Every foreigner in Japan learns one thing pretty quickly: This being the land of harmony, courtesy trumps candor. Hanging back works best, everywhere and every time.

Imogen M. Greene, though, has little time for such niceties.

That's because Greene -- an American who has lived in a leafy suburb outside Tokyo for years -- is on a mission to rid the world of pollution. He does it by bluntly telling drivers, one by one, to stop leaving their cars idling and polluting the air with exhaust fumes. To date, he estimates he has confronted an average of two drivers a day, over eight years, for a total of about 5,000 people.

"I've got a movement going," said Greene. "This is serious sh*t. This is something I believe in."

However, some things are best left unsaid about Greene, who worries that his campaign may start a brouhaha. As you might have guessed, his real name is one of them, and I.M. Greene is just a corny pseudonym.

It's not that this sturdy, middle-aged man is in any way threatening, and on topics such as food or sports his voice booms in the friendliest of ways. But when it comes to car exhausts, well, what with so many scientists blaming such carbon dioxide emissions for global warming, melting glaciers and numerous extreme weather events -- not to mention the possibility of mass extinctions of species (Homo sapiens maybe included) -- Greene doesn't mind ruffling a few feathers.

To see just how Greene goes about changing the world, this writer donned stout shoes and tagged along with him as he combined business and pleasure in search of stationary quarry on one of his regular country hikes.

No sooner had we set off on our safari than we spotted a white Toyota idling in a parking lot. Greene circled slowly at a distance to confirm whether the man inside was indeed willfully idling or just stopping briefly to attend to something. When it became clear that the man was indeed idling with no intent, Greene closed in, tapped the window and said: "Sumimasen, idoringu kinshi (Excuse me, idling forbidden)!"

The man nodded and switched off the ignition. No drama there, so with half the day's quota met, we moved on.

"That's usually all I say," said Greene. "Sometimes they'll look at me like, 'What the hell is he doing here? Get outta here!' "

Critics could argue that Greene has some nerve, considering that his (and my) native United States is the world's largest emitter of greenhouse gases. So I asked him half-jokingly, "What gives you the idea that you have the right to go tell people to turn their cars off?"

Not a national issue

Greene shot me a look. "What kind of a question is that? It's because I'm an earthling, and I have the right to clean air. We all do. It's not a national issue. Pollution here affects every country."

Is Greene an idealist? Maybe. But the scientific facts bear him out. Vehicle exhaust contains not only the greenhouse gases carbon dioxide and nitrous oxide, but other airborne nasties such as carbon monoxide and sulfur dioxide. Of the latter two, the U.S. Environmental Protection Agency says carbon monoxide exacerbates cardiovascular disease in humans and has cited automobiles as a main culprit.

As for sulfur dioxide, the EPA links it with "respiratory illness, alterations in pulmonary defenses, and aggravation of existing cardiovascular disease."

In Japan, vehicle exhaust accounts for a fifth of carbon-dioxide emissions, and despite its lofty Kyoto Protocol reduction targets, the country ranks fourth globally for carbon dioxide emissions -- behind the United States, China and Russia -- and experts say vehicle-related output is rising. Yet, as Greene is not slow to point out, Japan has no national law banning idling, and the local ordinances doing so lack teeth. Hence Greene's one-man movement.

So, as we trotted up a dirt path, I was treated to another dose of Greene philosophy.

"We Americans and Japanese, we live in a world where we have everything we could ever wish for. Why do we constantly need to be taking it to the absolute limit? Whatever happened to the idea that a car is something that gets people and goods from point A to point B?

"I mean, it seems like these people, they're all parked somewhere just sleeping in their cars. What are they doing? Don't these people have homes?"

Running after Greene while taking notes was no easy task, as he walked at the double along paths that would put a mountain goat to the test. Then suddenly, as we neared the end of our hike, the import of what Greene was saying hit me smack in the forehead -- almost literally.

For across our path lay dozens of giant trees that Greene said had toppled in a recent string of powerful typhoons. Though many experts believe it is still too early to directly link global warming with recent extreme weather events worldwide, that view appears to be gaining currency. Indeed, a report from the Association of British Insurers in June 2005 suggested that if no action is taken, climate change could send the cost of such events soaring in the decades ahead.

We had more immediate obstacles to contend with. After scrambling under the fallen giants, Greene and I finally arrived at the foot of the mountain -- where we found workmen standing by an idling truck.

With the exhaust from diesel fuel, used mostly in trucks, widely identified as a likely carcinogen, it was -- naturally -- time for Greene to intervene. He told a workman to shut off the engine. This time, though, he encountered resistance as the man launched into a long-winded explanation for why the engine absolutely needed to stay on. Communications bogged down. Greene let it pass, and the two parted amicably.

Violent encounters

Things seemed to be going almost too smoothly. I asked him if he'd ever had any violent encounters. Some people, he said, had shoved sharp objects in his direction, or threatened to throw things, but otherwise no . . . "Oh, oh, oh, oh! I remember one!" he suddenly blurted out. "There was a truck driver . . . " Turns out that several years ago, after Greene had been asking a fiftysomething driver over several weeks to stop idling, the man lost his temper, grabbed a golf club and ran after him down the street. Greene got away.

"Then I went back and continued to ask him, and he grabbed a screwdriver or a knife -- I couldn't tell which -- and chased me around the block before going back to his truck. When he got it moving, he charged toward me, then veered away and took off down the road."

As we were wrapping up, Greene seemed intent on erasing the impression of some kind of eco-grouch. "I'm a people person!" he insisted. "I don't mean to insult anybody."

That made him chuckle . . . and then he added: "Well, I do -- but I don't. It's like: 'Wake up, people.' "