The city medical examiner ruled yesterday that the October death of 45-year-old Leon Heyward was caused by cancer he got from breathing in toxic dust at Ground Zero. He becomes the first addition to the official total since a Staten Island woman's death was linked to dust inhalation almost two years ago. Heyward worked as an investigator and helped get handicapped co-workers out at the Department of Consumer Affairs Office on Church Street after the planes hit. As the years went on, he began having seizures, became delusional and suffered memory loss. His sister told the News that the ruling is some vindication after Heyward had been denied worker's comp. She told the News, "There were a lot of people that weren't first responders. There are a lot of people walking around sick for the same reason and fighting the same battle we did." The medical examiner has declined to make similar rulings in half a dozen other cases.