Portland-based

is the nation's top industrial air polluter, when accounting for toxicity and population exposure, according to a

released by researchers at the University of Massachusetts.

Precision, which has plants worldwide making aircraft and engine parts, tops corporate giants such as

(No. 2) and

(No. 3), the study says.

The ranking combines measures of pounds released with toxicity of pollutants and population exposure to yield a score for each company. One-third of Precision's score comes from three Portland-area factories that emit cobalt and cobalt compounds. Acute exposure to high levels of

, a natural element found throughout the environment, results in respiratory effects, such as a significant decrease in ventilatory function, congestion, edema and hemorrhage of the lung, according to the

.

Ranked solely on pounds of pollutants released in 2010, Precision drops low in the list of the nation's top 100 air polluters. But an author of the University of Massachusetts study says the company's high score raises questions that members of the public should ask companies and regulators.

"I would be concerned if I were downwind from the company that comes up at the top of this list," said Michael Ash, a UMass professor of economics and public policy.

Precision managers were reviewing the results Monday and declined to comment. "We are in the process of evaluating the study," said Jay Khetani, Precision vice president of investor relations.

Precision may have climbed in the rankings partly because it has been acquiring many companies, each with industrial plants. Precision placed No. 3 in a revised ranking last year. In 2010, it ranked No. 21.

The company's score may also be high because its plants in other states emit toxic substances such as nickel, chromium and chromium compounds. The EPA emissions inventory, which forms the basis of the ranking, doesn't differentiate between trivalent chromium and the far more toxic

, Ash said.

Ingestion of high amounts of hexavalent chromium causes gastrointestinal effects in humans and animals, including abdominal pain, vomiting, and hemorrhage, according to the EPA. Chronic inhalation or oral exposure to high levels of the substance may produce effects on the human liver, kidney, gastrointestinal and immune systems, and possibly the blood, the EPA says.

Pulmonary fibrosis and renal edema were reported in humans and animals following acute short-term exposure to

compounds, the agency says.

Annually since 1987, the EPA has required U.S. factories to report emissions of chemicals on a list of about 600 substances. The federal agency assesses human health risks by analyzing substances' toxicity, manner of movement and proximity to population centers.

Ash and the co-author of the UMass study, James Boyce, an economics professor, compile their ranking by tracing factory ownership as of 2012. In Precision's case, they found 40 plants nationwide owned by the company.

The study assigns each company a "toxic score" by multiplying pounds of substances released by their toxicity, and then by the size of the population that may be exposed. Precision's score this year is 16,563,535. That's more than double DuPont's score of 7,086,303.

The study found that in all, factories owned by Precision emitted 110,000 pounds of air pollutants. That amount is tiny compared, for example, to DuPont's 10,940,000 pounds of emissions. But multiplication by high toxicity and a large surrounding population could dramatically boost a score.

"I don't think people should panic," Ash said. "They should certainly begin a conversation with Precision Castparts about what's the nature of these releases and what efforts are being taken to limit the releases."

Asked whether he had proof that substances were emitted and people got exposed, Ash said the emissions data come from reports that the companies themselves submit.

"That people were exposed is based on a model," Ash said. "It's a high-quality model, but it's only a model. It's not the same as bio monitoring of exposed persons."

The reported emissions were almost certainly legal and within permits, Ash said. Asked for comment Monday, Oregon Department of Environmental Quality officials said they were still analyzing the rankings.

Precision Castparts, a Fortune 500 company, makes structural investment castings, forged components and airfoil castings for aircraft engines and industrial gas turbines. Almost every aircraft in the sky flies with parts made by the company, whose annual sales exceed $8 billion.

Precision employs more than 28,500 at 151 plants worldwide. The company's share price has soared since 2008. It closed at $216.25 Monday on the New York Stock Exchange, down $1.91 or 0.88 percent.