The aim of this study was to assess the relationship between particulate matter of diameter≤2.5µm (PM2.5) and airborne nicotine concentration as markers of second-hand smoke exposure with respect to the setting studied, the intensity of exposure, and the type of environment studied (indoors or outdoors). Data are derived from two independent studies that simultaneously measured PM2.5 and nicotine concentrations in the air as airborne markers of second-hand smoke exposure in public places and workplaces, including health care centres, bars, public administration offices, educational centres, and transportation. We obtained 213 simultaneous measures of airborne nicotine and PM2.5. Nicotine in the air was measured with active samplers containing a sodium bisulphate-treated filter that was analysed by gas chromatography/mass spectrometry. PM2.5 was measured with a SidePak AM510 Personal Aerosol Monitor. We calculated Spearman's rank correlation coefficient and its 95% confidence intervals (95% CI) between both measures for overall data and stratified by setting, type of environment (indoors/outdoors), and intensity of second-hand smoke exposure (low/high, according to the global median nicotine concentration). We also fitted generalized regression models to further explore these relationships. The median airborne nicotine concentration was 1.36µg/m(3), and the median PM2.5 concentration was 32.13µg/m(3). The overall correlation between both markers was high (Spearman's rank correlation coefficient=0.709; 95% CI: 0.635-0.770). Correlations were higher indoors (Spearman's rank correlation coefficient=0.739; 95% CI: 0.666-0.798) and in environments with high second-hand smoke exposure (Spearman's rank correlation coefficient=0.733; 95% CI: 0.631-0.810). The multivariate analysis adjusted for type of environment and intensity of second-hand smoke exposure confirmed a strong relationship (7.1% increase in geometric mean PM2.5 concentration per µg/m(3) nicotine concentration), but only in indoor environments in a stratified analysis (6.7% increase; 95% CI: 4.3-9.1%). Although the overall correlation between airborne nicotine and PM2.5 is high, there is some variability regarding the type of environment and the intensity of second-hand smoke exposure. In the absence of other sources of combustion, air nicotine and PM2.5 measures can be used indoors, while PM2.5 should be used outdoors with caution.