Nearly 80,000 chemicals are in use in the United States, and yet only a few hundred have been tested for safety, the report notes. It criticizes the nation’s regulatory approach, calling it reactionary rather than precautionary, which means that the government waits for proof of harm before taking action, instead of taking preventive steps when there is uncertainty about a chemical. Regulation is ineffective, the panel says, in part because of inadequate staffing and financing, overly complex rules, weak laws, uneven enforcement and undue industry influence.

The report looks at contaminants from a variety of sources: industry, agriculture, air and water, medical imaging and contaminated military sites. It also considers natural hazards, like radon gas in homes and arsenic in drinking water. The report concludes, “At this time, we do not know how much environmental exposures influence cancer risk.”

Dr. Michael Thun, an epidemiologist from the cancer society, said in an online statement that the report was “unbalanced by its implication that pollution is the major cause of cancer,” and had presented an unproven theory — that environmentally caused cases are grossly underestimated — as if it were a fact.

The cancer society estimates that about 6 percent of all cancers in the United States — 34,000 cases a year — are related to environmental causes (4 percent from occupational exposures, 2 percent from the community or other settings).

Suggesting that the risk is much higher, when there is no proof, may divert attention from things that are much bigger causes of cancer, like smoking, Dr. Thun said in an interview.