Firefighters sprayed foam and water on burning oil tanks Friday while environmentalists warned that pollutants whipped into the air from the four-day infernoposed a potential health hazard.

"Air pollution is a serious problem," said Dr. Richard de Dear, noting smoke and soot levels threatening the population of 2.6 million were mounting.Meteorologists predicted rain mixed with more grime and soot from the huge funnels of black smoke churning out of the three naphtha tanks.

Three firemen were hospitalized after a high-pressure hose disconnected, swung around and knocked them off a platform 6 feet above the ground. A senior fire official said 20 others have been treated for minor injuries since the battle to extinguish Singapore's worst offshore blaze started Tuesday.

"The latest injuries to the three firemen have been the most severe so far," he said, noting they received head and leg injuries.