Residential greenspace might modify the effect of road traffic noise exposure on general mental health in students

Traffic noise has been linked to mental ill-health but little is known about the impact of residential greenspace on this relationship. In the present study, we investigated whether residential greenspace modified the effect of road traffic noise on general mental health in students. We collected cross-sectional data from 399 participants in the city of Plovdiv, Bulgaria. Road traffic noise (L-den) level was calculated from the strategic noise map of the city. We tested several objective greenspace measures - the Normalized Difference Vegetation Index (NDVI), tree cover density, and Euclidean distance to the nearest green space. Mental health was measured with the General Health Questionnaire (GHQ-12). We conducted moderation analysis and used the Johnson-Neyman (J-N) procedure to identify the values along the continuous moderators (i.e., NDVI (500-m), tree cover (500-m), distance to green space), at which the linear relationship between L-den and GHQ-12 transitioned from statistically significant to non-significant. Results indicated that living in a neighborhood deprived of trees (< 5.84%) enhanced the negative effect of noise, whereas in neighborhoods with higher tree cover density noise had no effect. NDVI and distance to green space showed a similar overall trend, but failed to reach formal statistical significance. Living in a less green neighborhood may enhance the negative effect of road traffic noise on mental health. This observed effect modification may not only be due to disrupted propagation of sound waves, but also to higher recreational quality in greener environment.