Count up the hours you spend under a roof. Bed, kitchen, the car, the store, the good chair in front of the TV. The Environmental Protection Agency puts it at roughly 90% of your life.

Ninety percent. Indoors.

And here's the part that ought to bug you. That same agency says the air in a house often holds two to five times more junk than the air out on the street. Gas stoves, cleaners, dust, glue in the new rug, whatever that candle is burning.

So you buy a filter. Maybe a spray as well, or one of those plug-ins that smell like pine. You're out $40, $80, more, and you still can't make what one open door makes for free.

Because there's a thing in the air outside that no machine builds. Trees put it there. Your immune cells know the difference...

After the warning about some popular shampoos, one thing's clear — your scalp needs special care.

This 15-second routine helps clear buildup and awaken hair growth naturally.

Thousands say their hair feels thicker and stronger in just weeks.

In 1982, a man named Tomohide Akiyama ran Japan's forestry agency. His country was rich in woods and short on lumber sales. So he coined a phrase — shinrin-yoku. Forest bathing. Go stand in the trees and let the air wash over you.

It was a pitch. Akiyama had no hard proof, just a hunch and a lot of trees to promote.

Then a doctor went and tested it...

Dr. Qing Li works at Nippon Medical School in Tokyo. He took twelve healthy men, ages 35 to 56, on a three-day trip. Two nights away, about two hours of walking each day, blood drawn before and after.

Later he took the same men on the same trip again. Same days, same hours, same walking. This time, though, he ran it through a city. No woods. The air was the only thing he changed.

You carry a cell in your blood called a natural killer cell. Doctors named that one well. Picture a hunter cell: it roams, it inspects, and it takes out cells that have gone wrong. Age wears the hunter cells down, and so does a hard, stressed year.

After the forest trip, those cells were more active — and there were more of them. The city trip moved almost nothing. Best of all, the lift from the woods was still there more than a week later.

Li's team also tested the air itself. They found alpha-pinene and beta-pinene, the two compounds behind the smell of pine. Both sat in the forest air. City air held almost none.

Trees give off oils like these to guard themselves from bugs and rot. Scientists call them phytoncides, from the old words for plant and kill. I'll just call them tree oils from here.

The Environmental Protection Agency, on your house: Americans spend about 90% of their time indoors, where "the concentrations of some pollutants are often 2 to 5 times higher than typical outdoor concentrations."

That's the first thing a careful reader asks. Both trips had the same two hours on foot, so the walk can't be what did it. Li took the doubt one step further and cut out the trees, too.

His team booked twelve men into a city hotel for three nights. A humidifier ran in each room. Into the water went oil from the hinoki cypress, a Japanese tree in the cedar family. There was no trail, no forest, no hike — just a room and a mist of tree oil in it.

The hunter cells climbed again. Two stress chemicals, adrenaline and noradrenaline, dropped in the men's urine.

Hold that loosely. These are small studies, mostly men, mostly one lab in Tokyo. Nobody has run this on thousands of people, and twelve men is twelve men.

Something much larger backs the direction, though. Caoimhe Twohig-Bennett, at the University of East Anglia's Norwich Medical School in England, gathered up 143 studies. They covered more than 290 million people in 20 countries.

People with more green space near them had less cortisol, the stress chemical. Their resting heart rate ran lower. So did the bottom number of their blood pressure. Sleep ran longer. Rates of type 2 diabetes, heart disease, and early death were lower as well.

That's a link, not a promise. Green space brings other things with it — a reason to walk, a neighbor to talk to, some quiet. No one can hand you one clean cause. But when 290 million people lean the same way, it's worth ten minutes of your day.

Now for the food part. Pinene and limonene belong to one big family of plant oils. Pines breathe out pinene by the ton. Rosemary and basil hold it in their leaves, and lemon and lime peel is packed with limonene, which is exactly why the peel smells the way it does.

Crush a sprig of rosemary between your fingers and take a breath. That's a bit of forest, in your hand, off a windowsill.

It's not the same dose, and it's no swap for going outside. The idea is the same, though: a plant makes a compound to guard itself, and you get to borrow it. Eat close to the soil and you keep on borrowing.

Open a window on each side of the house, not just one. A cross-draft swaps the air in a room fast, and it costs you nothing. Morning and bedtime is plenty. Ten minutes in winter won't chill the walls.

Pine, fir, spruce, and cedar give off the most of those oils. Warm, still, damp days hold them best, so the hour after rain is prime time. Wander rather than hike. What you want is time in the air, not miles on a watch.

"Pine fresh" out of a spray bottle is not tree oil, and plenty of fresheners and scented candles add to the load in your rooms. Bin them and crack a window instead. One more thing: check the local air on smoky or hot ozone days. The woods are no help when the sky turns brown.

Akiyama was trying to sell a forest. Of everything in it, he picked the one product nobody can bottle, price, or ship — and that turned out to be the real one.

None of this is a cure. Air won't fix a bad heart or clear an illness. If your breath is short or your chest aches, that's a doctor's job and not a walk's, so keep your appointments and keep taking what you're given.

The free part, though, is yours today. Open the door and stand out there for twenty minutes. Do it again tomorrow, and the day after that. See how you sleep by Sunday.