The head of the largest program tracking the health of World Trade Center site workers said several have developed rare blood cell cancers, raising fears that cancer will become a "third wave" of illnesses among those exposed to toxic dust after Sept. 11.

Dr. Robin Herbert, co-director of the World Trade Center Medical Monitoring Program at Mount Sinai Medical Center, said researchers who have screened 20,000 of the estimated 40,000 ground zero workers are "most concerned" about lymphatic and blood cancer cases.