In the 19th century, Henry David Thoreau wrote about the problems of modern society, the importance of nature and the restorative benefits of spending time outdoors.

“We need the tonic of wildness,” he writes in Walden, after spending two years in the woods.

Here in the 21st century, an increasing number of health experts agree with Thoreau.

The varied physical and mental health benefits that seem to come with spending time in the woods or other wild and green settings is the subject of an increasing body of study and (some) scientific research.

A good old walk in the woods has been credited with reducing blood pressure, heart rate and anxiety, while improving mental health, cognitive abilities and sleep patterns.

While this data isn’t generally the product of robust, randomized controlled trials, it’s substantive enough to lead some doctors and nature advocacy groups to promote time outdoors as at least a partial solution to many growing health problems.

In the United States, organizations like Walk with a Doc and Park Rx America focus on increasing health and happiness by getting doctors to hand their patients “nature prescriptions” in place of, or at least alongside, drugs and other medical treatments.

The national health service in Scotland has approved something similar, with some doctors there prescribing outdoor activity for their patients. Japan has a long tradition of “forest bathing” and South Korea has established official “healing forests” to help its stressed-out citizens.

Here in Ontario, there’s Mood Walks, a province-wide initiative led by the Canadian Mental Health Association in partnership with others to promote activity in nature as a way to improve physical and mental health.

And there are numerous programs — including this Friday’s free parks day — associated with the worldwide Healthy Parks Healthy People movement.

So on July 19 there’s no admission charge to use any of Ontario’s provincial parks.

Really, then, there’s no better time to prescribe ourselves a healthy boost of nature.

There are hundreds of provincial parks covering 8 per cent of Ontario — that’s larger than the provinces of Nova Scotia and Prince Edward Island combined — so there should be something for everyone.

There are easily accessible provincial parks with gentle, barrier-free marked trails through woods and marshes, as well as the more rugged wilderness of Algonquin, Ontario’s first provincial park.

An afternoon walk in the woods to a weekend car camping trip to a back-country multi-day adventure where the city is a distant memory — they’re all options.

What’s still missing, of course, is better public transit to get out of Ontario’s cities and into its wilderness. There’s nothing like gridlock traffic getting out of Toronto on a Friday to raise one’s blood pressure before even getting to the park to lower it.

That’s starting to change, albeit not quickly or comprehensively enough. Parkbus makes it possible to get from Toronto to a dozen Ontario parks without a car and there’s a free weekend shuttle service to the Rouge National Urban Park, east of the Toronto Zoo in Scarborough.

When it comes to their kids, parents have long known that more activity and less screen time is what the doctor ordered. And increasingly unstructured play, ideally in some dirt to help develop a healthy immune system, is recommended as well.

It’s a prescription that adults need, too.

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The U.S. Environmental Protection Agency is now quantifying the effects of indoor air quality on human health because Americans, on average, spend 93 per cent of their time indoors and in automobiles.

Thoreau, who wrote “we can never have enough nature,” would not be impressed.

It’s time for us all to get outside. And with the heat forecast for Friday, the shade of a forest will be even more welcome.