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Background: Retrospective questionnaires are commonly used to assess exposure to unpleasant environmental stimuli, suchas sounds and odors, but they are vulnerable to recall and salience biases. Ecological momentary assessment (EMA) is oftenproposed as a superior alternative because it captures experiences close in time to their occurrence. However, self-initiated,event-contingent EMA protocols require sustained participant engagement, and little is known about their feasibility, reportingcompleteness, and implementation challenges under naturalistic conditions.

Objective: This study aims to evaluate a self-initiated smartphone-based EMA protocol for documenting unpleasant soundand odor exposures and to examine the implications of observed EMA use, the time between the first and last report, andreporting completeness for interpreting data obtained using the self-initiated EMA.

Methods: Participants completed a 3-month study using a self-initiated, event-contingent smartphone-based EMA protocolalongside baseline and follow-up retrospective questionnaires. Individuals with and without self-reported environmentalsensitivity to sounds or odors were offered access to a web-based EMA app for self-initiated reporting of exposure events.Reporting behavior measures included observed EMA use, the number of submitted reports, the time between the first andlast submitted reports, reporting completeness (self-reported missed events), and barriers to app use. Analyses were primarilydescriptive.

Results: Of 104 participants (environmentally sensitive [ES], n=42; nonenvironmentally sensitive [NES], n=62), EMAreporting was limited, with 15 (36%) ES participants and 22 (35%) NES participants submitting at least one report (χ²1=0.00,P=.98). Report submission was concentrated early in the study period, and no participant submitted reports across the full3-month observation period. Among EMA users, the mean reporting duration (time between the first and last submittedreports) was 19.9 (SD 17.6, range: 1‐51) days in the ES group and 7.9 (SD 9.5, range: 1‐32) days in the NES group. Missedexposure reports were common: 14 of 15 (93%) ES users and 14 of 22 (64%) NES users reported experiencing one or moreexposure events that were not recorded using EMA. In addition, many nonusers reported unrecorded exposure events duringthe study period (ES: 11/27, 41%; NES: 5/40, 13%).

Conclusions: Under naturalistic conditions, this 3-month self-initiated, event-contingent EMA protocol demonstrated limitedobserved use, declining report submissions over time, and substantial self-reported incompleteness. These findings arise from1 specific implementation and should not be generalized to self-initiated EMA protocols using different sampling procedures,onboarding, participant support, reminders, incentives, or technologies. Nevertheless, they highlight the importance ofreporting implementation characteristics when interpreting data from self-initiated, event-contingent EMA studies.