformulate their products or pull them from the market. The following year, Steinman wrote Living Healthy in a Toxic World: Simple Steps to Protect You and Your Family from Everyday Chemicals, Poisons, and Pollution (Perigree, 1996) with foreword by TV actress and Scientologist Kirstie Alley.

Since 1996, Steinman has been an advisory board member for The Green Guide Institute, a national non-profit, organization for consumer research and information run by Wendy Gordon Rockefeller. In 2000, Steinman served as Chairman of Citizens for Health, a national nonprofit consumer advocacy group known as the voice of the natural health consumer. He has also been featured as a consumer health advocate in the media on TV, radio and in the press.

On February 8, 2007, Steinman led a press conference at the National Press Club in Washington, D.C. regarding the presence of the undisclosed carcinogenic petrochemical 1,4-dioxane in children's and adult's bath and beauty products. Held jointly with the Campaign for Safe Cosmetics and with participation from the Environmental Working Group, the conference highlighted a range of products including name-brand baby shampoos and bubble baths that were found in lab tests to have the carcinogenic petrochemical in significant amounts, although it was never included in product labeling. The press conference also called for official FDA oversight of the cosmetics and personal care products industry, which is currently self-regulated and subject only to suggestions from the FDA.

On March 14, 2008, Steinman led a similar press conference with the Organic Consumers Association at the Natural Products Expo in Anaheim, CA regarding the presence of 1,4-dioxane in "organic" and "natural" personal care products. This press conference led to the California Attorney General suing four companies with 1,4-dioxane in their cleaning and/or personal care products in June, 2008. On March 6, 2009, Steinman and the Organic Consumers Association announced at the Natural Products Expo that 16 major brands had cleaned up 1,4-dioxane from 23 of their products, and that they would bring legal action against several brands whose products still had high amounts of the chemical.

Steinman also wrote the introduction to 50 Simple Steps to Save the Earth from Global Warming by the Green Patriot Working Group (Freedom Press, 2008).

In 1998, Steinman founded Freedom Press, a publishing house for books and magazines on health and the environment. In the same year he founded the science-based health magazine The Doctors’ Prescription for Healthy Living.

In January 2008, Steinman began to host Green Patriot Radio with David Steinman, a weekly one-hour Internet radio show on www.webtalkradio.net. The show features interviews with authors, politicians, journalists, scientists, corporate executives and other experts on a variety of environmental issues.