A fire that claimed the life of a 72-year-old Westfield woman this month was caused by the woman smoking while using an oxygen tank, said Jennifer Mieth, a spokeswoman for the state fire marshal. Margaret A. Lunden died on Feb. 2 in the high-rise elderly housing building where she lived. No one else in the building was injured, according to Mieth. Damages were estimated at $50,000. The fire was jointly investigated by members of the Westfield Fire and Police departments, and State Police assigned to the Office of the State Fire Marshal and the Hampden district attorney’s office.