Background:
While secondhand smoke exposure in outdoor spaces has been investigated before, few studies have used airborne nicotine data to quantify it. Such data could help policymakers and tobacco control advocates gain public en political support for smoke-free outdoor public spaces.

Objectives:
To quantify levels of nicotine exposure by non-smokers in public outdoor settings (café terraces, public transport stops, and building entrances) in The Netherlands.

Methods:
Between May and November 2021, we visited 24 outdoor locations in The Netherlands. We measured levels of airborne nicotine when smokers were present and when they were not. We rated the tobacco smell intensity during each measurement and counted the number of smokers. Airborne nicotine data were collected through active sampling on thermal desorption tubes. The tube contents were later analyzed using gas chromatography-mass spectrometry. Using linear mixed models, we investigated the association between levels of airborne nicotine and 1) the number of smokers and 2) tobacco smell intensity.

Results:
Nicotine levels were higher when smokers were present (B = 1.40, 95% CI [0.69, 2.11], p < .001). For each additional smoker present, we measured higher average nicotine levels (B = 0.23, 95% CI [0.10, 0.37], p = .001). We measured higher airborne nicotine levels when the smell of tobacco smoke was stronger (B= 0.85, 95% CI [0.44, 1.26], p < .001).

Conclusions:
Non-smokers are exposed to higher levels of nicotine in outdoor public spaces if the smell of tobacco smoke is stronger. They are also exposed to higher nicotine levels when more smokers are present. Overall, this study showed that airborne nicotine is useful in quantifying levels of secondhand smoke in various outdoor locations.