Societies widely recognize the marine environment's potential health benefits. Previous single-country studies indicated a correlation between coastal proximity and better self-reported health, attributed to factors like reduced environmental hazards, increased physical activity, and opportunities for psychological restoration. Coastal visit frequency, also linked to improved health through physical activity and stress reduction, typically decreases with distance from the coast. However, a 2017 systematic review highlighted insufficient evidence due to limited single-country studies and inconsistent health outcomes. This study aimed to address these limitations by examining the relationship between coastal proximity and visits with self-reported general health, considering the potential moderating role of income on this relationship across 15 countries (14 European nations and Australia). The study used a Bayesian approach for robust cross-country generalizability testing. The hypotheses were: 1) Living closer to the coast predicts better self-reported health; 2) More frequent coastal visits predict better self-reported health; 3) Coastal proximity mitigates the income-health relationship; 4) Coastal visit frequency mitigates the income-health relationship. The study also explored the generalizability of these hypotheses across countries.

Existing research showed mixed results. Some studies demonstrated a positive correlation between coastal proximity and better health in the UK, Belgium, and Spain, suggesting benefits from reduced air pollution, increased physical activity, and improved psychological well-being. A longitudinal study in England confirmed this association. However, a systematic review in 2017 concluded that the evidence was insufficient due to a small number of single-country studies. The potential for coastal access to buffer income-related health inequalities (equigenesis hypothesis) was also under-researched. While studies on green spaces showed mixed evidence for a buffering effect, some research suggested that coastal proximity might mitigate the relationship between income deprivation and health in England. This study aimed to address these gaps by employing a larger, multi-country dataset and utilizing a Bayesian analytical approach to quantify evidence strength and generalizability.

This study analyzed cross-sectional data from the Seas, Oceans, and Public Health In Europe (SOPHIE) and Australia (SOPHIA) surveys. These surveys collected data from representative samples (stratified by age, sex, and region) across 14 European countries and Australia. The key variables were self-reported general health (outcome), home coastal proximity, coastal visit frequency, and household income (predictor and potential effect modifier). Age and sex were included as potential confounders. A Bayesian multilevel cumulative probit regression analysis was employed, treating respondents as level 1 and countries as level 2. Home coastal proximity and coastal visits were modeled both categorically and monotonically, accounting for unequal intervals between categories. Four models were fitted for Hypotheses 1 and 2, with varying levels of random effects. For Hypotheses 3 and 4, models with fixed and random interactions between income and proximity/visits were compared. Model selection was based on the leave-one-out cross-validation information criterion (LOOIC) and Bayes factors (BF). The BF provided a measure of the relative support for the alternative hypothesis versus the null hypothesis. Generalizability across countries was assessed by comparing credible intervals (CrIs). Sensitivity analyses were conducted to check robustness against variations in priors, inclusion of data from Czechia, exclusion of fast responders, and inclusion of additional covariates (education, work status, and political orientation).

The study found very strong evidence (BF+ = 82.33) supporting Hypothesis 1: Living closer to the coast predicts better self-reported health. The largest improvement in health (37.9%) was observed between those living within 1 km and 1-2 km from the coast. The relationship generalized across countries, although evidence strength varied. Extremely strong evidence (BF+ = ∞) supported Hypothesis 2: More frequent coastal visits predict better self-reported health. Again, this relationship generalized across countries, though the magnitude of the effect varied. Regarding the moderating effect of income (Hypotheses 3 and 4), results contradicted the expected buffering effect. Very strong evidence (BF = 39.68) showed that the income-health relationship was stronger when living closer to the coast, although this effect was very small. Extremely strong evidence (BF01 = 2,348.96) indicated that lower household income was associated with poorer health regardless of visit frequency. Sensitivity analyses confirmed the robustness of the findings. Overall, the coastal proximity-health relationship was very strong and consistent across Europe and Australia, while the coastal visits-health relationship was extremely strong but with varying magnitudes across countries. The buffering effect of coastal contact on the income-health relationship was not supported by the data.

The findings provide strong evidence that coastal proximity and visit frequency are associated with better self-reported health across Europe and Australia. This supports the notion that coastal environments offer valuable health-promoting opportunities. However, the lack of a buffering effect on income-related health inequalities is crucial. The negative interaction between coastal proximity and income suggests that living close to the coast might exacerbate health disparities for low-income individuals. This could be due to poorer quality coastal areas being more affordable, leading to lower-income individuals experiencing less of the positive health benefits. This contrasts with some previous research but aligns with findings showing a negative effect of green spaces in low-income areas. The lack of buffering effect for visit frequency suggests that while coastal visits positively affect health, they don't necessarily reduce health inequalities. Interventions promoting coastal access should focus on equity and ensure accessibility for all income groups.

This study provides strong evidence supporting the positive association between coastal proximity/visits and self-reported health across multiple countries. However, it challenges the assumption that coastal access reduces income-related health inequalities. Policymakers should prioritize equitable access to high-quality coastal environments for all income levels to maximize public health benefits and address health disparities. Future research should examine coastal quality and explore the generalizability of these findings to low-income countries.

The cross-sectional study design limits causal inference. The study's reliance on self-reported data, internet-based surveys (potentially under-sampling low-income individuals), and a lack of coastal quality assessment are limitations. Future research should employ longitudinal designs, objective health measures, and incorporate coastal quality measures, ideally with more representative sampling methods, to strengthen causal inference and broaden generalizability.