On October 26, 1994, at 1800 central daylight time, a student pilot returned from a solo cross country in a Cessna 150, N7XC, operated as a 14 CFR Part 91 training flight by Weiss Aviation of Chesterfield, Missouri. The student complained she was nauseous, about to pass out, and had difficulty walking, but drove herself home. Four hours later, after discovering an exhaust muffler crack (the repaired muffler had accumulated 18 hours in service since installation), the operator contacted the student and recommended she obtain medical attention. Medical testing revealed an elevated blood level of Carbon Monoxide (17.5%). After 5 1/2 hours of 100% Oxygen treatment, the CO concentration had reduced to 1.5% and the pilot was released. The pilot stated that she had smelled fumes during the flight, was not taught of a potential hazard or connection between the smell and Carbon Monoxide poisoning, and continued the flight. AC-61-21A, the Federal Aviation Administration Flight Training Handbook, discusses the hazards of Carbon Monoxide poisoning and faulty heater systems as a cause. FAA-S-8081, the Private Pilot Practical Test Standards, requires under "Aeromedical Factors" that an applicant exhibits knowledge of symtoms, effects, and corrective action of carbon monoxide poisoning.