PITTSBURGH (AP) -- Aluminum manufacturer Alcoa is warning thousands of past and present employees that they may face a greater risk than previously believed of developing lung or bladder cancer.

Recent industry studies, including one by Canadian aluminum producer Alcan, found that exposure to coal tar pitch used in aluminum-smelting may be more likely to cause cancer than previously believed, Alcoa spokesman David Neurohr said Friday.

He said the company does not know how great the cancer risk may be, and he declined to say how many workers have been notified.

Neurohr said Alcoa will reduce the amount of coal tar pitch released in aluminum production at 22 plants worldwide. The company plans to reduce to one-quarter the limit on coal tar pitch releases set by the U.S. Occupational Safety and Health Administration, Neurohr said.

Alcoa will also introduce new safety measures, he said. The company already limits the amount of time workers are allowed to spend in smelting areas and requires employees to wear respirators when working in them. Alcoa also uses ventilation to capture fumes that are released.

New regulations will include increased handwashing and special protective clothing. Employees will also be barred from eating or smoking in work areas for fear that casual hand-to-mouth contact could increase their risk of exposure.

The Pittsburgh-based Alcoa began informing employees about the risk in November, said Mike Wright, director of health, safety and environmental protection for United Steelworkers of America, which represents about 2,000 U.S. employees who work for the company in aluminum production.

``This is quite a good step the company's taking,'' he said.

The union will also ask Kaiser Aluminum Corp. and Reynolds Metals Co., which is merging with Alcoa, to follow a similar course of action, Wright said.

Kaiser aluminum spokesman Scott Lamb said his company was looking into the issue. There was no answer at Reynolds to phone calls placed by The Associated Press after business hours Friday.