KABUL, Afghanistan » It’s long been a given that the air pollution in this city gets horrific: on average even worse than Beijing’s infamous haze, by one measure.

For nearly as long, there has been the widespread belief by foreign troops and officials here that — let’s be blunt here — feces are a part of the problem.

Canadian soldiers were even warned about it in predeployment briefings, which cited reports that one test had found that as many as 30 percent of air samples contained fecal particles. The Canadians were worried enough that the government ordered a formal investigation, officials say.

"I’ve heard that story for 40 years," said Andrew Scanlon, the head of the United Nations Environment Program here, who dismissed it as an urban legend. "I think the need by diplomats for danger-pay raises is what has kept reports of fecal matter danger very high."

He wryly added: "It’s a very compelling story."

The supposed threat does have a certain logic.

In Kabul, Afghanistan’s capital, a city built for half a million people, the wartime population has swollen to 5 million. Only 5 percent of Kabul’s homes are connected to actual sewage systems, while most household waste flows straight into open roadside drains. Often their effluvia doesn’t make it far enough to join the floating hypodermic needles and assorted other muck of the Kabul River. Thanks to decades of drought, the stuff dries out and becomes part of this place’s ubiquitous dust.

Plus, most homes are heated with a bukhara, the Afghan version of a multifuel stove — and one of the most commonly used fuels is dried animal dung, much cheaper than wood chips or logs.

Kabul, a municipal official declared in 2007, "has the highest level of fecal matter in the atmosphere in the world," according to a Reuters article that year.

However plausible that claim may sound, it just is not backed up by scientific evidence. No one has been able to find, for instance, the original air sampling studies on which various reports have based the 30 percent fecal figures, let alone the supposed world record.

"It’s just not true," said Kabul’s mayor, Mohammad Yunus Nawandish. "Kabul air is not as polluted with human feces, as they say."

When the U.N. Environment Program did a study that included air sampling, in 2008, it found plenty to worry about, but mostly what would be expected of a traffic-congested city: a lot of sulfur dioxide and nitrous oxides. Plus a very high concentration of particulates, known in the trade as PM10 — which means particles smaller than 10 microns, small enough to penetrate deeply into the lungs, and an important indicator of air pollution — but no specific fecal bits.

A study by the Asian Development Bank and the Afghan government’s environment agency in 2007 similarly found the atmosphere thick with the usual suspects in any city, especially in an underdeveloped country where fuel quality is very poor, but it made no mention of flying feces — although toxic levels of cadmium were noted.

In fact, when the Canadians investigated the matter in response to their worried soldiers, the investigators said that some fecal matter in the air was normal — even in Canada. Some of it could just be bird and flying insect droppings.

While Kabul might have more because of open sewers and burning dung, that still was not a problem for two reasons: Ultraviolet radiation from the sun kills most microbes, and even those that might survive in airborne fecal matter are not the type to invade the body by air; they are the type more likely to infect victims through the mouth or skin.

The investigators were also unable to find the original studies that supposedly found such high levels of fecal matter in the air.

"I’d like to point out that I have a vested interest in this," said a Canadian colonel who was among the debunkers, quoted by Canada’s National Post. "I’m breathing this air, too."

Kabul may not have an aerial fecal problem, but it is far from off the hook on air pollution.

"Any kind of pollution is a problem," the mayor said.

Kabul’s geography is a big part of that: The city sits on a 6,000-foot-high plateau that looks like the bottom of a bowl, encircled by much taller mountains. The result is atmospheric inversions during fall and winter that trap airborne pollutants.

Dust in dry places like Kabul is a major pollutant. It is not unusual to see police officers and pedestrians here wearing face masks as protection.

Nawandish was not sure how his city shapes up compared to others in overall pollution. But he said its PM10 count two years ago was 250 micrograms per cubic meter of air, and this year it has dropped to 190 micrograms. "That’s because we paved a lot of roads and we planted a lot of trees and those two initiatives brought down pollution a lot," he said.

Beijing, for comparison, on a bad day has a PM10 count of 250, and averages 121; the average across the world is roughly 71, according to World Health Organization statistics.

So Kabul, with an average of 190, is worse than the Chinese capital based on PM10 count alone. (However, researchers say there are other pollutants in Beijing’s air that add to its specific danger.)

It is a deadly serious problem for Afghanistan’s capital. The Asian Development Bank report calculated that such a level of air pollution — even without feces on the fly — would result in 600,000 additional asthma attacks annually, and lead to an "excess annual mortality" of 2,287 in Kabul.

That means that Kabul’s atmosphere is more than twice as big a killer of civilians as the war; civilian casualties in the conflict that same year were 1,400 — in the entire country.