One of the best things you can do to help your children grow up to be happy adults is to make sure they get time outside in nature while they're young. A recent study involving kids from four nations found that in addition to spending time in green spaces — from roof gardens to forests — blue spaces like ponds, creeks, rivers, lakes and beaches make a difference, too.

The study looked at nearly 3,600 adults living in cities in Lithuania, the Netherlands, Spain and the United Kingdom. People filled out questionnaires regarding their current and childhood exposure to green and blue spaces, and their current mental health was evaluated using a five-question scale on nervousness and feelings of depression.

Parents who want their children exposed to green and blue spaces should be ready to take a proactive approach. A walk in a park or trip to the beach would be a good start.

Adults who had less exposure to nature as children tended to have poorer mental health. They also attached less importance to nature and natural environments than those who had experienced them more frequently during childhood.

Neither exposure to nature as an adult nor the amount of greenery they lived near as adults affected this, but more exposure to nature during childhood meant less nervousness and depression during the past four weeks as an adult.

The study is based on the recollections of childhood of people who were slightly over 50 years old. While not the most precise type of measurement, it still strongly suggests that the mental health of adults is better among those on whom nature made the greatest impression in childhood. And that means they had exposure to it. Over 80 percent of people in the U.S. live in urban areas, areas where nature must be actively sought out. Pediatricians have been asserting for years that today's children spend too much time indoors. They don't go outside to play often enough, much less to enjoy the green and blue spaces scattered throughout the urban environment.

Children will never appreciate nature if they don’t get to see it. Parents who want their children exposed to green and blue spaces should be ready to take a proactive approach. A walk in a park or trip to the beach would be a good start. Parents shouldn't overlook the fact that time in nature is good for their health, too.

The study authors strongly encourage policymakers to make more natural spaces available to children. For a look at how this was done in Philadelphia, see “Turning Vacant Lots into Mental Health Oases.”

For more details on the four-nation European study, see the article, published in the International Journal of Environmental Research and Public Health.