The plume of finely ground sand rises in the chill New Jersey air like steam from a locomotive.

Elsewhere in the aged Unisil Corp. plant, an industrial contraption that would have delighted Rube Goldberg, the white powder coats surfaces thicker than soot on a grimy city windowsill. It is so fine that nose hairs don't catch it. It doesn't get held up where it can be coughed or spat out. The flour settles in the lungs.

When a truck is loaded at Unisil's sand-processing plant here, clouds of the flour drift up from the intake hatches, escaping as fine ashes do from a stirred fire.

The dust causes silicosis, and some of the workers at this small plant built on the sand flats of southern New Jersey are worried. Worried enough to talk to a reporter about their fears, but even more fearful that they may be identified by their superiors and fired.

Unisil is a tiny dot on the map of industrial America, and there is no hard scientific evidence that the plant is hazardous to its workers' health. Yet, some of the men say they have silicosis and they are torn between concern for their health and their continued livelihood.

No federal or state inspector has sounded an alarm about the Unisil plant, a fact that William J. Woods Jr., president of the parent corporation, Unimin, is quick to point out.

"I don't think we have a silicosis problem." Woods said by telephone from his corporate headquarters in New Canaan, Conn.

Woods acknowledges that a dust collector atop the silica flour storage bin is broken. But he stressed, "We got on it as soon as we could."

Some of the Unisil workers say the collector has been broken for three years. Others think it has been more like two years.

"That's not my understanding," Woods said. Asked how long the collector has been broken, he admitted he was not sure.

Workers said that when the collector was operational its suction controlled the otherwise free-flowing dust during truck loadings.

Some also remember that workers' complaints were greeted by management "advice" that they learn to live with the broken collector.

Most people think of silicosis as a disease of the 1930s and that in 1979 Americans won't work where they know they run a unnecessary risk of disease. The small plant here illustrates how wrong a lot of people are.

What outrages doctors who specialize in silicosis is that it is an entirely preventable disease.

It also was perhaps the first identified occupational disease, according to Dr. Hans Weill of the Tulane Medical Center. Tombstone cutters have suffered from it. So have sandblasters. Weill quotes a 1934 graffiti from Britain: "Join the navy and see the world. Become a slandblaster and see the next."

The most infamous case of worker death from silicosis occurred in 1930 when a Union Carbide subsidiary, the New Kanawha Power Co., began digging a three-mile water tunnel through a mountain at Gauley Bridge, W. Va.

The pay was good and times were hard, but what the workers weren't told was that the rock they were drilling was close to pure silica and that silica dust kills. Half of the Gauley Bridge workers may have died from silicosis, in what became a national scandal.

The Unisil plant is old and a few of its workers have been with it since the 1940s, working in the various sheds sorting and washing sand the company sells to the glass industry. The coarser grains, less suitable for glassmaking, are ground into silica flour.

Every new worker at Unisil is required to promise in writing that he will not sue the company if he contracts silicosis. The men are told to wear small respirators while at the plant and are given chest X-rays by the company every six months.

Doctors say the respirators are close to worthless, because they get clogged and are uncomfortable and workers take them off from time to time.

Woods said that when Unimin bought Unisil in 1972, the company found only a handful of workers with potential lung problems and immediately transferred them to jobs in dust-free parts of the plant.

The Mine Safety and Health Administration (MSHA) inspects Unisil regularly. A. J. Toscano is the most recent inspector to look at the plant. rToscano, however, would not discuss Unisil with the Post because of a MSHA policy that prohibits inspectors from talking to reporters.

An MSHA spokesman in Washington said Unimin, the parent company, has a good reputation and has been quick to address problems in its plant here or its other three sand-processing factories.

Since a reporter began inquiring about Unisil's faulty dust collector, repairs have begun. Woods said he expects the device to be operating soon, and that at least some of the workers' anxieties then will ease.

Silicosis generally requires several years of exposure to dangerous dust. Once it is contracted, however, the disease progresses in an irreversible fashion, even if a worker's exposure to dust ceases.

Sand is an unkind material. Plants that process it are always breaking down.

"The sand eats everything up. It tears the machines apart pretty quickly. I guess it can eat up people, too," one worker said.