This week's column is about air pollution, principally emissions from diesel engines. But first, the forest and cedar trees.

A cleared stand of Kitayama cedar at Kitayama, northern Kyoto, shows how 1-2 percent of these forests nationwide may soon look.

For those in Japan allergic to cedar and hinoki pollen, March and April are torture. Even the "greenest" allergy sufferers have probably wished for a blight to hit these barren monoculture plantations masquerading as forests.

If this includes you, help is on the way -- maybe. Next year, the Forestry Agency will begin efforts to reduce allergy-causing pollen in Japanese cities by trimming and clearing about 100,000 hectares of cedar and hinoki nationwide. Officials hope the effort will reduce pollen in metropolitan areas by between 10 and 20 percent within three years.

As both a pollen sufferer and monoculture critic, this plan suits me fine. Still, a few questions come to mind.

First, after trees are cut, will other species be planted to regenerate the natural diversity of Japan's forests? Second, will it (honestly) do any good to cut 1 or 2 percent of these forests when cedar and hinoki nationwide cover 7 million hectares? Finally, what if the primary irritant for allergy sufferers is not pollen after all, but anthropogenic air pollutants, specifically diesel-fuel emissions?

Four years ago, researchers in the United States found that particulate matter in diesel exhaust attaches to pollen in the air, exacerbating allergies and asthma. A year later, the state of California designated particulate emissions from diesel-fueled engines as toxic air contaminants: "TACs are those air pollutants that may cause or contribute to an increase in death or serious illness or may pose a present or future hazard to human health."

How bad can diesel be? According to the California Air Resources Board, "Emissions from diesel-fueled engines are mainly composed of particulate matter and gases, which contain potential cancer-causing substances such as arsenic, benzene, formaldehyde, nickel and polycyclic aromatic hydrocarbons." A fact sheet from the board adds that diesel emissions "currently include over 40 substances that are listed by the U.S. Environmental Protection Agency as hazardous air pollutants."

According to the Vermont Public Interest Research Group, an environmental advocacy group in the U.S., "In addition to containing benzene, a known carcinogen, diesel exhaust contains high levels of fine soot, known as small particulate matter, or SPM." This microscopic soot is easily inhaled deep into the lungs, remaining there for long periods, corroding cells and potentially leading to tumor growth.

VPIRG reports that the worst type of SPM is less than 3.5 microns in diameter; 90 percent of the particles emitted by diesel engines are less than 1 micron in diameter. "Bound to these tiny soot particles," says VPIRG, "are a variety of toxic polycyclic aromatic hydrocarbons that are highly carcinogenic. Recently, one of these PAH called 3 nitrobezathrone was identified in diesel fumes. It is among the most potent carcinogens known."

If that is not enough to keep you urban joggers home, consider these research results:

* The proximity of schools to freeways and major truck routes is significantly associated with chronic respiratory symptoms in children (Speizer and Ferris, 1973).

* Children admitted to hospital with an asthma diagnosis are significantly more likely to live in an area with high truck traffic (Edwards, Walters, et. al, 1994).

* Asthma symptoms have been positively linked to truck exhaust (Duhme and Weiland, 1996).

* The U.S. Environmental Protection Agency has found that exposure to diesel exhaust, even at low levels, is likely to pose a risk of lung cancer, as well as other respiratory risks (EPA Report, 1996).

* Children living near major diesel thoroughfares are more likely to suffer from reduced lung function (Brunekreef, et al., 1997).

Children, clearly, are at the greatest risk, perhaps even before birth. In February 2000, the Japanese press reported that diesel SPM had been implicated in negative effects on reproductive organs.

Most recently, researchers at Japan's National Institute for Environmental Studies reported having replicated U.S. research results linking high SPM levels with increased death rates. NIES scientists revealed this month that they have found the death rate rises in Tokyo's 23 wards on days when the SPM index is high.

But SPM is not just an urban problem. According to Environment Ministry data released earlier this month, SPM levels along the nation's roadways exceed environmental standards 44 percent of the time. The standard states that a daily average of hourly values shall not exceed 0.1 mg/cu. meter in ambient air, and that hourly values shall not exceed 0.2 mg/cu. meter.

Diesel comes from a variety of sources, primarily trucks, buses and construction equipment such as bulldozers and heavy machinery. Vans and passenger sports vehicles are other familiar culprits. In Japan, where door-to-door delivery is the norm, tailpipes belching diesel particulates navigate even the narrowest country lanes and urban alleys.

VPIRG reports that "short-haul" diesel use is the worst. "On average, short haul, 'stop and go' trucks and buses are the dirtiest. Because diesel trucks and buses release their soot at street level, pedestrians and people living and working along truck routes are most exposed."

Though Tokyo remains hazy with particulates even on nice days, help may be in sight. The capital city is leading the nation with new regulations that, by 2003, will see diesel-burning vehicles banned from the city unless they comply with strict limits on particulate emissions. Busing and trucking companies are huffing, but Tokyo residents may soon be able to breathe a sigh of relief -- without fearing the consequences of each breath.