Designing recruitment strategies for population-based human biomonitoring: evidence on participation and representativeness from Latvia

BACKGROUND: Human biomonitoring (HBM) is an important public health tool for assessing population exposure to environmental chemicals. However, national HBM programmes frequently face methodological and operational challenges in recruiting and retaining representative population samples, particularly when relying on voluntary participation and biological sample collection. Evidence describing the practical implementation of recruitment and communication strategies in real-world HBMsettings remains limited, especially in smaller countries with constrained resources. METHODS: The HBM4LV programme in Latvia implemented a voluntary self-enrolment approach combined with stratified participant selection and multi-channel communication. Members of the public registered via an online questionnaire capturing key sociodemographic characteristics. Between October 2024 and November 2025, 1,891 individuals registered, of whom 1,675 met the inclusion criteria. A total of 590 participants were selected to achieve balance across sex, age, residence type and urbanisation strata, and 404 completed all study procedures. Recruitment and retention strategies included personalised email invitations, automated reminders, telephone follow-ups and optional SMS/WhatsApp communication. Operational processes, communication records, participant feedback surveys and expert interviews were systematically analysed to evaluate recruitment efficiency, participant engagement and implementation challenges. RESULTS: Recruitment required substantial operational effort due to participant attrition and unequal representation across demographic groups. Nearly half of the participants required at least one reminder, and repeated follow-up was frequently necessary to support questionnaire completion and biological sample submission. Recruitment gaps were particularly evident among younger participants and some residential groups, requiring targeted outreach and replacement using comparable strata. Participant feedback highlighted time burden, complexity of study procedures and sample submission logistics as the main challenges, while expert interviews emphasised the considerable workload associated with participant communication and coordination. Despite these challenges, biological sample collection and shipment logistics were implemented successfully, with only a small proportion of pre-analytical issues requiring corrective action. CONCLUSIONS: The HBM4LV study demonstrates that voluntary self-enrolment combined with stratified participant selection can provide a feasible approach for implementing population-based HBM studies in resource-constrained settings; however, substantial operational effort was required to address recruitment gaps, participant attrition and selection bias. The findings highlight the importance of structured communication systems, repeated personalised follow-up and flexible logistics for maintaining participant engagement and data completeness. The study also illustrates methodological challenges related to representativeness, digital recruitment and participation burden that should be carefully considered when designing future national HBM programmes.