JMIR Publications

ABSTRACT

Background:

Outdoor secondhand smoke (SHS) remains a public health concern, particularly around designated outdoor smoking areas where non-smokers may pass through or linger nearby. While previous studies have quantified outdoor SHS concentrations, fewer have examined the number of people potentially exposed in real-world settings. Estimating exposure opportunity at the population level requires methods that are feasible, scalable, and minimally intrusive.

Objective:

This study aimed to evaluate the feasibility of using passive Wi-Fi packet sensing, calibrated with brief on-site observation, to quantify the number of smokers and passersby within a plausible SHS exposure range at a public outdoor smoking area in Japan.

Methods:

We conducted a formative field study at a designated outdoor smoking area of the Asia Pacific Trade Center (ATC), Osaka, Japan. A passive Wi-Fi packet sensor was installed adjacent to the smoking area, collecting timestamps, anonymized device identifiers (hashed MAC addresses), organizationally unique identifiers (OUIs), and received signal strength indicator (RSSI) values from October 13 to 29, 2023. On October 28, a high-traffic event day, a 30-minute manual count (15:00–15:30) of smokers and passersby was conducted within a 25-m radius to calibrate sensor-derived estimates. Records outside business hours were excluded, and devices transmitting outside business hours were treated as fixed devices and removed. Detected signals were aggregated into presence episodes, screened by dwell time, and classified as likely smokers or passersby using empirically derived RSSI thresholds. Calibration ratios from the observation window were applied to estimate hourly and daily counts during business hours.

Results:

During the 30-minute observation period, 14 smokers and 207 passersby were visually counted within the 25-m radius. On the same day, sensor logs yielded 659 eligible presence episodes during business hours. Applying classification rules and calibration ratios, we estimated that 262 smokers and 3,907 passersby were present within the plausible SHS exposure range over the course of the day. Temporal patterns indicated bimodal peaks in smoker presence and a midday peak in passerby traffic, corresponding to event-related footfall.

Conclusions:

This formative study demonstrates the feasibility of combining passive Wi-Fi packet sensing with brief manual validation to quantify population-level exposure opportunities to outdoor SHS in a real-world setting. The approach offers a low-cost and privacy-preserving method for assessing outdoor SHS exposure and may inform the design, placement, and management of smoking areas in public spaces. Further multi-site studies are warranted to refine exposure estimation and support evidence-based tobacco control strategies.