S-sulfonate levels were measured in the nasal lavage (NAL) fluid of humans exposed to sulfur dioxide as a potential biological marker of exposure. These levels were determined by treating NAL fluid protein with cyanide to cleave the S-S linkage and release the sulfite. The cyanolytically released sulfite was measured by ion chromatography. In two experiments, humans were exposed to air or 1 ppm SO2 for 10 minute, and to air or 7 ppm SO2 for 20 minutes and lavaged immediately after exposure. Releasable sulfite levels in NAL fluid were 1.06 +/- 0.24 and 2.61 +/- 0.55 micrograms SO=3/mg protein, respectively (mean +/- SE, n = 5), for the first experiment, and 1.16 +/- 0.37 and 4.91 +/- 0.76 micrograms SO=3/mg protein, respectively (mean +/- SE, n = 8), for the second. The subjects in the former study were persons with asthma. In both experiments, S-sulfonate levels were statistically elevated in the exposed group compared with the control groups (p < 0.05, paired t-test). The same individuals in the second experiment received five additional 20-minute exposures to 7 ppm SO2 every other day, for a total of six exposures. NAL fluid taken at the conclusion of the final exposure had releasable sulfite levels of 4.99 +/- 1.36 micrograms SO=3/mg protein; these levels were statistically elevated relative to controls but were not elevated relative to the 1-day exposure (mean +/- SE, n = 8). The lack of accumulation of S-sulfonates after 6 days of short-term exposure suggests clearance of these compounds from the nasal passages within 24 hours. The levels of S-sulfonates observed in NAL fluid in this study are almost three orders of magnitude higher than those measured in plasma following similar SO2 exposures. Measurement of S-sulfonates in the nasal passage may be an effective short-term biomarker of exposure to SO2.