The spectral method for quantifying hemoglobin fragility caused by smoking is based on decreased concentration of tryptophan and elevated concentrations of the biomolecules nicotinamide adenine dinucleotide (NADH), flavin adenine dinucleotide (FAD), and porphyrin in the presence of hemoglobin fragility. The method involves the steps of obtaining a blood sample from a patient who is a smoker; separating blood plasma from the samples; obtaining synchronous excitation spectra (SXS) of the blood plasma with a spectrofluorometer at a scan offset of 70 nm and at a scan offset of 10 nm; comparing the patient's SXS with the SXS of normal control samples; and diagnosing hemoglobin fragility when the excitation maxima of NADH, FAD, and porphyrin are between 30% and 70% higher than the maxima for these metabolites in the normal control samples, or when the excitation maximum for tryptophan is 60% of the control sample.