PURPOSE: To evaluate the effect of high levels of environmental air pollution on tear osmolarity and its possible correlation with clinical signs and symptoms. METHODS: This was a panel study involving 71 taxi drivers and traffic controllers from São Paulo, Brazil. Mean individual levels of 24-hour exposure to nitrogen dioxide (NO2) and particulate matter smaller than 2.5 µm (PM2.5) were assessed on 4 different occasions. On the first and third visits, subjects were submitted to clinical evaluations including the administration of the Ocular Surface Disease Index questionnaire, slit-lamp examination, estimation of tear breakup time (BUT), the Schirmer test, and vital staining of the cornea and conjunctiva. On the second and fourth visits, tear samples were collected for osmolarity assays. Statistical analysis was performed using generalized estimating equations. RESULTS: Although the taxi drivers and traffic controllers in our sample were exposed to high levels of NO2 and PM2.5, few symptoms were reported on the Ocular Surface Disease Index questionnaire. BUT values were reduced, whereas vital staining and Schirmer test mean results were within normal limits, despite considerable variability. A significant and negative correlation was found between PM2.5 levels and tear film osmolarity levels (P < 0.05). An increase of 10 µg/m(3) in PM2.5 was associated with a 10.9 mOsm/kg decrease in tear osmolarity. There also was a negative correlation, although not statistically significant, between NO2 and tear osmolarity. CONCLUSIONS: Exposure to air pollution reduces tear film stability and influences tear film osmolarity. Combining clinical examination with the assessment of tear osmolarity may help understand ocular surface response to high levels of air pollution.