Background <p>Emergency responders are regularly exposed to potentially traumatic events, placing them at risk of developing posttraumatic stress symptoms (PTSS). Within this group, lifeguards remain an understudied population. The present study examined the building block effect in volunteer emergency responders of the German Lifesaving Association (DLRG), hypothesizing that a higher number of stressful events would be associated with greater PTSS. In addition, potential influencing factors on symptom severity and on the relationship between event exposure and PTSS, including gender, age, perceived social support, and peer support measures, were investigated.</p> Method <p>Data were collected via a web-based self-report survey, resulting in a final analysis sample of 365 lifeguards from the DLRG. Participants were nested within 198 local subgroups. Generalized linear mixed models were constructed, with predictors entered in a stepwise manner.</p> Results <p>The findings support the building block effect of stressful experiences, indicating that greater exposure is associated with higher PTSS among lifeguards in Germany. Hypothesis testing further revealed protective effects of higher age, male gender, greater perceived social support, and stronger peer support measures in reducing symptom severity. Interaction effects of these protective factors on the relationship between the number of traumatic events and PTSS were non-significant. Exploratory analyses also suggested that higher rank within the organization may be associated with lower symptom severity.</p> Conclusion <p>The study highlights the importance of addressing mental health challenges among volunteer lifeguards, acknowledging that PTSS do not necessarily stem from a single worst traumatic event but can develop cumulatively. The findings also indicate target groups for additional training efforts, particularly younger women, and underscore the value of fostering support networks.</p>