SEDRO-WOOLLEY, Wash. - Kids, go ahead and dig in the dirt.

That messy message is from a new National Wildlife Federation report which outlines how getting dirty outdoors benefits children - from behavior to health and grades. The timing coincides with Earth Day, with plenty of opportunities for youngsters to get their hands dirty in Washington this weekend.

Meri Margaret Deoudes, who oversees the Federation's "Be Out There" initiative, says it's natural for most children to want to be messy, and there are good reasons to let them dig around - including stress reduction and exposure to bacteria which helps build their immune systems.

"Some of the research is showing that there is something in dirt that actually triggers serotonin, which makes kids more happy, more relaxed. Also, some of the studies are showing that, that helps them perform better in school."

For young children, Deoudes says, there are tactile benefits to playing in the dirt. Encouraging them to make mud pies and create forts also counts as exercise, she says.

Jeff Giesen, the North Cascades Institute's acting executive director, sees the beneficial effects firsthand in North Cascades National Park. When children come to such outdoor programs as "Mountain School," the difference in their attitude is almost instant, he says.

"You can talk a little louder. You can move a little more freely. You're not stuck into your chair. You're not stuck in the classroom. So, you see a big change in kids when they realize that there's a lot of open space for them to explore."

For children and parents, the Washington Department of Ecology is keeping a list of this weekend's Earth Day events around the state at ecy.wa.gov/earthday.

The report, "The Dirt on Dirt: How Getting Dirty Outdoors Benefits Kids," is online at nwf.org.