Last August, a release of poisonous hydrogen sulfide from a natural gas well in Parkdale, a commercial and residential district just outside the city of Manistee, sent 11 people to the hospital. The accident, which occurred while workers were attempting to plug the well, caused several victims to collapse and left at least one person with serious lung damage.

Neither the Department of Environmental Quality, which regulates oil and gas development, nor the well's owner, Petrostar, Inc., have acknowledged responsibility for the accident. Both the state and the company maintain that not enough gas escaped to cause the injuries victims say they sustained.

This position, they acknowledge, is based on computer models. To date, neither the DEQ nor Petrostar has conducted a thorough public review, or interviewed victims or emergency workers.

An investigation by the Institute has found that there is ample justification for a formal investigation by local and state authorities:

• The Parkdale accident caused the most injuries to the public from an oil field operation in the history of Manistee County.

• The Department of Environmental Quality did not immediately respond to emergency calls. The injuries would have been even worse if a quick-witted employee of Michigan Consolidated Gas Co., who was called to the scene by a business owner, had not urged victims to evacuate and seek emergency assistance.

• The Parkdale accident also followed by just two years a similar release in northern Mason County that caused four children and an elderly woman to be hospitalized.

The Institute's investigation comes as concern about hydrogen sulfide is growing in Manistee County. Several new wells containing high concentrations of the dangerous gas have been drilled in residential neighborhoods in the city of Manistee and in Filer Township (see article on page 20). New pipelines and processing plants that would handle hydrogen sulfide also are planned.

"The issue here is the health and safety of residents," said Sharlene Wild, chairwoman of the Manistee County Board of Commissioners, who has called for a task force to study the health risks from hydrogen sulfide. "The county has the responsibility to protect its citizens. There is much more about this that we don't know, and that we need to know."

Hal Fitch, chief of the Geological Survey Division, (the DEQ unit that regulates oil and gas), said his staff is completing an internal investigation of the accident, but declined to make public its conclusions. When asked whether the DEQ would issue sanctions, Mr. Fitch said the agency is considering "what kind of action is appropriate." He added that the procedures that led to the poison gas release "did not violate any specific rule."

In conducting its research, the Institute interviewed witnesses, state officials, energy industry executives, and independent engineers. The investigation reached the following conclusions:

• Petrostar appears to have broken a key provision of the state air quality law -- Rule 336.901-- that prohibits emission of an air contaminant in quantities that cause "injurious effects to human health or safety." The rule provides for fines of up to $10,000 for each violation, according to a DEQ spokesman.

• Both Petrostar and the DEQ appear to have ignored state-sanctioned guidelines for managing poison gas in Michigan's oil fields. The guidelines, which have been in effect since 1976, note that hydrogen sulfide is lethal at relatively low concentrations, and that care must be taken in handling the gas near populated centers. On the day the accident occurred, stiff winds blowing from the south blew the gas directly toward nearby businesses.

• In contrast to public statements by the DEQ and Petrostar, there was nothing "routine" about the maintenance operation that led to the accident. The Petrostar well was being temporarily plugged, an unusual operation in Michigan, according to state geologists.

• At least 5,500 cubic feet of natural gas saturated with hydrogen sulfide was released, 30 times more than the amount that Petrostar has publicly acknowledged. It is not known how much hydrogen sulfide the employees of the Parkdale businesses were exposed to.

Gregory Blanche, vice president of Petrostar, said his company conducted an internal review and found nothing unusual about the maintenance operation. He said Petrostar does not believe it did anything wrong, but as a courtesy agreed to cover the cost of medical treatment for the victims.

"Based on the volume of gas released, and the distance from the location, and wind dispersion models, the exposure the individuals had to hydrogen sulfide would have been minimal," said Mr. Blanche. "We had two people on location, who were closest to the source and who would have been exposed to the greatest concentration. They experienced no ill effects."