Potentially dangerous levels of dioxin are being released into Michigan's Tittabawassee River in waste water from a Dow Chemical Co. plant, the Environmental Protection Agency concluded in a preliminary report released yesterday.

"This is going to become the ticking bomb for human beings if it is accumulated over years and years," EPA Region 5 Administrator Valdas Adamkus told a news conference.

The EPA study, concluded in September, 1981, showed more than 40 toxic chemicals, including the most dangerous form of dioxin, are being released into the river by the Midland, Mich., plant. Of particular concern is the fact that dangerous levels of dioxin were found in caged fish placed in the river below the Dow waste water outfall.

"The key is that dioxin is being bio-accumulated by the fish to the levels that cause us concern," said Dale Bryson, deputy director of the Region 5 water division.

The announcement follows recent charges by Adamkus and scientists in EPA's Chicago office, who said they were strongly pressured by high-ranking EPA officials in 1981 to change an earlier draft study on dioxin contamination in Michigan that had blamed Dow Chemical.

Dow official Sarah Rowley said the company has not had an opportunity to study the latest EPA report and was withholding comment. Dow previously has denied discharging dioxin.