Urban green space has many well-known health benefits, including longer life expectancy, fewer mental health problems and better cognitive functioning. However, exactly how much green space is needed to improve people’s health remains an open question.

A study led by the Barcelona Institute of Public Health that included 3,145 Barcelona inhabitants ages 15-97 evaluated the relationship between better mental health and the 3-30-300 green space rule. According to this rule of thumb, everyone should be able to see at least three trees from their home, have 30% tree canopy cover in their neighborhood and not live more than 300 meters away from the nearest park or green space.

Mental health status was assessed with the 12-item General Health Questionnaire.

The study showed that full adherence to the 3-30-300 green space rule was clearly associated with better mental health, less medication use and fewer visits to a psychologist, although the association was statistically significant only for the latter. Residential surrounding greenness, but not visibility of trees from windows or access to a major green space, was significantly associated with better mental health.

Only 4.7% of the surveyed population met all three criteria of the green space rule. Just over 43% of respondents had at least three trees within 15 meters of their home, 62.1% had a major green space within 300 meters and 8.7% lived in an area with sufficient surrounding greenness. However, nearly 22.4% had none of these elements.

Eighteen percent of participants reported poor mental health and 8.3% reported having visited a psychologist in the previous year. In addition, 9.4% reported using tranquilizers or sedatives and 8.1% reported having used antidepressants in the previous two days.

"There is an urgent need to provide citizens with more green space," said lead author Mark Nieuwenhuijsen. "We may need to tear out asphalt and plant more trees, which would not only improve health, but also reduce heat island effects and contribute to carbon capture."

To read the article, published in Environmental Research, click here