We've all seen smoke billowing from a big truck and assumed it's a big polluter. We've been right. Trucks are a major source of smog, of toxic chemical pollution, and of fine particle soot linked to cancer and respiratory problems.

It doesn't have to be this way. Most big trucks don't even use pollution control devices -- even though they could, and should. In fact, big trucks could be more than 90 percent cleaner than they are today -- especially if we clean up the fuel.

The U.S. Environmental Protection Agency has an opportunity to make big trucks cleaner. But will they -- or will they succumb to pressure from engine manufacturers and big oil companies? Here are some basic facts about pollution from big trucks:

What's a "big truck?"

Right now, the federal government classifies any vehicle above 8,500 pounds as a "heavy-duty" truck. That includes everything from some of today's largest pickup trucks to the biggest of big-rigs. Buses are also legally classified as "heavy-duty" trucks.

How much pollution comes from a big truck compared to a car?

Big trucks are legally allowed to emit as much pollution as several dozen of today's cars. But many big trucks actually emit as much pollution as 150 cars!

What sort of impact does big truck pollution have on the air we breathe?

Big trucks and buses are one of the biggest sources of pollution -- especially in urban areas. Diesel soot is a toxic air pollutant linked to human cancer. A new analysis by state and local clean air regulators concludes that more than 125,000 Americans will get cancer from diesel fumes. Other studies have linked diesel exhaust to the development of asthma. In many cities, diesel exhaust is the biggest source of fine particulate soot. For example, in New York City, diesels (mainly trucks and buses) constitute more than 50% of the particle soot. Diesel engines (including construction equipment) also emit about 30 permit of all smog-forming nitrogen oxides (NOx). That percentage will grow as future cars get cleaner

What else is wrong with fine particle diesel soot?

In addition to causing cancer, health studies have shown that fine particle soot can shorten our lives in other ways -- in fact, that up to 50,000 people die prematurely each year because of it.

Is it true most big trucks don't use exhaust pollution control devices?

Yes -- even though equipment is available. Starting in 1994, many new middle-sized trucks started using a device called an oxidation catalyst. But the pollution standards are so weak that even these trucks don't have to use exhaust pollution control equipment anymore. Instead the engine makers have learned how to manipulate the emissions that come out of the engine -- a tactic that may have led to an increase in emissions of dangerous "ultra-fine" particle emissions.

Wasn't there a pollution settlement involving trucks and the Justice Department?

Yes. In October 1998, the Justice Department (along with EPA and the state of California) reached a billion-dollar settlement with seven big-truck engine makers who were caught putting "cheating" devices on their trucks. (They rigged the trucks so they could pass a pre-sale emissions test, but then polluted much more once they were actually on the highway.)

What's wrong with the settlement?

It let the engine makers off far too lightly. (After all, they had been cheating since 1988). The settlement fails to recover more than 12 million tons of additional pollution caused by the cheating. It also requires state governments to beg the companies to give them money supposedly promised for pollution-reduction projects. And now to top things off, the engine makers are trying to weaken the terms of the settlement so they can continue polluting more!

What about future pollution standards for big trucks?

In 1997, the EPA set new standards due to take effect in 2004. (The legal settlement noted above moves up the date for the affected companies to late 2002. EPA is expected to reaffirm those standards this year.

More critical, however, are standards that EPA hopes to propose this year for trucks sold in 2007 and thereafter. Health and environmental groups are urging EPA to require that trucks and buses be more than 90 percent cleaner than those sold today.

More than 90 percent cleaner? Is that possible?

Yes, if diesel fuel is cleaned up. Today diesel fuel contains high levels of sulfur, a contaminant that prevents the use of advanced cleanup technologies. It's a lot like lead was for cars in the 1970s. Tests show that advanced pollution controls can remove more than 90 percent of the fine particle soot if low-sulfur fuel is available. That's why Europe is moving to require extremely low-sulfur diesel fuel. (Needless to say, the oil industry isn't happy about the idea of requiring cleaner fuels. The industry is trying to kill the EPA plan.) Reducing sulfur also would enable use of advanced technologies that could eliminate more than 90 percent of smog-forming NOx from big trucks.

What should EPA do?

In the short term, don't allow the engine makers to weaken the consent agreement and the related pollution standards. Longer term, remove most of the sulfur from diesel fuel and require more than a 90% reduction of fine particle soot and NOx from big trucks. Finally, EPA must make sure that trucks on the road actually meet the standards they're supposed to.