New York City’s chief medical examiner, Dr. Charles S. Hirsch, has for the first time directly linked a death to exposure to dust from the destruction of the World Trade Center.

In a letter made public yesterday, Dr. Hirsch said that he was certain “beyond a reasonable doubt” that dust from the twin towers contributed to the death of Felicia Dunn-Jones, 42, a civil rights lawyer who was engulfed on Sept. 11 as she ran from her office a block away from the trade center.

She later developed a serious cough and had trouble breathing, and she died five months after the terrorist attack.

Dr. Hirsch said he had decided to amend Mrs. Dunn-Jones’s death certificate to indicate that exposure to trade center dust “was contributory to her death.” The manner of death will be changed from natural causes to homicide.