The modern environment is being polluted by an ever-expanding number of chemical hazards that make ordinary living increasingly dangerous in many areas, the smog-filled air is unfit to breath; the water and its marine life are polluted; the food supply is adulterated. Often it is caused by industrial indifference or negligence, but sometimes food or drugs can become contaminated by a tragic accident.

We have learned of a major mishap involving one of the country's most widely used products. Large amounts of Ex-Lax, the popular laxative, have been secretly withheld from the market because they have been contaminated by the toxic chemical PBB. Federal investigators suspect that a number of other food and drug products also may have been contaminated.

The Food and Drug Administration has lauched a confidential investigation into the contamination threat, and some of th companies involved may be referred by criminal prosecution, according to informed sources. Meanwhile, inspectors are analyzing other product samples for possible contamination.

The amount of PBB discovered so far does not represent a serious public health problem, our sources maintain. But investogators believe it is likely that the highly toxic chemical has spread to other foods and drugs.

The problem first surfaced in June, when Environment Protection Agency officials in New Jersey and New York discovered PBB in soil, water, fish and even human hair samples from barber shop floors. This was the first such discovery outside of Michigan were accidentally mixed with cattle feed, contaminating much of the state's meat and dairy products.

The discovery prompted Rep. Andrew Maguire (D-N.J.) to ask the Food and Drug Administration to inspect the plants that were processing PBB in his state. The inspectors found that Fluidized Processing, Inc., of Newark was processing drug products through the same equipment it used to produce the flame retardant PBB.

The inspectors examined three batches of a chemical the company was processing for the Ex-Lax Company. No trace of PBB was found in the samples before they were processed, but all three samples contained PBB after being run through the machinery.

The inspectors recommended that the three batches, which amount to about 18,000 pounds, be seized by the voluntarily withheld by Ex-Lax, our sources say.

But the president of Ex-Lax, Robert Howell, denied to our associate Howie Kurtz that PBB has been found in any of his products. "I've heard of no batches being contaminated," he said. "It sounds like a wild rumor to me."

Yet inside sources report that FDA is working closely with his company. For example, FDA has given Ex-Lax a method to determine whether any finished batches of laxatives, processed by Fluidized, contain PBB. And the general manager of Fluidized, Hank Jacobowski told us that Ex-Lax officials have visited his plant to examine the data.

FDA inspectors informed Jacobowski, he confirmed, that PBB has contaminated the material used in laxatives. He insisted, however, that PBB "was never run on the same day as other products. Any contamination is almost improbable. We have taken all precautions for this reason." He added that state health inspectors failed to find any contamination.

But the FDA finds it unacceptable to use the same machinery for both products because residues of PBB can be left behind on the equipment. The agency will recommend criminal prosecution of Fluidized and the drug companies involved if it determines that the firms were negligent.

An FDA spokesman refused to comment on the case because of the ongoing investigation. But our sources say a decision on whether to seize other foods and drugs processed at Fluidized is awaiting the outcome of further tests. The agency is also considering safeguards to prevent such contamination in the future.

Meanwhile, hundreds of Michigan residents of Michigan residents found to have PBB in their blood have suffered from headaches, fatigue, sleep disorders and other ailments.