Dioxin, the poisonous contaminant found in the military defoliant Agent Orange, is also present in the emissions from coal-burning power plants, diesel engines and municipal incinerators, according to recent scientific studies.

Dow Chemical Co., which conducted a four-month study of such emissions, said that the levels of dioxin they found pose no significant danger to human health, but some scientists say there is evidence that dioxin accumulates in living organisms over a period of time, increasing its concentration.

The concern over the dangers posed by dioxin in power-plant combustion comes at a time when the Carter administration is urging greater reliance on coal.

Spurred by Dow's research and studies by Swedish and Dutch chemists, the federal Environmental Protection Agency has begun a major new testing program to determine how much dioxin is produced by the nation's coal-burning power plants and by municipal incinerators.

Two laboratories, at the University of Nebraska-Lincoln and at Wright State University in Dayton, Ohio, are doing similar studies and checking the level of dioxin in diesel exhaust.

Until now, public attention has focused on dioxin only as a contaminant occurring in some herbicides. Dioxins are a large class of chemicals, but the most infamous and most toxic member of that class is 2, 3, 7, 8 tetrachlorodibenzo-p-dioxin, more commonly known as TCDD, or simply dioxin.

TCDD has been known to kill some laboratory animals at levels of exposure as low as 50 parts per trillion over a period of time, but no one has been able to determine precisely what dosage would be lethal to humans. As a contaminant in the Agent Orange used in Vietnam, its concentration was as high as 40 parts per million. More commonly it was used at a level of less that one part per million, approximately the same concentration found in the domestic herbicide 2, 4, 5-T, which the EPA temporarily banned last spring.

The TCDD found by Dow Chemical in combustion byproducts was at the level of one-to-two parts per trillion -- a concentration that Dow regards as insignificant in terms of dangers to humans. But some scientists say there is evidence that dioxin accumulates in living organisms over time and may thus reach more dangerous levels.

The Dow study showed that the higher the efficiency of combustion, the lower the level of dioxin contamination. Dow researchers said that inefficient burners, such as outmoded trash incinerators in apartment houses, could increase dioxin levels a hundredfold.

Rick Cothern of EPA's Office of Toxic Substances, said: "I think it's obvious that if Dow is correct it could have a substantial effect on our way of thinking about 2, 4, 5-T. But in the scientific world you don't make decisions of this magnitude on one study."

(Cothern said that EPA had some questions about the scientific methodology used by Dow. He said his office will be actually testing for dioxin at different sites where there are coal-fired power plants.

When Dow's study first appeared last year, the EPA, independent chemists and environmentalists were skeptical of the company's motives for the combustion research, which some saw as a self-serving attempt to divert public attention from TCDD in herbicides.

At the time, Dow already has embroiled in a class-action lawsuit brought by Vietnam veterans who claim they suffered a variety of problems as a result of exposure to Agent Orange. Dow was the major producer of the defoliant. A Dow spokesman said, however, that the company's dioxin research was prompted by a desire to prove that dioxin found in fish near a Dow herbicide plant came from emissions rather than the manufacture of the herbicide.

Cothern said that one aim of the federal testing program would be to determine which types of combustion would produce more contamination, which fuels or trash would be more likely to produce dioxins, and what ranges of contamination are produced nationally.