NATIONAL BRIEFING / NEW YORK

Researchers say a small number of young law enforcement officers who participated in the World Trade Center rescue and cleanup operation have developed an immune system cancer.

The numbers are tiny, and experts don’t know whether there is any link between the illnesses and toxins released during the disaster.

But doctors who coordinated the study, published today in the Journal of Occupational and Environmental Medicine, said people who worked at the site should continue to have their health monitored.

The researchers looked at 28,252 emergency responders who spent time amid ground zero dust and found eight cases of multiple myeloma. Normally, researchers would expect to find about seven cases in a group as large as the one examined in the study.

But four of the people who fell ill were under age 45; under normal circumstances, researchers would have expected to find only one case of the disease in that age group.